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Thread: Large format Astrophotography

  1. #21
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    Quote Originally Posted by Dan Fromm View Post
    Drew, if the current C-90 is the astigmatic dog that the two (one purchased, the other a warranty replacement) I've had were, it won't do at all for astrophotography.

    To be fair to Celestron, I got dog 1 in 1978 and dog 2 in 1983 after I realized that there was a warranty. The current one may be an improved design. The ones I had are the worst and next-to-worst lenses I've ever tried to use.
    Most new scopes are pretty good. Can always get a telecompressor or coma corrector to fix issues at the edges.

    Refractors are better and RC scopes as good and better.

  2. #22
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    I am gonna try eyepiece projection so to speak using a large format 150mm lens or smaller and see how that works. I just don't know how the Fstop of scope will combine with Fstop of lense.

  3. #23
    Tin Can's Avatar
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    Re: Large format Astrophotography

    I am a big fan of empirical science on all levels

    AKA WYSIWYG
    sin eater

  4. #24
    umop apsidn
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    Re: Large format Astrophotography

    Quote Originally Posted by cuypers1807 View Post
    Those of you who are doing this... how are you focusing the LF cameras? Are you marking infinity during the day?
    A good quality 8x or 12x loupe/magnifier to focus on the stars on the ground glass just before exposure. A miles-distant radio tower or tree for infinity isn't quite the same, so you have to focus on the stars themselves. It saves a lot of grief if the loupe is carefully pre-focused to the textured side of the ground glass during the day. Several times I've picked up the magnifier from the light box and it was set for thin film; re-focusing it on the camera for the thicker ground glass at night with the faint image on the glass can be frustrating.

    The focus of the camera can drift ever so slightly with temperature changes too. Starting on a mild evening and then working into a cold night I've seen my focus shift so it is important to check. I've ruined many a sheet of film in the camera to this not double checking the focus thinking it was still set.

  5. #25
    Drew Bedo's Avatar
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    Re: Large format Astrophotography

    Quote Originally Posted by Dan Fromm View Post
    Drew, if the current C-90 is the astigmatic dog that the two (one purchased, the other a warranty replacement) I've had were, it won't do at all for astrophotography.

    To be fair to Celestron, I got dog 1 in 1978 and dog 2 in 1983 after I realized that there was a warranty. The current one may be an improved design. The ones I had are the worst and next-to-worst lenses I've ever tried to use.
    Thanks for that. This C-90 is an older item bought used. I have not used it much, and only as a terrestrial spotter to familiarize myself with the mechanics of how it all goes together. It came with a bunch of eyepieces, prisms and T-Adapters. I was not too impressed. But then, it is pretty old. It has the orange finish and takes the 1.25" eyepieces. There was a period when the Chinese sourced C-90s were black, focused differently and took the 0.95" eyepieces.

    In any case, this one will probably not be used for anything beyond recreational spotting from the balcony of a beach house somewhere. I just cannot bring together the project I envisioned for any time in the foreseeable future.
    Drew Bedo
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    There are only three types of mounting flanges; too big, too small and wrong thread!

  6. #26

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    Re: Large format Astrophotography

    Very cool and not easy with the reciprocity response of film. Maybe try some hypersensitivity techniques? Astronomers have used that technique for decades with stunning results. Next stop: color film!

    The Harvard-Smithsonian has a collection of ~500,000 glass plates.

  7. #27

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    Re: Large format Astrophotography

    Quote Originally Posted by Steven Ruttenberg View Post
    Reason for wanting to cover 4x5 at minimum is for research. Using glass plates provides a fairly stable medium to make measurements once scanned in.
    What are you gonna measure? Angular distances between stars? They have been measured with space telescopes to within miliarcsec already. Not really sure what you trying to achieve for "research" here.

  8. #28
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    Many things, spectroscopy is one, looking for Supernova is another, studying nebulae.
    This will be part of my 2nd BS in Astronomy thesis/research project. As well as a masters and hopefully a PhD.

  9. #29

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    Re: Large format Astrophotography

    Quote Originally Posted by Steven Ruttenberg View Post
    I am gonna try eyepiece projection so to speak using a large format 150mm lens or smaller and see how that works. I just don't know how the Fstop of scope will combine with Fstop of lense.
    There's no simple rule for how to combine the f-stop of the reimaging lens with the f-ratio of the telescope - they don't add or multiply, etc. The whole system will certainly be slower when you try to magnify it to 4x5, because you're taking the same light entering the telescope and spreading it out over a larger area of film.

    Some care is needed in the design of a reimager. One rule of sorts is that the reimaging lens should be faster than the f-ratio of the telescope, otherwise the lens will vignette the on-axis beam. However, if you want to reimage a large field of view, then the reimaging lens usually has to have a large aperture (be very fast) to avoid vignetting the off-axis beams.

    Here is a rough picture to illustrate. You have a refractor with about 800mm focal length and 100mm diameter, so that's f/8. I'm not sure what the field that it images is, but let's say it's 25mm diameter and you want to enlarge it to 100mm diameter. Click to enlarge:

    Click image for larger version. 

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    The telescope lens is out of the diagram to the left. f/8 ray bundles come in from the left, converge at the focal plane, pass through the focal plane, and enter the reimaging lens. I drew a singlet lens, but clearly one would use a more complex lens. The lens then forms an enlarged image at the new focal plane on the right.

    This is not really to scale, but a couple of points:
    - this is a lot like darkroom enlarging; you're trying to make a 4x enlargement of the focal plane. (It's not exactly the same because of the direction of the incident ray bundles.) For this reason you may find it better to use an enlarging lens than an LF lens not designed for 4:1 ratios.
    - you may need a very fast aperture to avoid vignetting the edges.
    - I didn't draw any curvature of the telescope focal plane, but if either the telescope or the reimaging lens has curvature of field, and those curvatures are not matched, image quality at the edges may decline sharply.

    Whatever setup you try, I encourage experimenting with it and trying it out in the daytime to understand issues like the vignetting, which will be harder to see in the dark.

    Unfortunately, all of these kinds of problems are easier to solve in slow (large f-number) systems, while astrophotography has a strong preference for fast systems for speed.

    Reimaging cameras for large professional telescopes that cover wide fields of view can be extraordinarily involved because of these issues and leads to some exotic designs (large CaF2 crystals, super-fast f-ratios, etc).

  10. #30
    Steven Ruttenberg's Avatar
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    Re: Large format Astrophotography

    I am going to enjoy this very much. Most scopes, SCT and RC are f/8 to f/10 natively. M9st Refractors depending on cost can get pretty fast f/4 or faster.

    I will look you info over more closely when anesthesia wears off. Just got out of shoulder surgery.

    You gave me some ideas to consider.

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