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  1. #1
    Donald Qualls's Avatar
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    Building a sky tracking mount for astrophotography?

    I've seen many online plans at various times and places for so-called "barn door" mounts, simple sky tracking mounts to allow any camera with B or T shutter to be used for tracked sky photos -- tracked, so that stars, planets and nebula will appear as points (or at least small slightly fuzzy circles) instead of long trails, as would be the case with exposures over a few seconds using a fixed camera.

    What all seem to have in common is some combination of somewhat complicated geometry requiring considerable precision in fabrication, and tracking time limited to an hour or so before significant rate errors crop up (even discounting any errors in aiming the rotation axis at the celestial pole). It seems to me that a simpler mount could be built using a plain motor drive (shaded pole synchronous motors are common, have good torque, and hold rate very well if not asked to work against too much resistance), suitable belt reduction (belts can be readily slipped to move the mount between exposures and don't have periodic errors as gears can), and a counterbalanced platform to minimize torque requirements.

    Has anyone seen a plan for such a device, or had any experience in building one? Or would it be simpler to modify an old style telescope clock drive with a tripod ball head in place of the telescope tube?
    Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.

  2. #2
    rbarker's Avatar
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    Even the "clock motor" driven mounts for (time-exposure) astrophotography usually include a device for making minor adjustments in the tracking by placing a magnified "guide star" within a target of some sort. Commercial versions of these are available from astronomy shops like Orion Telescopes, but are usually intended for use on equatorial-mount telescope tripods to get the dual-axis movement required.

    I'd think it would be easier to adapt an equatorial mount tripod to wide-field photography use, as opposed to the other way around.
    [COLOR=SlateGray]"You can't depend on your eyes if your imagination is out of focus." -Mark Twain[/COLOR]

    Ralph Barker
    Rio Rancho, NM

  3. #3

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    The "ultimate" barn door tracker (or Scotch Mount) would be one of Dave Trott's double arm designs (Sky & Telescope: February 1988 and April 1989), driven by a stepper motor using Ray Grover's circuit. I managed to etch a circuit board for the drive circuit and got the mount half finished before I came across a deal for an equatorial tripod for my scope. The stepper motor route is much simpler than making anything involving reducing drives, belts etc. The mechanics of the whole mount can be rather crude. I think the single arm/single hinge driven by a hand-turned screw is very accurate up to 15 minutes. I would guess that any motorized scope tripod should hold a 35mm camera and up to a 200mm lens, and would certainly be easier unless you're very handy.

    Dave Trott's Homepage:
    http://hometown.aol.com/davetrott/page17.htm

    Quartz Controlled Scotch Mount by Ray Grover:
    http://www.mikeoates.org/mas/projects/scotch/

    Another page with lots of info:
    http://www.astronomyboy.com/barndoor/index.html

  4. #4

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    Also, get a copy of Michael Covington's book "Astrophotography for the Amateur", ISBN: 0521627400, and check Jerry Lodriguss' site www.astropix.com.

    I'd recommend a simple single arm tracker to see if you really enjoy this aspect of the hobby before you go spending a lot of time to build a double arm unit, or spend some money on a decent mount. You can probably build one in an afternoon for $20.

  5. #5
    Donald Qualls's Avatar
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    Well, it's the limited exposure time of the single arms, and even the double arm Scotch mounts that bothers me. I don't need guiding -- that's for high magnification; I'm talking about putting, at the heaviest, a 35 mm SLR with 135 mm lens and teleconverter or 9x12 cm plate camera on this mount, but I'd like the option, where skies are dark enough, to stop down a bit and leave the mount running for a couple hours.

    And an electronically driven stepper motor has the serious disadvantage that it requires me to work with electronics. I don't have a problem with that, as long as it a) doesn't require handling a soldering iron, and b) doesn't require handling discrete compnenets or integrated circuits. I don't even mind soldering irons, as long as I'm not soldering items that will be damaged by leaving the iron in place long enough to flow on a little extra solder.

    But a shaded pole motor can be taken from an old desk fan or similar, gotten from a thrift store for next to nothing.

    I suppose I could use a shaded pole motor with suitable reduction to drive the screw in a Scotch mount, but I'd need to make something adjustable to allow for calibrating to the actual running speed of the motor -- otherwise, even if I use a motor/gear set from a clock (and even if such a unit has enough torque -- clock motors are notoriously weak), I'll need more precision than my woodworking normally exhibits to get accurate tracking.

    I'm asking this because I'm trying to decide if it would be simpler to build a platform for the camera to replace the telescope in the mount for my little Meade reflector, and recalibrate the (wrong model) motor drive to match sidereal rate, then live with the little jumps that happen periodically with the (cheap, better than hand tracking for visual observation) tracking drive.

    The other consideration, of course, is whether I could align the mount well enough to be good beyond an hour in any case. A simple sight tube sized to put Polaris at the edge, along with a chart to know which direction to offset, should allow aligning within a few minutes of arc -- is that enough to avoid problems visible with a normal lens?

    Yes, I could probably build a manually operated single-arm barn door for under $20 (one piece of hardwood 1x6 a couple feet long, one hinge, threaded rod, assorted nuts and fittings, and, initially, wedges, hooks and rubber bands to mount and point the camera, plus a stopwatch or similar to know when to turn the screw), and from my yard, where the sky has a significant glow (even on the edge of a town of about 80,000) a five minute exposure is pushing things a bit. But I'd like to avoid building the whole thing again when I'm ready to drive out in the country a bit and try exposures of an hour...
    Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.

  6. #6
    rbarker's Avatar
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    It would seem to me, Donald, that without precise and almost constant tracking, you'd be likely to get some "wander" of individual bright stars. Doing the tracking for multi-hour exposures would get pretty tedious, I'd think. So, I think the requirements really revolve around what you want to accomplish with the images, and where the dividing line is between a circle of confusion and what might be more like a nebula of confusion.

    But, I've only poked at the problem enough to realize how difficult it is to precisely align a regular telescope's equatorial mount, thus enabling the clock drive to function properly. You really need to get input from someone with far more experience than I have. An astronomy forum might be a better place to seek such experience.
    [COLOR=SlateGray]"You can't depend on your eyes if your imagination is out of focus." -Mark Twain[/COLOR]

    Ralph Barker
    Rio Rancho, NM

  7. #7
    Donald Qualls's Avatar
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    Alignment is one of the big issue, alright. With a telescope, and photography though the telescope, it's a *really* big issue, because of field rotation -- even with accurate guiding, the field will appear to rotate around the guide object and you'll get trailing on objects in the edges of the field.

    However, the equation has changed, a little.

    I developed some images yesterday, and scanned them today, taken with a fixed camera -- "star trails" -- and it seems to me there are other limitations on exposure time, at least when shooting from my yard in town. I found the sky background at EI 800 was quite bright after only a five minute exposure at f/8 (even with the relatively poor reciprocity failure characteristics of Tri-X); five minutes at f/2.9 was enough to render the sky nearly as bright as in a normally exposed daylight image. So, I'll either need to stop down a good bit (like to f/16) or shoot at a lower EI, or both.

    My next experiment with this will likely be with my Zeiss-Ikon 250/7 Ideal plate camera -- not only will I get a much larger negative, but I'll be able to use ISO 100 film (I don't have any 100 speed film on hand for 120 or 35 mm), and stop down, if I choose, to f/45. And, of course, I can shoot and develop individual frames instead of dealing with a roll format. That will be much better for trails -- the trails will be longer if I can expose for, say, 20 minutes without the sky background starting to compete with the star trails -- but also suggests that tracked images could be effective with as little as five minute exposures, which is well within the capability of a single-arm barn door mount.

    So, perhaps I can get some wood and make a mount sometime soon.

    Can anyone recommend a (very) reasonably priced ball head that would let me aim the camera in an arbitrary direction while the mount axis remains pointed to celestial North?
    Attached Thumbnails Attached Thumbnails frame07.jpg  
    Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.

  8. #8
    Lee L's Avatar
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    Donald,

    Are you using one of the Meade ETX models? It might be simpler and easier to use that for a drive system. I've heard they're pretty accurate drives, but haven't used one myself. If you can build a camera cradle for the ETX mount that balances around the same axis as the OTA (optical tube assembly), then you should be in pretty good shape. Keep your camera as light as possible and the setup as balanced as possible.

    Another idea is to use a curved bolt drive in the same "barn-door" configuration as the Scotch drives. You just need to get threaded brass rod and curve it to the correct radius, usually around a routed wooden form. I use an old AstroKits drive that does this with good results on up to 180mm lenses and 15-20 minutes. The tangent error is eliminated in this form of drive, but you'd need better than 30 degrees of arc to get to two hours. Putting a tripod head on the end of an axle that is polar aligned and driven by a belt & disk has also been done, but can get bulky, and requires accurate machining and motor speed.

    If you have a library nearby with back issues of Sky & Telescope, you can look through them for ideas. They used to have a column called Gleanings for ATMs (amateur telescope makers) that you could browse for ideas.

    You might find that you can't get to 2 hours very effectively anywhere near where you live, and with the newer films with lower reciprocity failure, exposures of that length will almost certainly be unnecessary, or will be overwhelmed by skyglow. I'd expect that you'd need to be under very dry clear desert skies in the SW of the US or some other very remote location to get away with a two hour exposure.

    Even a well constructed commercial drive with a polar alignment scope costing $800 only promises 35 minutes with a 135mm lens on 35mm film. So you might be asking a bit much unless you can DIY with very high accuracy. http://www.sciencecenter.net/hutech/...memo/index.htm

    I've had excellent luck with an unguided Synta drive, available from Orion as the Atlas mount with lenses up to 100mm and 20 minutes (and probably longer if skies permitted). Nothing but pinpoint stars across the field.

    Oh, yeah, I assume you already know this, but use a camera with mechanical time exposure shutter operation, and pull batteries if you have LEDs in the finder that can fog the film.

    As I wrote this, you posted again, and I see you found the skyglow problem.

    The field rotation is not an issue with a properly polar aligned and driven equatorial mount, but is with an alt-az mount.

    Read Robert Reeves' site for recommendations on astro films.
    http://www.robertreeves.com/filmtest.htm
    It's a little out of date, but worth a look.

    Hope this helps,
    Lee

  9. #9
    NikoSperi's Avatar
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    Wow. I salute your courage if you undertake this endeavour. Precise polar alignment (or lack thereof) will bring you crashing back to earth, even if, and it's a really big one) you manage to DIY a clock drive sufficiently precise to track at 135mm.

    Even a $10,000 Astrophysics GTO requires guiding - i.e. some feedback mechanism to iron out the tracking irregularities inherent in any drive mechanism.

    The Jerry Lodriguss reference is a good one to get feel for the degrees of precision required for photographic quality guiding.

    For your sky glow issue, as you're using B&W film, one thing that will help you keep it down (depending on the type of illumination causing the glow) is to use a red 25 filter. If you want to invest a little to get something specific astro, then look into a night sky Hydrogen alpha filter (not to be confused with the solar hydrogen alpha daylight filter used to view eclipses etc.) . With that on your lens, you no longer care about moon/skyglow... but your exposures will get long.

    Good luck!
    If you tone it down alot, it almost becomes bearable.

    - Walker Evans on using color

  10. #10

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    Any drive you choose will still need to be monitored and corrections will need to be made. Have you thought about piggybacking the camera on an existing equatorial mount and scope? That way you could use the scope for guiding and checking. With a Scotch drive, you would still need a finder scope or a small telescope for alignment and tracking, and it all might cost more than a decent second hand setup.

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