* 12/18/2019: The following is long and often repetitive, but it might point to approaches that you'd like to pick up on. What I've ended up with is making simple fixed tripod "snap shots" and the occasional "barn door drive" shot --which I like to call "astrographs".
* 1/7/2022: When I last perused advice to beginners at the AAVSO (American Association of Variable Star Observers --nice people), I saw a gap --'twixt visual observation and DSLR based photographic methods --namely: the hand camera methods I go on and on about on this page. Their idea is to go for all the gusto that a DSLR might allow of, through recording "RAW" format images (over 1000 brightness levels), referencing "flats", "darks" and other calibrations, and then processing your star imaging through a specialized computer graphics astronomy program.
That would leave me feeling like a small appendage, along for the big ride. Also: I gather that darks^, flats^, vignetting, Gausian fall-off, and geometric distortion correction are built into many digital hand cameras (^at longer exposures).
The killer argument against using a consumer type hand camera: it records JPEG format images --which compresses the image information into the scant 256 levels needed to end up with a smooth looking photo print of your sweetheart or his/her birthday cake. That doesn't take into account, however, how magnitude has traditionally been gauged via astrographic film and plates: image spread. My graphs (below) show a transfer from image density range (what little there is) to star image spread^, which I measured using standard, close fitting (digital graphics) masks (getting the average brightness within a mask's confines), and calibrating against reference stars. (I use old, general purpose, bit-map graphics programs).
^ It's my impression that star image spread has traditionally
been attributed to halation --in the photographic emulsion of film or plates.
That may well have been the main factor, but I'm suspicious that rapid
image displacements due to air cell turbulence plays a roll --which, along
with clouds/haze, necessitates comparisons to nearby reference stars.
* How might one affordably (time and money) go about
a personal "program" of doing astronomy? I've nothing against "sky
appreciation" and getting your Messier certificate, but I wanted at least
a humbly defined ability to locate stuff ("astrometry"), measure brightness
("photometry"), and be open to the possibility of discovery --using charts,
reference images and my own records of past observations^.
Given my (in)abilities to sketch, remember/recognize patterns, and orient myself in the sky (which I need to work on), photography is very appealing --but I've here-to-fore needed more light grabbing power than what was at hand and a way to do it that's not a pain in the butt. I had fair magnitude reach and an easier time of it (thanks to those old manual, mechanical camera settings which stay put!) when I did darkroom photography and used an SLR film camera, but that really wasn't really enough light grabbing to work with either.
* What held promise for magnitude reach was "image stacking". A person might patrol portions of the sky using a camera, a tripod, normal charting/sky program resources, organized effort --and little else. I think there's good prospect in that approach under decent night skies --well inland from where I live and observe. Here near the Pacific coast, however, the atmosphere is turbulent --especially for the small cameras I try to use. 23mm of lens aperture might be tolerable, but I suggest a Canon T6 or T5 with the 55-250mm lens (44.6mm of aperture at full zoom and wide open).
* After noticing things like a missing star and a star that looked multiple in my astrographs, I placed an 8mm (diameter) stop over the objective of a 70mm refractor and watched a bright star while using a 150 line-pair per inch reticle.
The star kept randomly staggering around, often with +/- 8 arc-second excursions. This is a phenomenon that larger apertures (and/or many more stacked frames) should largely average out --depending on the image magnification. It must also be a major reason why brighter stars bloat/spread out on digital sensors and film emulsions. (I use to think it was just the brightness spread-out of "halation" to contiguous pixels (or emulsion grains).)
** I sent Jerry Oltion (the Astronomer's Workbench
editor for Sky & Telescope) a piece of this same printer film and he
got right on it --but under a very nice sky with steady seeing (no twinkling
stars). There were no star excursions with an 8mm stop. It's welcome news
that small camera astrography will likely work with a decent night sky.
^ Given my generally horse crap
night sky, my advanced years, the importance of being close to my wife
--for a while I pretty much gave up on the notion of "doing astronomy"
--in favor of just a little "sky appreciation" now and then (and being
a fair weather observer at that :-) --However: a better hand camera
came my way (thanks Chuck!). (No: I haven't hands-on explored the possibilities
of using a spendier digital camera with a significantly larger aperture
--more like that of my old 35mm SLR lenses).
* Another approach would be to lower my image resolution, such that the stars would wander --pretty much within the confines of a throughput pixel, but I hate to lose arc-minute resolution which is a bit better than night adapted eyeball resolution.
* As mentioned above, part of my problem was in the stacking of fairly high resolution images was that a dim star might move about and average away with the noise, while a bright star can look like a close multiple (unless many-many images are stacked).
* Perhaps my eagerness to "see more" through stacking is simply another version of "aperture fever" --which is a distraction from studying what I can see --and that's plenty, even if I'm looking through a pair of toilet paper tubes.
* I'd rather not mess with northing and tracking (while standing outside in the cold), nor do I want to be subtracting "darks" and "flats" --which can often be automatic when using an intact digital "hand camera" or DSLR.
* Also, my methods will be more accessible to others if I use a rather ordinary camera --perhaps an affordable DSLR (I keep expecting the prices of "mirrorless" DSLRs to plummet, due to their mechanical simplicity) --and easily replicated methods.
* Let's be positive. Most everything/method has an advantage. Where-as pro-class astrography, reaching down into "DSO" magnitudes, might record brighter objects as spread-out halation (or: as rendered, and despite the great mag range of CCD sensor based systems), my humble methods tend more to resolve them (and any nearby objects) more as points. Where-as long exposures and/or multiple exposures (stacked) see only the average of an area's luminaries, my short exposures record more of "what happened when". Whereas what's watched and noticed in the night sky has a lot to do with fashion, lists and popular literature (but take note, for "mining" commonly posted photos), patrolling neglected regions of the night sky might be more rewarding.
* Maintaining my web pages will continue in any event,
answering to new developments/revelations and in hopes of finally getting
most of my pages right --or at least free of glaring errors and repetition.
*
In light of the revelation
that stars are seen to wander when imaged through small (up to 8mm) apertures,
perhaps that was why I saw what I took to be rotation drift in my fixed
tripod polar shots.
So let's consider a typical 2448 x 3264 pixel sensor, centered on Polaris. What matters is not the zoom or field size, but the length of the exposure. Let's assume I'm using only a central circular area of the sensor with a radius of 500 pixels. At 39mm (my Samsung S850's max zoom focal length), 1000 pixels is a 12240 arc-sec (3.4 degree) field.
1 pixel of motion across a radius arm of 500 pixels is 0.002 radian, or 412.6 arc/sec --which, at the Earth/sky turning rate of 15 arc-sec per second of time, would require a 27.5 second exposure. For simply monitoring the north celestial pole out to 1.5 degrees, the maximum 15 second exposure of a typical digital hand camera should be free of apparent rotation.
* Now consider the problem of wandering stars (when using such small camera apertures under my terrible coastal night sky). This has been bad enough that dim stars don't record, and bright stars can appear multiple --when manually stacking just a few frames. Since I don't want to stand out there in the cold and dark shooting a long run of photos, the solution might be a larger taking lens. This could be achieved by afocally coupling to a telescope or (more sanely) by breaking down and getting a DSLR with an optional long lens (per that Canon item in the below chart).
* Why Polaris, and why Polaris only? Aside from being
always up (where I live), easy to orient on, and slow moving, I suspect
that the polar vicinity isn't being given much attention these days. (It
use to be watched and referenced a lot.) Although my photometry and astrometry
are pretty rough, I might be able to notice gross changes and visiting
bodies. As my friend Chuck suggests: one patch of sky is as good as another.
Although a more densely populated part of the sky holds more potential
for "happenings", circumpolar changes must be easier to notice, especially
if I become very familiar with the sparsely populated neighborhood up there.
Also: that's the best/darkest part of my backyard sky here.
* My Samsung S850's manual infinity focus will hold
if I set it last --and given the depth-of-field with today's hand cameras,
one doesn't have to toy with this manual setting (but I sure miss the stable
mechanical manual settings of my old film cameras!).
It's just too dang bad that the Bradford
operation shut down. It was a nice learning facility for school students
(world wide!) and a wonderful option for would-be "arm chair astronomers"
like myself. Some such folks were doing serious work. There is that rent-a-scope
operation but, aside from our household not being on-line (the ISP
here charges $55/month just for Internet, so we snag public WiFi by day),
simply requesting images of specified fields with specified filters and
exposures (they then schedule it and run the scopes/cameras) --made excellent
sense.
* In the past, reporting discovery was a rather arcane
process run by the Bureau for Astronomical Telegrams, which has since become
much less formal. However, it might still be best to work through a regional
observatory, one which makes a sincere public outreach effort --for confirmation
and the forwarding of an apparent discovery. Your concerns about credit
and priority might be answered by early disclosure, via "publishing" what
you've found to your own web page/s. That can be a no-cost undertaking
(with tutorial help) thanks to NeoCities.
* A goal of mine is to employ methods and equipment which could at least be a starting point for others who have limited spare time and funds. That doesn't mean doing bad astronomy, it probably does mean humble astronomy, and it certainly means doing honest astronomy.
* Like that perpetual pariah: the "department store telescope" (--but hey: the several I've owned have all been diffraction limited) --the worst feature of our night sky efforts (with camera, binoculars, telescope or direct vision) is usually the shaky way we mount our instruments, the way we seat ourselves, and the ways we plant our cheap tripods.
Speaking of tripods, how 'bout that 100 year old, common mechanical item: a single, 1/4 inch x 20 tpi bolt and receiver --which is often poorly located and seldom tight enough?
* Let's pause and think about what it is one is trying to do.
If it's VSOing (variable star observing --the traditional opportunity for amateurs to contribute to the data basis of serious astronomy), photography through a telescope might not provide a large enough field of view. A normal point-and-shoot isn't likely to serve, and a DSLR with a normal lens/zoom setting is more suitable for "pretty sky photos" than actual "astronomy". A tracked DSLR and a medium telephoto/zoomed lens, however, could be just right --if you don't mind the expense, setup effort, and spending some time with your images afterwards. (I still favor a regular digital graphics program for this work, but I've tried using stacking programs.)
* Looking back, using my Minolta SLR film camera seemed
so
simple: focus, shutter, aperture, film speed --and those settings would
stay where you put them. Minolta's 1974 manual (a worthy hardback book)
covering
all of their SRT series SLR cameras, lenses, accessories
and associated products spans 191 pages. The Canon T5i DSLR's manual --alone--
runs 380 PDF pages --!-- and my Samsung S850's manual runs 220 pages.
* Then there's "sketching at the eyepiece". I'd need
a good grid reticule, a corresponding grid lined sketch pad, and a way
to read/judge relative star brightness to pull that off. Higher power work
would be impossible without a visually accurate tracking telescope.
* What I need is a field of view that's "astrographically wide" --more like the 6 to 9 degrees that I used from the center of my cameras' sky shots. That's quite wide for a telescope, but about right for binoculars.
* When you think about it, not much of an objective is needed. Assuming a 6mm pupil in your eye, a typical 135mm telephoto (f/2.8 ~ 48mm effective diameter) camera lens^, a (max'd out, standard 1.25" barrel)) 32mm focal length Plossl eyepiece with an apparent field of 50 degrees --then we're talking 4.2x power and an actual field of nearly 12 degrees --close to what the camera lens was designed to cover --and about what I'd want for including some comparison stars in a VSO field.
Given that 6mm eyeball pupil, however, only 25mm of the lens' diameter would be used (the camera lens effectively being at nearly f/5.6 now). There's "room to grow", should your dark adapted iris be wider --but rather too much (comfortable as a nice, wide, 11+mm telescopic exit pupil might be). A 25mm (50 degree) eyepiece pencils out to 5.4x, 9.26 degrees of actual field, 32.4mm of effective lens diameter (f/4 now) --and you still have room to grow --into a nearly 9mm exit pupil. (Or: maybe you own a 60 degree, 25mm ocular, for an 11 degree field.)
For alt-az manual tracking, it would be necessary to turn the eyepiece (and its reticule) to keep a selected grid line crossing two prominent stars --something which would be a constant fuss --so maybe a visually accurate, table top "Poncet platform" under the 'scope setup would be in order and worth the trouble.
^ I tried an eyepiece behind an old 135mm telephoto
and the magnitude reach was poor --too many glass surfaces and too much
design compromise, perhaps --but it was a great image. Best I stick to
binoculars --or maybe cobsnobble the optics and eliminate the prisms. (Nah:
I really like a "correct"/erect image.)
Mirrorless DSLRs:
** The photography market is in flux (has been for a long time). Film and SLRs like my old Minolta are gone (of course) and even the "point and shoot"/"compact hand cameras" market is being gobbled up by smart cell phone camera capabilities --perhaps set up for astrophotography with a downloaded application. It could be that low end DSLR cameras are also among the walking dead --as changeable lens "mirrorless DSLRs" and quality APS-C class, fixed lens, hand cameras replace heavy-on-the-neck, DSLR albatrosses --but not yet (year 2016). Mirrorless DSLRs have only been around since 2009 and (it seems) the market for their premium priced novelty value has yet to be tapped out.
It seems obvious, however, that significantly smaller and lighter weight cameras, without the mirror, the shock inducing mirror lifting/locking mechanism, the pentaprism, associated manufacturing tolerances --and perhaps no moving parts in the camera body --will eventually smoke traditional DSLRs on price, performance and durability. But it's the mirrored DSLRs (so far) which are at all price competitive.
** I read that mirrorless cameras heat up their sensors and are hard on batteries --as compared to their regular DSLR counterparts (if not to smaller hand cameras). It must be that mirrorless cameras are designed to keep their sensors, displays and other parts cooking and generating heat --but won't a regular DSLR's sensor rise to a temperature plateau during long astro exposures anyway?
* Operating a DSLR in "live view" mode (when using the display instead of the optical viewfinder) has to amount to the same thing: a cooking sensor, display and the in-between electronics. More-over: that's the whole point of buying a DSLR with a swivel-out tilting display --so you can see the view, the focus, exposure and settings --without having to crane your neck (say: your camera being connected to the focuser of an oddly oriented telescope, or simply aimed up at the high sky from a low tripod).
* Meanwhile, and aside from craning your neck to use the optical view finder, you'd normally trouble to lock up the reflex mirror in order to prevent vibration.
* Do think on it before buying a camera with a fixed back display. Even though it might cost you an extra $200 for (say) the Canon "i" version, can you live with a back display that can't be swung out and comfortably angled toward your standing or sitting line of sight?
* Regardless the type of camera used, one appealing fall-back would be to make a "piggy-back" perch atop my scope, such that a rather ordinary digital camera can be mounted there, reliably aligned to the telescope. This would at least be a way to know for sure what part of the sky I was looking at when I logged an observation or something unusual. One still needs to trouble with camera adjustments (some of which --maddeningly-- revert to default settings when a manual setup's idle power-on period times out).
* Best I be doing and focusing upon one or the other, but should it be observing or photography?
* Howsabout sketching what I see --an appealing option. (See inspiring examples by Howard Banich and Mel Bartels.)
** All-in-all, the most sustainable effort seems to
be the taking of individual, fixed tripod camera frames (plus a couple
backup frames at each setup) --using setups spaced an hour apart. Given
my night sky situation here, I'd be shooting Polaris.
This is moth-to-the-flame, time consuming, crazy-making stuff.
Judging from past experience (with an A590 Canon camera and a 25mm Surplus Shed "500 Series" Plossl), this simple, swing-up camera mount should work fine for Lunar photography (with spot photometry on, delayed shutter release, and something between 2x to 4x of zoom).
* That swing-up mount is such that I can't crash the zoom lens into the eyepiece. Also, I've sized and glued in a plastic bumper such that the swing arm stops when the camera is aligned. We'll call this the "up-down" alignment --per the photo. (It should have been an adjustable screw stop, but I got lazy. The fine adjustment was done by adding 1/4 inch Dennison, self-adhesive dots.)
* For the other "left-right" alignment, I physically squared the camera with the aluminum arm (and therefore with the 'scope), then adjusted (rat tail filed) the 1/4-20 thumbscrew hole until the camera (set to widest angle) would display an evenly illuminated circle of light ---
~ --all with the telescope well collimated and aimed at our brightly illuminated ceiling.
* Preliminary tests on a standard line pair (Air Force type) target indicate I've got as good a setup as before (with Dog-1) and can reach 1.63 arc-seconds of resolution: half the resolution I'm able to see (0.81 arc-sec) and pretty good, considering I'm only running about one pixel per arc-second when using a 25mm ocular.
Again: past efforts to use a 10mm ocular/eyepiece turned out not so good. Also: were I to approach full resolution (difficult, due to that old "rule of thumb"), then my maximum exposure times (without tracking) would be halved.
* When shooting at a camera zoom lens setting of 9.9mm, the camera can manage an f/stop of f/3.5 --which means a physical (or effective^) aperture of 2.83mm. Meanwhile, the exit pupil of that 25mm eyepiece is about 2.9mm. That's a good match --if I'm projecting the exit pupil to the nodal point^ of the camera's lens --where (presumably) the camera's aperture occurs. If not, I might be restricting a wider cone of light, ahead or behind the exit pupil.
~ I shot at the A590's native sensor speed of ISO "80". (The best I could do with Dogson-1 and a similar (3.1 megapixel) Sony hand camera was 2 arc-seconds.)
^ We've come a long way from our old film cameras and "first principles" photography. I've reviewed a camera that basically keeps the lens wide open (and it should), throwing in a neutral density filter instead of stopping down.
There use to be a mark atop good cameras indicating
the film plane --from which one could measure forward to the nodal point
of a lens --perhaps for to pivot a panoram series of shots. It would be
nice to have that line for matching an eyepiece exit pupil location to
a camera lens' nodal point.
(Apologies! his whole page has been a poorly matched
mix of editing vintages --"a work in progress". Hopefully, my on-going
rewrites are doing more to discipline its words-in-a-row, than to increase
its length.)
Afocal and "barn door" driven/tracked astrographs:
I did calculations to anticipate how much exposure the turning night sky would allow --depending on the camera's zoom lens setting --or whether it's afocally coupled to my telescope. Since I realize about 2 arc-second resolution telescopically, the math is easy. Along the ecliptic (close to zero degrees of declination) the sky is turning at a rate of 15 arc-seconds per second of time.
A camera's moving "circle of confusion" (which is often the theoretical diffraction disc limit these days) describes both a 2 arc-sec circle at the camera's focal plane and in the sky --would immediately depart the camera sensor's trailing pixels, but would seem to "dwell" across the initial leading pixels --belatedly reaching new pixels at the end of the exposure. Imagining this, you can "see" how a dim (dimmest) star's 2 arc-second resolved image might still look pretty good, even though it underwent (say) 4 arc-seconds of drift. Consequently:
* I anticipated a 1/4 second maximum telescopic exposure near the ecliptic.
Similarly, my throughput resolution for simple tripod shots was anticipated to be about 2 arc-minutes, with an expected maximum exposure time of 8 seconds --maybe 10 seconds. On or near the celestial pole, however, I expected that the full 15 second exposure capability of my A590 could be used, and at any zoom or telescopic magnification (reasoning).
However (and again): I could see too much evidence
of star trailing, so I'll be barn dooring it.
Throughout this web page I often refer to the pixels, microns and diffraction disc sizes of my cameras. Best I set the main ones down together here for easy reference:
Camera Min
Max Sensor Aperture at
Aperture at Pixels Mn Diff Mx Diff Mn/Mx Rez
Zoom Zoom Pixels Min Zoom
Max Zoom Spaced Disc Disc
per Pixel
Samsung
S850 7.8mm
39mm 2448x3264 f/2.8~2.8mm f/4.5~8.7mm 2.3u
~4 mic ~6 mic 61/12 a-sec
low-rez 7.8mm 39mm
1536x2048 f/2.8~2.8mm f/4.5~8.7mm 3.7u ~4 mic
~6 mic 97/19 a-sec
min-rez 7.8mm 39mm
768x1024 f/2.8~2.8mm f/4.5~8.7mm 7.3u ~4
mic ~6 mic 194/38 a-sec
Canon
A590 5.8mm
23.2mm 2448x3264 f/2.6~2.2mm f/5.5~4.2mm 1.76u
~4 mic ~8 mic 63/16 a-sec
Canon
T5i/T6 18mm 55mm
3456x5184 f/3.5~5.1mm f/5.6~9.8mm 4.31u ~5 mic
~8 mic 49/16 a-sec
option 55mm 250mm
3456x5184 f/4 ~13.7mm f/5.6~44.6mm 4.31u ~5 mic
~8 mic 16/4 a-sec
Canon A590 sky test shot results --with and without
tracking:
* Even when centered on Polaris, the dimmest stars (approaching 8th magnitude) seem to be degraded by a scintilla of motion --within a 10 degree field (at 15 seconds and maximum zoom). (update: might have been "star wander".)
* Since my "barn door" tracker works well (quite well with a lightweight digital hand camera, and especially for short 15 second exposures), I'll trouble to use it.
* After the onset of "astronomical night", I ran barn door drive tracked exposures on the fields around Polaris (my "Observation Area 09c") and the beta star of Ursa Minor (OA-13a).
* Preliminary results indicate that, magnitude-wise, I do about as well with 9.9mm zoom as with 23.2mm (full zoom): to magnitude 6.5, and that both zoom settings seem able to locate individual dimmer stars to within a pixel. It also looks like I'd be able to resolve similar binaries at a separation of 2 pixels for the dimmest stars.
Astro-photography:
Some kind of a camera arrangement seems like a sane, self-documenting way to approach stellar photometry and other astro pursuits. I'd want to use an actual, self-contained camera --either direct to the sky or afocally coupled to my telescope.
One of the toughest aspects of doing prime focus astrophotography
with a film type SLR was critically focusing the image. However, my Samsung
and Canon hand cameras manually focus at infinity and can automatically
focus themselves on bright objects when afocally coupled to a telescope
(once a bright star or the Moon is within their auto-focus grasp). These
cameras also automatically subtract a dark field after longer exposures
(getting rid of "hot pixels") and I believe they do field "flattening"
as well --compensating for normal lens "Gaussian light fall off" and the
sensor fall-off peculiar to digital cameras (which is less important when
only the central part of a frame is used).
* It's usually best to run a well mounted digital
camera with its micro-jittery image stabilization turned off. While I can
imagine image stabilization doing some good on a single, bright, central
object, when shooting a field of stars, air cells will only affect small
parts of a field (if at all, non-telescopically). Consequently, should
a camera's image stabilization be able to lock onto the star which wiggles,
all the others get wiggled instead. I've seen frames with jittered star
images due to "image stabilization".
* For tripod or barn door shooting, I get about 1/2
arc-min images of 4th to 8th magnitude stars, but I feel that I can judge
their position to as little as 16 arc-seconds (one image pixel). Brighter
stars spread out more, of course.
* There's no point in trailing stars, so anywhere near equatorial declinations I'd use no more than a 10 second exposure (maybe 5 seconds at 9.9mm of zoom) if not tracked.
I once managed 2 arc-second resolution with afocal
coupling into my Dogson-1 (25mm ocular, 3x zoom worked best). Without using
tracking near the celestial equator, that would seem to forbid more than
about a 1/8th second exposure, but 1/4 second might work (per my previous
reasoning).
Centered on the celestial pole
(tripod mounted and at any zoom, or afocally coupled to a telescope at
any reasonable magnification/field), the field rotates at 73 micro-radians
per second. With a 5.76mm wide sensor in a hand camera, and 1.76 micron
pixel spacing, that should allow at least a 10 second exposure, and easily
the whole (typically the maximum hand camera) exposure of 15 seconds. However:
I seem able to detect the pesky effects of even sub-pixel image travel^
("egged out" stars), so I've ended up always using the barn door drive.
^ This might have been due to "star wander", caused by turbulence and such a small camera aperture.
I've gone to using only a 3 degree circle for my "observation areas" (882 pixels for the Samsung S850 --about right for printing out stereoscopic pairs) out of the center of an exposure. (Update: I'll probably be going to "blinking" image pairs, rather than stereoscopically fusing them.)
Originally, I used nearly the full 10 degree height of an SLR's 35mm frame, shot with a fixed 135mm telephoto lens (which turned 48mm of glass to the sky at f/2.8, a 33.8mm aperture at f/4).
* At "4x zoom" (actual focal length: 23.2mm) using our Canon A590, I get nearly the same field, but (and interestingly), the effective, maximum camera lens aperture (at "f/5.5") is nearly twice as wide (4.2mm) as the full (f/2.6) aperture (2.23mm aperture) at minimum zoom (5.8mm focal length). Theoretically, then, this camera should have better magnitude reach at full zoom, and when I ran more test shots both ways, it does.
* Canon's APS-C sensor'd T5i (and the newer T6 model)
DSLR --has a full zoom of 55mm at f/5.6, which equals 9.82mm of aperture,
so I'd expect something like a 2 magnitude gain --and at its 30 seconds
maximum exposure --instead of the A590's 15 seconds. The T5i also has a
timed/"bulb" setting, during which elapsed time counts up in its display,
and 2 minutes is do-able with my barn door drive. That should reach 10th
magnitude --if the noise level allows. (It had always seemed --to me--
best to run film and digital cameras at the native ISO --usually 80 to
100. However, I've found that much of the noise at higher ISOs can be averaged
out with stacking.) I've since reached 11th magnitude with my Samsung S850.
(edited
to here: 3/30/2018)
* The alternative of "stacking"
high
definition video frames with the T5i is interesting --but messy (IMHO),
due to the compression shortcuts used in MPEG formats. Detail gets borrowed
from one frame to the next and magnitude reach isn't great, due to noise
(if I understand it all correctly). Never-the-less, the bad frames can
be discarded --and we've all seen some amazing planetary and lunar imaging
results.
* The alternative alternative is to option several automatically sequenced exposures (our A590 does up to 10 shots) and then try stacking them. I did just that (January 16, 2017) and got gainful results by (manually) stacking just 5 frames, using a general purpose digital graphics program.
* Using a Samsung S850 (still only a 15 second exposure), stacking just 4 frames reached 11th magnitude, per:
* Previously, it's been my impression that stacking more than 5 to 10 frames leads to diminishing returns --aside from being rather tedious to do it manually. The procedure is to precisely align your frames in a digital graphics program, adjust the 2nd frame to 50% "transparency" ("opacity", or whatever vernacular your graphics program uses), the 3rd frame to 66%, the 4th frame to 75%, the 5th frame to 80% --and so on.
However, some very solid looking tests (with graphed results, and see the links below) indicate significant noise reduction up to 25 frames --using 12 to 14 bit RAW frames from modern DSLR cameras (or 16 bit, astronomy dedicated CCD cameras). You probably want a quality camera tracking mount and a pro grade tripod to go along with that.
To make so many frames a lot less labor intensive, you also want an automatic stacking program with astro-friendly features --like "DeepSkyStacker" or "Registax".
My oldest (general purpose) graphics program merges frames/images one at a time. Newer programs let you set up the frames as so many "layers", for which you'd set the merge for each layer to 100% divided by the number of frames for opacity. (Photoshop has some degree of automatic stacking.)
* Again: I'm shooting $100 to $150 dollar class hand held digital cameras --which only output 8 bit (per RGB color channel) tonal depth, for which stacking more than (say) 8 frames might not make sense to anybody. Another factor is that I want to preserve image resolution and see exactly what's going on as I register and combine my images.
* With all digital photometry, it's best to have your faintest study star images span several pixels, such that there's more of a voting consensus from them. Also, and if the field isn't crowded, a somewhat larger photometric averaging mask might be used to take in the whole of a somewhat dappled stacked star's image.
** This business of stacking images has opened up new vistas for amateur astronomy. Again: you can probably do more and do better with a Canon DSLR, a tripod, a good, polar aligned tracking head, a dedicated stacking program and a good sky program on your computer --then by spending the same amount of money on a commercial telescope --plus: your observations end up well documented.
** That all being said/stated, I can imagine tracked still exposures and untracked video stacking going well, but a series of untracked, timed exposures, each (say) 15 seconds long, presents significant sky rotation from shot to shot. Derotating shots afterwards in a graphics program --even if it's a sky program, means resampling the images with resolution and magnitude losses. The only way I can imagine comparing one such shot to another is by the klunky approach of printing out and physically rotating pairs of images into best registration ---then: either (somehow) "blink" the pair, and/or fuse and rotate them together stereoscopically. (Must rotate the pair through 90 degrees to detect all directions of motion.)
* Blinking use to be done by carefully aligning glass negative plates in devices (I imagine) with partially silvered//aluminized mirrors --which alternately illuminated and presented each of the pair. Print-outs would work the same way --or: one might try blinking them through a stereoscope.
One can't actually blink his/her eyes alternately, nor otherwise completely block one side and then the other --since our eyes then lose alignment (we all have a bit of aphoria, plus a stereoscope adds tension biasing). However: by simply blocking part of the view (with a hand or gray paper stock) for the dominent eye, eyeballs alignment is maintained and a portion of the print pair can be effectively "blinked". (I've lost confidence in only viewing pairs stereoscopically, but will still do that to supplement blinking.)
here - here - here - here - and here are links to what look like great web pages on stacking.
Magnitude --and "Poisson distribution"
* Although I was aware that CCD/photo receptors have long been getting down to tallying individual photons (say: with the venerable old GE-931 photomultiplier tube), until reading nitty-gritty stuff about photometry in the Sidgwick handbook recently, it didn't "click" in my head that the totals of such counts affected the certainty about a star's magnitude --in the same way that I error bracket my Geiger counter totals.
For 5+ years I've been logging gamma photons --as "background" radiation (alleged gamma radiation --another story). It arrives in ample amounts, but since a Geiger-Mueller tube only fires on about 2% of them, the mean sea level counts-per-minute is in the range of 10cpm to 35cpm --depending on the size of one's G-M tube. At higher altitudes and over more radioactive soils, the count averages can be higher.
* What one reports about logged counts can only be stated with some plus or minus degree of certainty, depending on how high the count rate and for how long it was counted. No doubt you're all familiar with this statistical stuff --so essential to running tests and experiments.
If (say) the long term average was 10cpm, and one logged any given total count over a period of 100 minutes, then that count has a high probability of varying by the square root of 1000 = +/-31.6 --for a 10 +/-0.3cpm average, but a low probability of varying by (say) 10 +/-1.0 cpm.
* When it gets down to counting arriving light photons, it's the same thing: you might only have enough photons to confidently state a star's magnitude to one place beyond the decimal point.
* Visual variable star observers are encouraged to work well above their minimum magnitude, in order to achieve valid results.
* When the logged evidence for a distant star's planet gets presented in terms of magnitude variations of only a percent or two, it must take a lot of collected light to stay above the statistical noise level.
* I won't be doing any such delicate photometry here,
but I'm looking forward to recalibrating our Canon A590 camera and having
at the area around Polaris --my northern meridian being much darker than
my southern. That, and maybe 4 more of my old "observation areas" (checking
for AAVSO targets) --well spaced through the seasons, areas in the sky
which pass over or near my zenith.
* My thinking has been: "it is what it is", and my focus has been on stars, which aren't as hard to fish out of the sky glow as are "dim fuzzies" and DSOs --which so many amateurs list and pursue. However, the "seeing" is so bad here that I joke about trading in my telescopes for meteorological gear --in order to take up the study of weather, clouds and states of overcast. But if I'm going to be griping about my night sky, maybe I should at least stand ready to put some numbers to my laments --something like: "magnitude per square arc second" (Mag/sas). That's also similar to how the "surface brightness" of "extended objects" (planets, the Moon, Messier nebulas) are rated.
* You might not care to get into the weeds of determining the dimensionless magnitude of extended objects. As to "sky glow", expressed as so much Mag/sas, more telescopic magnification (with the telescope's aperture held constant) makes the sky appear darker, since the available light per square arc second gets spread out across a wider (to the eye) patch of sky. (Do see Jerry Oltion's "Understanding Surface Brightness" in the December 2017 issue of Sky & Telescope.)
--This is a good time/place to review:
* The obvious "starting point" is bare-eyed vision, in which (say) a 60 degree field of view (use length = diameter toilet paper tubes if you wish) --flows into your 6mm dilated eye pupils at (we'll call it at) "100% light flux".
* Useful (for darkening the sky) telescopic power runs up to about 120x, at which point we're probably up against the "seeing" limit of an average night sky (so further magnification based sky glow darkening doesn't help).
* Were we to have kept making bigger objectives --to support increasing telescopic power --by delivering a 6mm exit pupil (we'd need a 28 inch primary at 120x), then --save for the telescope's transmission losses (which also apply to the stars), there would be no sky darkening, but the stars would get brighter, per the increased area of the primary.
* However: were we to stop making/installing bigger objectives at some point (say: we stopped at only a 60mm objective, used in a binocular at 10x power), then the night sky would start darkening as we added power --either with shorter focal length eyepieces --or with ever longer focal length 60mm objectives --but the stars would shine as bright (until the stars begin to spread out as diffraction disks).
* Another aspect to keep in mind (also covered elsewhere on my pages): how much aperture you can use in a telescope depends on how much sky you want to see in its field. Assuming (again) an affordable eyepiece with an apparent field of 60 degrees, a maximum dilated eye pupil of 6mm, and that you want to see 6 degrees of sky in your actual field of view, then the above example of a 60mm objective at 10x power is all the glass you can use. If you managed to shoehorn in a 5x eyepiece, you'd then be looking at 12 degrees of sky, but only through 30mm of the 60mm objective. For an 8 inch (200mm) objective, you'd be stuck at about 33x and just short of an actual 2 degree field.
* If your quest is for stars and planets, it could be that your best, sharpest vision is with an exit pupil in the range of 2.5mm to 3mm. In that case, you'd only need a 12 to 14 inch objective at 120x --or maybe stay with the crisp image you get at about 60x --needing only a 6 to 7 inch diameter objective --which telescope is a lot easier to get out of the house, set up in the back yard, requires less demanding build quality/rigidity, allows of an easily collimated longer focal length, can aspire to refractor-like quality, and bargain priced Plossl eyepieces will give excellent results.
~~~~~~~~~~~~~~~~~~~
Back to sky metering:
* I guessing that "/sas" (per square arc-second) was chosen because --traditionally-- the night sky and "good seeing" has supported only arc-second resolution, regardless of a telescope's aperture (and previous to very high altitude observatories with "active optics").
* The approach I've taken has been to get my meter onto the same page as that familiar "SQM" device, by using a "standard" lamp: a common, 4 watt, C7, 120V "night light" --at given distances from a sheet of good (95%+ reflectance) stationary. Below are the graphed results of tests run by Jerry Oltion and myself.
Westinghouse and GE spec their 4w C7s as having about a 14 lumen output. Since there are 4pi (12.57) steradians around the lamp, one would expect a light flux of about 1 foot candle at a distance of 1 foot, so those lamps are within hailing distance of being a "standard" (1/6th pound, whale sperm wax) candle. Indeed: my C7 balanced the light of a typical, tall, dinner table candle.
* Jerry read his C7 lamp light at 7 feet, 14 feet and 28 feet: which should have been two 4-to-1 ratios, or 1.5 magnitude differences (Mag = 2.5xLog (L1/L2). The jump from 7 feet to 14 feet read exactly right. The next jump should have been to 16.6m, but it read 16.82: a "shade" darker, but surprisingly close to the "curve", given that the SQM is only critically calibrated between 16 and 23 mags.
Final(?) useful range calibration
One might make a dial graduated in milli-foot-candles.
* I find the concept of sky brightness being measured in "magnitude per square arc-second" off-putting. Is it necessary? Is citing the total/lumped brightness of extended objects (planets, the Moon, dim fuzzies) useful to the amateur?
As to the night sky, I humbly suggest that it can be more meaningfully read in milli and micro foot-candles of light flux, something a very sensitive photographer's light meter might accomplish --or: my "sky meter", based on the old "holophane lumeter" and your great-grandpa's photographic darkroom "spot photometer".
* I like the "basic" nature of my sky meter. It seems adequate for urban skies, but one might try making a simple, lensed meter using a photo-resistive component or a photo-transistor.
As to reading the brightness of "extended objects", it seems reasonable to make an eyepiece version of my sky meter and cite measurements as so many micro or milli-foot-candles, profiling large objects from core to extremities.
~ 10k-Ohm, 1/4 watt audio/log taper potentiometer (#271-1721)
~ White LED, 5mm round (#276-0017)
~ A fresh alkaline 9 volt battery (any name brand source, relied upon to maintain 9+ volts --for years)
~ A 9 volt battery connector cap. (#___________)
~ A 9 volt battery holder (atop
the holometer, for easy access and voltage checking). (#____________)
Or: Simply
Gorilla Glue down a piece of aluminum (as I did), then tack glue the battery
to it with Duco
Household
Cement (a colloid glue which dries fast, holds well, and snaps off with
a pocket knife).
A briefly
used alkaline 9v battery should go its rated 4 year shelf life.
~ 100K-Ohm 1/4 watt resistor
~ 2.7k-Ohm 1/4 watt resistor.
~ Momentary, normally open push button switch. (#_______________)
~ Neutral gray card stock (or print your own).
~ A short piece of black plastic tubing with a frosted plastic diffuser and an orange filter.
~ A simple, plano-convex lens about
1 inch in diameter, with a focal length of 6 inches. I can help supply
that item (to U.S. addresses) if you can't get ahold of one. However, a
different focal length should
work as well, as long as the LED distance from the gray card is about the
same. Should you change
that distance or the circuit, simply calibrate what you end up with, using
a common, brand name, 4 watt,
frosted white "night light" at 10 feet from a piece of standard, 95% reflectance
stationary), such that
we can all get our meters somewhat onto the same page --and with at least
a rough comparison to the
digital read-outs of a standard "SQM" --then get the rest of the dial by
halving the current.
More photos:
An hour before "astronomical night"^, I read the dial at 75 degrees = 7.5mFC = mag 14.66/square arc-second.
At the start of astronomical night I dialed 182 degrees = 1mFC = mag 16.84. About 20 minutes later: 185 degrees = 0.95mFC = mag 16.9. That last took less than a minute of dark adaptation. The dial position seemed unambiguous.
9/2/2018: The sky was clear and a bit darker last night at 0.87 milli-foot-candle, for an honest mag17/sas. That was well after the onset of astronomical night. A minute of dark adaptation was essential for getting fairly consistent sky meter readings, the spread of which was from 189 to 193 degrees of dial rotation, averaging 190.6 degrees. (Each degree is worth about 0.0125mFC or 12.5uFC on that half of the dial.)
* I've a feeling that mag18/sas (about 0.35mFC) is going to be a bit of a reach (if my night sky ever gets that dark).
Mag 17/sq-arc-sec is a really crappy, big city, urban level of sky glow. I have medium city glow here, but lots of haze to fog and overcast stuff, being so near the coast and our encircling bay.
^ Defined as the Sun passing beyond 18 degrees below the horizon.
^ See Tony Flanders' Sky & Telescope article: "Rate Your Skyglow" for a good treatment of natural and artificial night sky glow. (The text of S&T articles seems to be Google-able now.) See also:
> http://en.wikipedia.org/wiki/Airglow
9/3/2018: Polaris (magnitude 1.97) first became visible to my naked eye when the twilight sky checked at 6.62mFC^ --which works out to: mag14.79/sas. (So ---what?)
9/4/2018: 'Twas a very nice, clear nightfall. I was first able to look directly at and clearly see Polaris when the sky (in that vicinity) checked at 10.5mFC --or mag 14.29/sas. (Yes: other, more to the zenith, brighter stars had already appeared, like the triangle of Deneb, Vega, and Altair.) (I feel that I've learned something --not sure what.)
* Here's a link to a web page with a helpful, cross-light/lumenence units nomogram --per this thumb:
10/16/2018: ** Per the above (linked) nomogram, it's been bothering me that a 15.6mag/sas sky can extinguish a 2nd magnitude star. The star is supposed to be over 275,000 times brighter --than those little square arc-second ("sas") patches of sky around it.
* With reference to the nomogram and my comments about how a telescope works, let's take it a step further --or rather: backward --from the starting point of a good 60x refractor with a 6mm exit pupil (therefore: it must have a 14 inch/360mm objective). With such a 'scope, "arc-second-seeing" becomes (20/20 vision) arc-minute seeing.
Going from this telescope's eyepiece to just looking at the sky --is like de-magnification ("minification"). If increased magnification (with the same diameter objective) darkens the sky (but not the stars), then minification should brighten the sky (but not the stars --since our retina's central rods and cones are effectively spaced for arc-minute resolution).
But switching from the eyepiece of that 60x telescope and looking directly at the sky --doesn't "concentrate" the 15.6 mags of light in each square arc-second --into the one square-arc-minute "points" that our eyes can resolve, since the imagined telescope neither makes "extended objects" (including the sky) brighter or darker --if the exit pupil (say: 6mm) is kept constant --by way of the objective's diameter keeping pace with magnification.
(I'm hammering these points because they seem so slippery to me.)
* Of course a star's disc, seen as a "point" on your retina, is (theoretically) about the same actual diameter of an arc-minute either way --but goes from being "fed" by that 360mm diameter objective to just the 6mm aperture of your eye (which is the same as looking through a perfect 1x power telescope with a 6mm objective). One would then expect it to be 1/3600 as bright --an 8.9 magnitude hit. Mag-2 Polaris then drops to mag-10.9 --against (say) a mag 15.6 sky --which somehow occults/blends it to invisibility at that point.
* We're talking 6mm exit and eye pupils --for which the eye's resolution drops off (it's best at 2.5mm or so). So let's assume 120x through the 'scope (to really nail that arc-second seeing), and 2 arc-minute eye resolution. That's 120x120 = 14,400 times gain/loss, or 10.4 magnitudes --for Polaris dropping to mag-12.4, leaving 3.2 mags unaccounted for.
* There's an excellent chance that I'm laboring under a misconception or two here.
* Again: I've been assuming here that the telescope's objective is kept large enough to produce a 6mm exit pupil --as telescopic power increases. Disregarding transmission losses through the telescope, it can then make "extended object" things look larger, but not brighter --nor does the telescope make the sky darker (than naked eye). Darkening the night sky requires increasing power, but holding the objective's diameter constant (therefore effectively reaching higher focal ratios --like stopping down a camera lens).
^ * Yes: I'm aware of "lumens" (same as the foot-candle), "candelas" (same as the candle but rigorously defined), cd/m2 and "mcd/m2", but I don't want to wander too far away from just "candles" --a concrete concept that I've at least simulated. Legend has it that the whale sperm/wax based "standard candle" was significantly brighter than a regular, paraffin, candle stick --the latter of which I was able to balance against the light from a 4 watt C7 lamp.
# As to foot-candles (FC) versus magnitude/sas: mag/sas is certainly a different kind of unit. Never-the-less, a target illuminated to a degree measured in either cd/m2 or FC (or milli-FCs) --can appear to be at some equivalency (to the eye, or to a spectrum compatible light sensor) --to a patch of sky --that's said to be illuminated to so many "Mag/sas". (At least: that's the implicit assumption of the above nomogram, old style "halophane lumeters" --and my assumption as well.)
As to cd/m2 versus FC: Candelas were first
established as being something like 0.95 foot-candles, then quietly promoted
to a 1-to-1 equivalency --decades ago. But the light flux of a standard
candle at one foot distant, seems to be a different concept than that of
1 candela --spread evenly and completely over a square meter. (Are they
talking solid angle square meters at a meter distant from a point source?
I try to study that stuff, but meanwhile, the "foot-candle" serves me well.)
(All these units strike me as more practical than "scientific", since it's
about eye sensitivity and mixed spectrums, but we try to apply them logically
and consistently anyway.)
Happy star trails!
Variable Star Observing
( newer approaches)
I built a lightweight tracker --which was too light (obviously) for my old 35mm SLR and its honking 135mm telephoto lens --even with that lead ball counterweight.
This tracker has an "arm pit" drive that's good for about 3 minutes. The (made in Tunisia) motor was taken from an older "Lux" brand wind up kitchen timer. The current version of this timer can be purchased almost anywhere (including Amazon.com) but it's now made in China, so good luck on getting the same thing with a nice metal shaft. The Lux timer's a bit spendy. Maybe something cheaper would work as well. Features and issues:
~ The humble escapement of this timer will slow down under load, so I added that sliding counterbalance. It weighs a pound. With that heavy old SLR film camera and its 135mm x f/2.8 lens, it ranges from barely enough (extended) to a little too much, so I keep a couple rubber bands handy as "helpers" (and to keep the works from flipping over in transit). For 95% of my aim points, however, first balancing the weight on that crank arm to zero, then sliding the counterweight in 1.5" (for about 40 grams of pressure --when aimed at the celestial pole) --works fine.
~ That old camera tripod is a honey and can almost handle the load, but you'll either want to use a lighter camera or get a heavier tripod --if you're buying a new one.
~ The second head under the camera was cut off of a typical $18 department store tripod and shows some strain. Again, use something heavier, go to a lighter camera, or cobble something up that will do the job.
~ To make the camera turn at sidereal rate, the lower arm is made of 2 pieces. The outer end, with the motor and arm, slides in and out against a short length of aluminum angle stock. It's secured at the right length by tightening a screw.
Curiously, this design has a bonus in that the effective length of the crank arm initially lengthens as it lifts against a glass slide on the underside of the upper arm, which (at first) compensates for the cosine loss of its up-angle.
~ Should one want to expose longer than 3 minutes, you'd build a more traditional barn door tracker that uses a tangential, chord, or curved screw drive and some sort of a variable speed motor --such as this honey:
> http://www.garyseronik.com/?q=node/52
--by Gary Seronik. Note that he uses a variable voltage regulator to the motor, which is far better than just a rheostat in series. Supplied with a "stiff" (meaning your power supply has a low source impedance), constant voltage, DC motors do their damnedest to run at a constant speed. If the regulator/controller isn't on the barn door itself, then use heavier lamp cord to run the power up and a stout automotive jack-and-plug. Possibly, this arrangement is immune to various loading presented by the camera's angling. If not, consider a counterweight arrangement, such that the motor always labors under about the same load.
* Seronik points out that while a double or compound arm barn door drive design seems an attractive solution, it's pointless unless you can figure out a polar alignment routine to achieve correspondingly high accuracy.
* For solid photometric results, I've found it best to stay 1 or 2 magnitudes from the limit of what I can capture on film or sensors, which happens to be consistent with general AAVSO advice about staying 2 magnitudes this side of what can be reached visually with a given telescope or binoculars.)
* Reciprocity failure sets in hard for fast films as exposure times increase, along with color "cross-over" problems. Hypering techniques are helpful, but only for longer exposures than I care to deal with --for which tracking and guiding become much more critical --which makes astrographs a lot less fun. It might be saner to get a good DSLR or a real astro CCD camera --but that leads to all sorts of expense and complexities. So --I endeavored to make modest exposures (60 to 180 seconds) work, using a simple tracking device. (Later I went to 15 second exposures.)
Here are yet a few more thoughts about barn door trackers.
~ The white wire pointer in the camera's hot shoe is a real eye hazard, which got replaced with a shorter version of that Polaris "glow finder".
~ The Polaris finder's "pupil splitter" lens "jumps" and projects that little 3/4 degree circle of glow film (per the above stereo photo) into the sky, where Polaris gets placed on it --according to the lay of the following chart:
Chart for locating the celestial pole
--which you turn to roughly match the night sky. The true celestial pole is in the middle of that little circle, on an imaginary line connecting Kochab and Polaris --so here we see Polaris located at about "10:30", which is where you place Polaris on the projected circle.
* Clearly, my BD Tracker is underspecified --when loaded up with my old Minolta SLR and a 135mm x f/2.8 lens. Something much more modest (and modern, by the way) is called for: like our nice little Canon A-590.
* Update: I've gone to a very similar looking Samsung S-850 camera --which turns 4 times as much glass to the night sky and just reaches 11th magnitude (stacked) at ISO-400. Stacking 4 or 5 frames knocks down the noise agreeably. Here it is mounted up on my improved tracker:
Normally, I'd pop a "glow finder" (see above) or a formed-to-fit white pointing wire into the hot shoe (avoiding any hot shoe contacts) --but: modern hand cameras don't have a hot shoe.
I tried the (as pictured) sighting glass (window plastic) again, eventually discovering that on the power management "Save" setting, the display dims way down nicely after 30 seconds.
* I also tried bending some 1/16th inch bronze brazing rod so as to snap around the base of the taking lens --to support and aim that white painted pointer. (The bent-around forward end is less of an eye hazard.)
* The sighting glass works just as well when stuck to the front of this camera --per:
* The paper-punched two dots of long-lasting, foil-backed, premium glow tape (from www.scopestuff.com/) are stuck to the upper edge of a lens in a pair of reading glasses. The idea is to "split" the night time pupil of my eye at the top of my glasses --so as to have distant objects and the reflection of the glow dots in focus at the same time.
Again: I barely reached 8th magnitude with the A590
(not stacked), 9th mag stacked, and 11th magnitude with my Samsung S-850
(ISO-400, 4 or 5 frames manually stacked). With both of these cameras,
much more physical aperture is turned toward the sky at maximum zoom and
aperture --than at minimum zoom. Both cameras perform at their theoretical
resolution (diffraction) limit and I can locate dimmer stars to within
a 15 arc-sec pixel (in relation to area reference stars).
However, and to my dismay: some
stars wiggled, while others held still --due to those damnable turbulent
air cells here (near the coast).
> http://www.garyseronik.com/?q=node/52
--is very appealing. The size, shape, curved threaded rod, and cleanliness of it is worth copying. Here's a more current hot link to the supplier for his motor:
> http://www.sdp-si.com/d790/html2/D790C14027.html
--and I see that it costs about $30. Should you want to shop around, here's another supplier:
> http://www.techmax.com/small-electric-motors/
Hmmmnnnn --- - -
* Since practical barn door trackers are used for exposures of only a few minutes (unless you want to fret about polar alignment --and aligning your Polaris telescope to the hinge within a minute of arc when you set up), and since you're going to be standing by anyway, then you might as well be the "motor" yourself --per:
> http://petesastrophotography.com/index.html?mainframe=http://petesastrophotography.com/barndoor.html
* But that only works if you've got the tracker bolted down to something sturdy --like a tree stump. So for use on a camera tripod, how about just any sort of a small DC motor to turn that big drive wheel? You let it advance to the next 5 second mark (using a normal camera lens/zoom) or go by half step intervals (135mm equivalent camera lens, or 4x zoom on a typical digital camera) --each time you close and hold down a remote pendant switch.
* Not to worry about exact dimensions --just make the threaded rod curve smooth and true, then "chase" its threads with a die to make sure the nut won't bind. I'd make the rig short and compact like Seronik's.
* After it's finished, park it under the stars (polar aligned) with a rifle scope in place of a camera --aimed at something bright near the celestial equator. Find out how far to turn the wheel at the end of a (say) 120 second interval and place a light mark on the face of the wheel (along the spiral, under the cursor wire).
Any little "gear motor", a battery, and some lightweight
lamp cord to a remote hand switch.
(Well, okay: a thin O-ring would be a lot better,
and maybe you want to file a groove into the motor shaft.)
* Figure out how many degrees it is each 10 seconds, then mark the wheel (per the drawing), and at each 5 seconds.
June 8th, 2016: --Been thinking --about photometry.
* For my last photometry/magnitude graph (from shooting on the night of 4/26/2016), I adopted the quaint old practice of simply fitting a least kinky, mechanical drawing type "flexible curve" to the dots. This approach "worked" well enough, but I had to admit to myself:
~ I'm simply not getting much star light onto these images. I can't hope to do +/-0.1 magnitude estimation for less than about 5th magnitude stars --and that's a very low star population, even in my large "observation areas".
~ Setting up a tripod with my barn door drive (nice as it is), and trying to keep my camera settings current is a pain in the ass --since the Canon A-590 loses patience with my dawdling and shuts down, plus it forgets my manual focus setting if I turn the blinding light of the LCD display off.
Hmmnnn: Does it really matter if the stars trail a bit? On or near the celestial pole, I suppose it does, since some trail more than others. Still: if I put a mask around the whole egged out image ---hmmnn. Elsewhere in the sky, trailing is more similar within a given field.
What if I defocused the camera a bit, such that the trailing is barely noticeable? I'd lose some magnitude, of course. I'd also depend less on halation and more on the dynamic range of the sensor pixels --for brighter stars. Moreover: there'd always be more pixels "voting" per star --a very good thing.
Dispensing with the barn door hassle ("northing" the
contraption and not subsequently upsetting that aim point, resetting a
drive run and restarting the timer motor) --would help a lot. Of course,
both trailing and defocusing should attenuate the magnitude reach of my
imaging, but that's how I use to do it with film. My intuition is that
what results I can get that way would be more consistent.
* These cameras are expensive to purchase new, but might be had at considerable discount used --since they're subject to that "latest greatest" fashion syndrome. Considering how cameras get "customer engineered" to cram more pixels onto the sensor and more supercilious features onto the program wheel (Canon recently added a "foodie" program to their T6 --for blogging flattering photos of your breakfast --sheesh), a reconditioned used camera might be a good buy.
* It use to be that affordable (film days) zoom camera lenses were crappy, but the comes-with (or built in fixed) standard zoom camera lenses on modern digital cameras are excellent and cover what use to be a (film days) full range of focal lengths (35mm wide, 50 to 58mm standard, 135mm tele --all with [film days] macro capability).
* There are two big considerations in favor of buying a DSLR. They're popular, plentiful, somewhat price competitive, and DSLRs don't (technically^) have to cook the sensor between shots --so less noise and more battery life. It might make dollar sense to buy a DSLR, always lock up the mirror, and never remove its standard lens (thereby gratuitously exposing your camera's sensor and other innards to dust and condensation deposits --and for that reason, you might plan on the expense of having a used DSLR professionally inspected and cleaned.)
^ To take advantage of a swivel-out camera back display, however, you'd be running a DSLR in "live view" mode --and cooking the sensor anyway --right?--, so you might as well buy a "mirrorless DSLR", or even a regular, fixed lens camera (and worry a lot less about dust on the sensor).
* I've the impression that DSLRs are not generally well sealed against the weather and moisture --unless such a feature is specifically listed. That doesn't bode well for going deep into the night, often with a great deal of condensation/dew forming on any surface exposed to the sky. (Another argument in favor of short exposures.)
Here are starting points for the blizzard of setting options on modern, APS-C class cameras.
> http://www.learn.usa.canon.com/resources/articles/2012/astrophotography_techniques.shtml
> http://downloads.canon.com/CDLC/Canon_CDLC_Astrophotography_Quick_Guide.pdf
Canon makes some spendy astrophotography cameras, but most of their advice translates to lesser cameras.
* I think about getting a better camera, but the prospect of using it for Telescope mounted prime focus astrophotography doesn't hold much allure, since it's a helluva bother and (again) I'd rather keep a good camera sealed up. (The exit pupil of my 25mm ocular seems to be a fair afocal match to the effective mid-zoom iris/nodal aperture of a compact digital hand camera.)
Tripod and barn door mounted sky shots/astrographs are a lot easier to manage and the full zoom of the standard lens (hand/compact camera or DSLR) runs 10 to 15 vertical degrees. That's a nice sized patch of sky to work with, orient on, find comparison stars within, and appealing for trying to image sub-fuzzies/"DSOs" and nebulous stuff.
I'm finding that almost any serious astrophotography seems to require an equatorial drive --piggybacking on a driven scope, or riding on the barn doors I describe in this page. I'm guessing that a good camera with a sensor in the APS-C class will reach 10th magnitude with maybe a 30 second exposure (Canon's DSLR maximum, short of bulb/timed). That's short enough to not require critical "northing" of an equatorial telescope or a barn door mount to keep the stars tight. (However: one might want to go a lot longer to get at nebulous fuzzies.)
** Aside from my standing amazement at the pixel spacing of modern sensors --and the diffraction limited zoom optics (the light passing through umpteen coated surfaces!) which try to match their resolution, even common digital hand cameras will automatically do the "flats and darks" for longer exposures, as well as having a tailored routine to suppress longer exposure and/or high ISO noise. (Just so the truth of the image doesn't get tailored out as well.)
* One thing missed in guides/advice to astrophotographers is that the common advice to "drop down a stop or two^" from maximum aperture incurs not only a serious light and noise hit, but often a resolution hit. Pixel spacing is on the order of 2 microns for hand cameras and 4 microns for (the Canon) APS-C cameras (that I've looked at), while the best diffraction disk diameters (which are much smaller than the old "circle of confusion" standards) for focal ratios of f/3.5 or f/4 is about 5 microns (which will be the size of dimmest stars --halation making bright stars much wider). For photometry, you want to spill out the stars across 4 or more pixels, so f/5.6 doesn't hurt too much, but your magnitude reach has taken a double hit.
Nebulous stuff, of course, gets a wide open lens --and maybe a push into upper regions of the ISO choices. (Again: not to worry about the vignetting of typically over-specified zoom camera lenses, thanks to automatic "flatting", but peripheral stars might end up a tad noisy thereby. I use just a central 10 degree or less circle of the image.)
* I'm talking about Canon T5i/s and T6 DSLR cameras with their standard 18mm to 55mm zoom lens, for which the maximum respective apertures are f/3.5 and f/5.6 respectively. Divide those numbers and see that minimum zoom allows of a 5.14mm effective aperture, while full zoom allows a 9.82mm aperture. Also noted: the theoretical, diffraction limited "circle of confusion"/diffraction disk is 4.6 microns at f/3.5 (about equal to the sensor's pixel spacing), whereas the diffraction disk reaches 7.4 microns at full zoom. Assuming that the lens is near perfect ("diffraction limited" --and I've found lesser Canon cameras to be so), that's a good thing, since you want at least 4 pixels illuminated for doing photometry --which might be the case for the dimmest stars in an image.
** That being said, and while I certainly covet a medium (APS-C) size sensor, I (again) question the wisdom of removable lenses. That was fine for film SLR cameras, since each exposure used a new "sensor"/frame of film, but out here near our windy, misty, rainy, dusty, salt air Oregon coast --hmmnnn.
Another problem are those rather small effective apertures: 5mm, 10mm; and for Canon long zoom options as well. The 18mm to 135mm zoom (f/3.5 to f/5.6) tops out at only 24mm, while the 55mm to 250mm (f/4 to f/5.6) tops out at just under 45mm of glass to the sky. That makes efforts to "digiscope" with a $50 to $100 binocular or spotting scope in the 50mm to 80mm range more appealing (despite such a lash-up being cumbersome to manage and losing the "flats" compensation feature of a DSLR).
* What I'd like --is to do some meaningful photometry/VSOing with simple tripod shots. It's a pain in the butt to set up/north an equatorial drive, locate the star field, then work through a raft of manual settings (often against the deadline of an automatic power-down/resets).
* And all that having been said: I've downloaded and am contemplating the Canon's whopping 380 page T5i manual. (Does anyone, even an astrophotographer, really need so many options?)
* Speaking of specific cameras, the top-of-the-line Canon T6s (at $800+) is most helpful for using and displaying manual settings, plus it has a button for locking in a "mode". If that means locking in your settings --and preventing an automatic power-down, that might be worth the premium price in itself. From my own experience, anything less is crazy-making.
^ Manufacturers are attempting to reach beyond the
once typical f/3 (or so) of digital camera lenses and we're seeing it in
non-zoom lens options, but review commentary has been in good agreement
that wide open should only be used when you need the light. It might be
that the resolution of (say) an f/2 digital lens is just too coarse for
astrographs --unless set down a stop. My old Minolta SRT-101 SLR film camera
could image 20 micron dim stars at f/1.4, but that's 5 to 10 times worse
resolution than what's expected of today's digital cameras.
** So: what's the conclusion here? How
the heck am I going to go about "doing astronomy"?
~ As noted elsewhere, I have a problem with using a digital astrophotography program which automatically coughs up magnitudes and IDs for everything in a camera frame. I guess I even have a problem with a modern digital camera that does darks and flats --much as I like the precisely timed image files and all the EXIF data. Of course, this all differs from the old days only by degree (a lot of degree), since Kodak made my film, Japan made my camera, and I didn't even mix up my darkroom chemistries "from scratch" --like my Mom did. (I'd be in a basket nearby as she printed pictures for Dad --who was far away --fighting WW-2.)
Ohmeohmy. I'm old now, tired and usually hurting somewhere.
* I do not disparage sky touring/appreciation at all, since simply beholding such wonders, like admiring art, is really the end result of all astronomical endeavors --toward better imaging of, relating to and understandings of what we're looking at. I envy the typical sky tourist's pattern recognition and familiarity with the heavens.
Yet, and despite how feeble my efforts are --especially compared to today's volume of research, done with incredibly large and precise instruments, I don't feel right unless I'm "doing" actual astronomy: recording stuff, timing events, astrometry, charting, photometry. That's more important to me than (say) having a better eyepiece.
It's turned out to be unlikely that I'll notice something --worth noting, but like my many failed business efforts: "I did my part" :-)
* Another prime motivation is understanding what I'm doing --even to the point of not doing what I don't understand, or doing without what I can't build, or don't want to invest the time to build and/or understand. Of course I can't make film emulsions, today's image sensors, the hardboard for my 'scope's OTA, nor the screws which hold it together --so let's put quotation marks around "build".
* For what little I do build and understand, another motivation is social: that it's within the reach of interested others --and thus these web pages.
** What makes astronomy most interesting to me is being in pursuit of a quest.
~ Perhaps watching a neglected spot in the sky --for who knows what to manifest.
~ Trying to recover a long lost, aberrant observation (like "Madler's Square" on the Moon).
~ Catching the light curve or gamma count evidence of a nova or GRB event.
~ Something remarkable in our atmosphere --like a formation of Canadian geese crossing the Moon (perhaps at a record altitude), or some sort of a "UFO" event.
~ _________________________ (Fill in the blank --for what you would like to nail.)
** Documenting what one sees is a vexatious problem which easily dominates and distracts from both the doing and the enjoyment of astronomy. After decades, I still haven't resolved this issue. For many years I practiced fixed camera and tripod "celestial snapshots" --which neatly compliments binocular observations and notes. Although small telescopes reach to an arc second or two of resolution --compared to a minute or so on my old negatives (which is also naked eye resolution), my photography reached to about the same magnitude (almost to 10th mag) as a small telescope or binoculars. The exposure was 10 seconds at f/1.4 (58mm lens) on pushed T-Max-400 --and later: 5 seconds at f/2.8 (135mm lens) onto Fuji 800 or 1600 color print film. That sounds simple enough, but it's unlikely that amateur astronomers are going to set up a darkroom and/or scan in film negatives --now-a-days.
For reference, I've glimpsed a 13th magnitude star with my 6 inch telescopes here in the Cities of Coos Bay/North Bend, but I sometimes have trouble spotting 10th magnitude stars on average nights.
Off-the-shelf consumer type (ie: affordable) digital cameras (to my knowledge) usually don't allow of time/"bulb" exposures and the consistent manual control one needs in order to obtain good results. Our affordable Canon A590 allows exposures to 15 seconds (a typical limit) at which point (I presume) sensor noise and "amp heat" become limiting factors. Such an exposure (full zoom, wide open) barely reaches 8th magnitude with our A590.
However, a recently acquired
Samsung S-850 (thanks, Chuck!) with 4x zoom (39mm) at a whole stop wider
(f/4.5 --turns twice as much aperture to the sky) gives tight stellar images
and reaches 11th magnitude with its 15 second exposures. I'm making stereo
pairs again with it --using my barn door drive,
of course.
I've gotten surprisingly good
Moon
shots by afocally coupling a common digital camera to my telescope
(maybe 2 arc-sec resolution). For star fields, the most practical method
for me has been to just aim a mechanical 35mm camera (the venerable Minolta
SRT-101) at the sky (as previously described). Your camera-film/sensor
might capture minimally moved star images near the celestial equator at
(say) 5 to 10 seconds, but you can use the same lens/setting centered on
the celestial pole for longer exposures. My old imaging would just "egg
out" (due to Earth's rotation) 20 micron images of dim stars.
Note that if they're moving across the film, they won't get any brighter with more exposure anyway --right?
With the Minolta's excellent f/1.4 x 58mm lens wide open (no problem with sharpness!), the limit was just short of the 10th magnitude (T-Max 400, pushed to 1600 EI), which gave plenty of stuff to study on a frame. (These weren't meant to be "pretty pictures", but the means of doing astronomy: astrometry, photometry, VSOing.)
When I had a darkroom and before digital images, I printed my frames to Agfa TP-6wp, a very high contrast graphics process paper that nicely split star images out of the fog. One could easily see the curve of the light fall-off (common to all lenses) toward the frame corners, especially when I used a 15 degree circle out of the center. At a 6x enlargement, that was about a 3.5 inch diameter circular print, which I used two of, side-by-side, as stereoscopic pairs. (I later switched to a 135mm lens at f/4 and 10 degree circles, which practically eliminated light fall-off.)
A 58mm lens comes pretty close to 1 degree per millimeter on the film --since there are 57.29 degrees to a radian. Fainter stars can be resolved down to 1 or 2 minutes of arc (20-30 microns). These fainter images started blending into the film grain for 8th mag stars and are gone by the 10th magnitude.
I did several experiments to "hype" various films and found it most efficient to use pure hydrogen --in which I "baked" the film using minimum volumes and explosion shielding. I carried the hyped film into the field in a desiccation jar and even tried using it with a dry ice cooled camera. Hyping does make a (reciprocity failure) difference for long tracked exposures, but seemed to offer little advantage for non-tracked "snapshots" of less than a minute's exposure.
Whilst having a life and a job or business, a "spare time" program of astrophotography with the claptrap, time and effort it takes to set up even a "barn door" drive arrangement didn't seem to be sustainable, but taking an ordinary camera out and "going click" --that's something which only needs a spare 15 minutes to accomplish. Later: I came to appreciate that extending exposures to (say) a couple minutes requires only casual "northing" of an equatorial mount/drive. See: barn door tracker.}
If you're using a camera with a flash shoe, the easiest way to aim it is to bend up a piece of coat hanger wire so it fits the flash shoe of your camera, cocks up an inch, then carries forward about 6 inches --which points toward your target star. Paint it white or with glow paint and be careful to prevent damaging someone's eye with the (bend it round) wire end. {Better still}
* One of my many failed strategies is to photograph where others aren't looking: "uninteresting" areas and straight at bright stars (which diminish the visual observer's magnitude reach).
* Interestingly, the Bradford robotic scopes in the Canary Islands (Tenerife) were programmed not to do exposures below 31 degrees of local altitude, and to instead wait until the requested object is higher in the sky (which is never for Polaris, so request a target just to the [astronomical] south). That's because you're looking through twice as much atmosphere, turbulence and dust at 30 degrees elevation. So: given the tropical locations of so many large scopes, I have to wonder how closely the area around Polaris is being watched --plus it makes a nice quarry: always up for us northern hemispherians.
Again: I've used stereoscopy and a trained eye to fuse pairs of circular prints, sometimes taken years apart. After registering them, I turned both circles together through 90 degrees, examining all possible shifts in the star field --as well as watching for variables. Aside from the limitations of film and paper media, the human eye has 10x the acuity for stereoscopic parallax, being able to notice shifts of only 6 seconds of arc! (I've discovered --nothing.)
Although my magnitude reach was poor and magnification low, I ended up looking at as much "stuff" as a photograph can hold. It's a mental test to pay attention to it all as the disks turn and all that autonomous parallax processing capacity in one's brain/"cyclopean center" chugs along.
* Of course: one might argue that "all the discoveries at such scale and depth have been made", but then again there are "visitors", and I doubt that many others are looking at this field stereoscopically.
* Comparing my "then and now" star fields stereoscopically didn't work very well, due to film defects, disparities in sky transparency and other vague factors. Also, I've lost confidence in only comparing fields stereoscopically --and am finding a way to "blink" my pairs as well --by simply waving my hand to cover part of the stereoscopic view to my dominent eye. (Stay tuned for how that's working out.)
I've resumed doing this work, but digitally --using a Samsung S-850 hand camera, ISO-200, a fixed tripod mount --no stacking or tracking. That's almost reaching 10th magnitude under a poor sky. (Stacking 4 frames reached 11th mag, but I thought it likely I'd average out precisely what I'm looking for.)
The disks were made with a hole saw. They rotate on the shoulder of two short flat head 1/4x20 bolts which pass through threaded holes in that 7 inch wide base (which has a finger hole as well). Their centers (3-1/4 inches apart) are well marked. Each disk has two small dabs of office tacky clay to hold those (bright star centered) circular prints --which were cut out with the shown yellow-black circle cutter. The 3/16" wide belt was made from two lengths of 3M/Scotch brand fiberglass "Mailing" (strapping) tape --sticky sides together. (It's the 2nd belt now. The first one lasted 15 years.) That short black area marks where I've placed some double-stick tape, so I know which disk can be clutch-slipped in order to register the print pair. That thing with a tall prong mounts the stereoscope so that my hands are free. I'm using a standard, traditional stereoscope with 8" focus (5 diopter) "prismatic" lenses --about as close as you need/want to work to 300dpi (image) print pairs made with a 600dpi+ ink jet office printer.
The T5i would be used for barn-door drive and fixed tripod "snap shots"/astrographs, with particular attention to the area near Polaris. At full, 55mm zoom (f/5.6) it turns the equivalent of a 9.8mm aperture of glass to the sky (only a 5.1mm aperture at 18mm minimum zoom [f/3.5]) --but I've nearly got that now with a Samsung S-850 (8.7mm at 39mm of zoom and f/4.5).
Although the T5i's pixel spacing pencils out to 16 arc-seconds (at full zoom), the diffraction disk at f/5.6 is about 8 microns wide, which covers 4 pixels (the minimum pixels you'd want for good photometry), so I'd expect a nominal resolution of 30 arc-sec --but probably with the (apparent) ability to locate a single dimmer star center to a given pixel or between two pixels (as I've done with our Canon A590 and the Samsung S-850).
Taking just a 3 degree circular field out of the center of the (full zoomed) 15 degree high frame should find the lens at its best behavior, with little need to correct for Gaussian light fall-off to the field's edge (although I believe such field "flattening" is automatic in digital cameras, along with dark field subtraction).
UNfortunately, even my Samsung's and a Canon T5i's apertures are tiny --in comparison to the lenses of good old 35mm SLR film cameras (say: 58mm x f/1.4 = 41mm), and while the diffraction disk at the sensor is still tolerable, the downside has turned out to be "wandering stars". I confirmed that with a small refractor and an 8mm stop (same as the Samsung when wide open at full zoom). The stars (here on the turbulent ocean coast) move about randomly, maybe +/- 8 arc-seconds --sometimes enough to make a star look multiple or to make a dimmer star disappear. I saw little such motion when using a 23mm or larger stop.
At this writing, it seems that the only cures would be to use a DSLR with a lens that turns more glass to the sky (say: the 55mm to 250mm zoom), or to use a rather large number of well-tracked frames, so as to average the star images out. Again: that might also average out my quarry.
"Keep Looking Up"
(Depiction of an old-time mariner finding his position by the stars.)
* The original content on these pages is unencumbered by copyright.
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