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  1. #11
    Donald Qualls's Avatar
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    What I've found so far is that five minutes is near the practical limit with Tri-X pushed to EI 800. I need something that will put the sky glow farther down the reciprocity curve than the stars to keep them separated; I intend to try on the next clear night with Fomapan 100 through an f/4.5, which ought to let me use exposures up to 20 minutes without sky glow being too intrusive (though ten minutes would be more comfortable in terms of trying to print down the sky without losing the fainter stars, at least for trails). Of course, I can also stop down, but that will tend to lose the fainter stars due to reduced aperture. Also, the night I took the test shots (including the one I attached a couple replies back) might not have been the clearest in terms of transparency, nor was it much more than an hour past dusk at the time I made those shots; I can probably do a bit better on the clearest of nights by staying up quite a bit later.

    I was rather surprised at how faint the stars can be and still be visible; with a 50 mm f/2.9, I was recording stars in a trail image that I have trouble seeing with naked eye. A tracked image should pick up faint stars better (I expect) since they will burn into the same point on the film rather than spreading along a trail.

    However -- with the practical upper limit still under 20 minutes, I can pretty readily get away with a simple barn door for now.

    For longer tracking, the mention of a curved bolt made me wonder -- if the bolt is curved, doesn't it tend to move (something) around an awful lot as it rotates? Oh, no, I guess one would turn the nut in this case. But one could also use a threaded rod as a worm to drive a gear made with JB Weld on the edge of a disk -- I've heard of folks doing this to drive telescopes, and as long as the radius is right it works well; one can, in fact, bend the rod around the edge of the gear to get improved engagement, as long as the radius of bend is long and the rod isn't permanently deformed. This one, in fact, might be workable from the (slightly adjustable) clock drive motor I have, which runs on AA batteries.

    Bottom line at present: I"ll start with a manual barn door and short exposures and see what I can get. I'd like to be able to do things like make printable images of objects like the Andromeda galaxy (M31), Orion Nebula, and the Pleiades as well as the Moon (much easier, though, since it's a short exposure sunlit subject). Longer term, it sounds if it would be less work to recalibrate my Meade drive (it's a Meade 4400, a cheap consumer "Christmas special" telescope from shortly before the computer drives came out in that class) and make a platform to replace the telescope tube.

    Eventually, I'd like to make an adapter of some kind and use my telescopes for this, but the Meade's mount doesn't do well handling the extra mass of a camera on the focuser, and my 8" is on a Dobsonian mount, so I'd need a complete mount rebuild to use it for tracking. Rich field photography works better with much shorter focal lengths, anyway...
    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. #12

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    The 8 inch Dob may work. Build an "undercarriage" for it and tilt it according to your latitude. In short, make it work more like a fork equatorial.

  3. #13
    Donald Qualls's Avatar
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    Quote Originally Posted by SteveGangi
    The 8 inch Dob may work. Build an "undercarriage" for it and tilt it according to your latitude. In short, make it work more like a fork equatorial.
    I've thought about that, just to make it easier to follow objects when observing visually -- but even here in the South, 36 degrees of tilt is enough to put serious stress on the base bearing, which will most likely make it very hard to get smooth movement in right ascension. On top of that, most of the weight of the tube will then be on one of the two bearing pads on each side of the mount, making the declination axis stiff as well (though that's less of an issue). Finally, there's still no drive, though that could be managed by inserting a disk with another teflon/formica bearing in between the base plate and existing box, driven by a variable speed motor mounted in the box and a belt around the intermediate disk.

    My dob isn't a low-slung ultralight, I'm afraid; the balance point is less than a foot from the middle of the five foot tube, which is made of steel furnace duct. It was easy, and it was cheap, and five feet doesn't weigh that much (it's no big deal to put in and out of my van) -- but it doesn't balance six inches from the mirror like a truss tube ultralight might. That gives a rather long lever arm -- my base would need an extended and very stiff plate on the ground just to keep the whole thing from tipping, and the forces on the R.A. bearing might well be too much for teflon...

    However, it isn't out of the question to design a fork mount that would use the same tube bearings I use with my rocker box. That's a long term project, though, that will take considerable thought, time, and probably money (and money is very tight right now).
    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.

  4. #14
    nick mulder's Avatar
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    I'm quite keen to build a scotch mount myself - I happened along this website:

    http://www.astunit.com/tonkinsastro/...otch.htm#Photo

    ... when googling 'CMOS 4060' for another photography project I'm upto (time-lapse drive for a 16mm cine cam)

    It got me thinking and I have a question for the astro types:

    could you have two or more of these mounted on top of each other so when the top one (or bottom one, whatever) completes its full action it triggers the next one and so on ...

    the first would be aligned with the pole for the time it started and it follows that my question is:

    >> if I knew my mounts wound down after 60 mins could I align the next in the stack for polar alignment an hour ahead ? ... and the next two hours ahead etc ??

    Actually, i can see it working fine, just need a bit of foresight and i have starry night pro (http://www.starrynight.com/) here on the powerbook to help me with that, but has it been tried before ?? easy ?? obvious issues i'm thinking of ? weight ?

    We have good dark nights just out of town here but i'd try just two for starters, very keen to try it with cine film, seeing the earth rotate around the stars ...


    (;
    cheers,
    nick

  5. #15

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    I have a 5 inch celestron Schmidt Cassegrain with equatorial mount. I have never used it for astro photography. I agree with those who indicate the problems with precise alignment.

    There is a filter (not red 25 and fairly expensive) for the sky glow problems that you mention.

    A 135 mm lens is probably going to include a fairly substantial chunk of sky and it will be pretty slow for night sky photography. I would want to be up in the 1000 mm+ area and have the capability to capture more light.

  6. #16
    NikoSperi's Avatar
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    Quote Originally Posted by Donald Miller
    There is a filter (not red 25 and fairly expensive) for the sky glow problems that you mention.

    A 135 mm lens is probably going to include a fairly substantial chunk of sky and it will be pretty slow for night sky photography. I would want to be up in the 1000 mm+ area and have the capability to capture more light.
    Actually, a plain Jane red 25 filter will do a world of good for B&W astro in cutting out skyglow. Worth using. Specific astro filters for camera lenses are expensive as they need to be cut to front filter size, rather than the standard 1.25" for eyepiece or 2"...

    Speed is not dependant on focal length. In fact in terms of astro, it is absolute aperture that is king. Wide field astro is alot of fun and alot easier to track. 1000mm and up means your guiding and alignment woes are going to be multiplied. Guiding at 1600mm on my (reduced) LX200 is practically beyond the mechanical capabilities of the fork mount and motors.

    Start with short focal lengths - trust me!
    If you tone it down alot, it almost becomes bearable.

    - Walker Evans on using color

  7. #17
    Lee L's Avatar
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    Michael Covington suggests the use of a didymium filter called a Hoya Color Intensifier to kill the sodium emission lines from high and low pressure sodium lamps. This is often called a Redhancement filter, and is available in the Tiffen and B+W lines as well. It passes much more light than a Wratten 25 or 23A. Search on Covington didymium in google to find sample shots. It's much cheaper than a typical LPR (light pollution reduction) filter made specifically for astrophotography. Unfortunately, lighting designers and mfgrs are now promoting full spectrum metal halide lamps that these filters don't work on as well. The metal halides are also less efficient and their blue spectral content scatters much more, contributing up to 6 times as much skyglow as the same wattage high pressure sodium lamp. (Why is the sky blue?)

    Niko is correct about aperture area (square millimeters) alone being what counts in astrophotography, and a 135mm f:2 or f:2.8 can be a nice astrophoto lens. Go for primes, not zooms. Going anwhere near 1 meter or longer focal lengths will require a very solid mount and long guiding sessions staring through a high magnification optic. That length is totally unsuitable for a barn door drive. Most folks using longer focal lengths (1 meter or longer) are now going to computer guiding using a CCD chip and software that keeps a star at sub-pixel registration by making adjustments to tracking motors. This is definitely not what Donald is asking for.

    Nice fast lenses from 35mm to 135mm or maybe 200mm with a solid and reasonably accurate barn door drive can get you a lot of beautiful shots. I have a curved bolt barn door drive (yes, you turn the "nut", actually a gear with a central 1/4 20 thread, not the bolt) that I've gotten consistently good results from with up to 180mm at up to 12 minutes.

    Film is also important. You need sensitivity at 650nm for red nebulae, and low reciprocity failure as well. Most of the really great astrophoto films are out of production, but Provia 400F and Elite Chrome / Ektachrome 200 do reasonably well. You can find the spectral sensitivity of most films in the tech data sheets, but you have to watch for reciprocity failure swamping red sensitivity. Robert Reeves suggests HP5+ for B&W astrophotography. In color film, you need to watch for differential rates of reciprocity failure in the different emulsion layers, which can cause radical color shifts. Ever shoot Kodachrome 200 at 1 sec or longer? YUCK!

    Film recommendations (all somewhat out of date, but the best info to my knowledge) at:
    http://www.covingtoninnovations.com/astro/films.html
    http://www.robertreeves.com/filmtest.htm

    The P factor in Reeves' tables is the value for the Schwarzschild exponent used in determining reciprocity adjustments, and is an indicator of the rate of reciprocity failure.

    For construction ideas go to "Gleanings for ATMs", a regular column in older Sky & Telescope magazines. Probably at a library near you. Lots of tangent drive designs, melted plastic teeth formed with a heated threaded rod driven with a worm gear made from threaded rod, Poncet and D'Automne(sp?) platforms for "equatorializing" your dob mount, etc.

    Lee

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