I am considering using a 50mm f1.8 maybe even try aN 85mm f1.8 on my Astro FI 102MM Telescope have you any thoughts or suggestions?

I have never taken images via a telescope so this is a whole new learning curve. Need to get my brain back in gear, stopped shooting over 5yrs ago due to the gear being too heavy and my back injury not coping so this will be a much easier way of holding the camera...I HOPE.

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    $\begingroup$ Hello! This question might be better on Photography SE. Also, do I understand correctly that you wnat to use both a telescope and a camera lens for astrophotography? Thanks! $\endgroup$ – Jonas Aug 5 '20 at 19:14
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    $\begingroup$ @Jonas It might get better answers over there, but it is certainly welcome here. No need to migrate unless the asker requests it. $\endgroup$ – called2voyage Aug 5 '20 at 20:22
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    $\begingroup$ @called2voyage I did not know that questions about astrophotography are on-topic here. Thanks for the information! $\endgroup$ – Jonas Aug 5 '20 at 20:36
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    $\begingroup$ @Jonas Since private beta first launched it has been seen as falling under the first bullet on our topic guidelines: "Setting up, using and maintaining your astronomy related equipment" astronomy.stackexchange.com/help/on-topic $\endgroup$ – called2voyage Aug 5 '20 at 20:39
  • $\begingroup$ @JohnLumb: amazon.com/… I know none of these books, but may a little theory won't do harm ? $\endgroup$ – user34599 Aug 6 '20 at 12:27

I'm not an astrophotographer but I'd like to make sure that you know that there are (at least) two ways to put a camera on a telescope from an optical perspective.

  1. The first is to keep the eyepiece on which produces a virtual image at infinity, and keep the camera lens on, focused at infinity.

  2. The second is to remove both the eyepiece and the camera's lens, which leaves the camera's CCD sensor right on the primary focal plane of the telescope.

The first method is the quickest, and it seems to be called Afocal photography (I didn't know that!). If you are careful and the image is very bright, say for example the moon, then you can just set the camera focus to infinity, hold it up to the eyepiece, moving around sideways until the inevitable vignetting is at least centered, and click! You can try to manage the vignetting by moving the camera closer or farther from the eyepiece. Ideally you want to place the entrance pupil of your camera's lens on top of the exit pupil of your telescope's eyepiece.

I predict you will quickly move to the second method which is called Prime Focus Photography. Find an eyepiece-to-camera body adapter and then you are a bona fide astrophotographer!

If you want to take advantage of your very nice f/1.8 lens then lose the telescope and just photograph the milky way or meteor showers. This works best if your camera can be programmed to take a sequence of regularly spaced images, because you can merge them later with a computer with offsets so that the motion of the stars doesn't produce trails.

For more on all of these, search questions tagged photography here and astrophotography in Photography SE.

For completenes' sake only, I'll mention a third way, called Eyepiece projection. While a properly focused eyepiece produces a virtual image at infinity, if you move it out, away from the telescope, it will produce a real image out past the eyepiece. If you are looking at the Moon or something very bright you could put a piece of paper out a ways away from the eyepiece, say 6 to 20 cm, and with some practice produce an image on the paper. You can then imagine having your camera's sensor at that plane. This gives you more magnification compared to prime focus photography, but it may not be useful magnification!

Further reading:

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    $\begingroup$ I'd like to add that with prime focus photography, you are simply using the telescope as your "camera lens." In the case of your telescope, this is effectively a 1325mm f/13 lens. You can use Barlow lenses as teleconverters to extend the focal length, but as with ordinary teleconverters this will cause the image to be dimmer (the f-number increases by the same factor as the focal length). Depending on the telescope, prime focus photography may not be possible (at least without serious adjustments to the telescope) if the image plane is too far inside the telescope to put on the camera sensor. $\endgroup$ – Puk Aug 6 '20 at 6:49
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    $\begingroup$ @Puk, exactly. I allways thought using the scope as a camera (and a good scope for photography is deliberately constructed with exactly that in mind) is much simpler than afocal because it only uses one focus and a simple optical bench-like setup with scope (the objective), corrector, distance tubes and pieces (there are never enough :-)) filter drawer and camera, especially for Newtons with their weird mass distribution and things sticking out at the side (cont) $\endgroup$ – user34599 Aug 6 '20 at 8:23
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    $\begingroup$ Newtons need the possibility to move the main mirror cell, so that the prime focus can be brought close to the focusser outlet where the corrector usually sits. Isn't there a good book to describe the possible setups ? Cloudynights.com is also an inexhaustible source. $\endgroup$ – user34599 Aug 6 '20 at 8:25
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    $\begingroup$ @a_donda I'm not aware of a book on the matter unfortunately. It is worth mentioning that collapsible Newtonian reflectors can also be used for prime focus photography, without having to move the primary mirror. The image plane position can be conveniently adjusted by collapsing the telescope: the more it is collapsed, the further out the image plane moves. $\endgroup$ – Puk Aug 6 '20 at 9:29
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    $\begingroup$ I use a small pocket size theatre scope to shoot the moon via smartphone. At first I got nice pics by keeping the phone on autofocus. Shouldn't be better to use the macro mode as for the camera lens is a couple of centimeters away the image formed on the scope eyepiece? When I have tried this second logical option I failed.... No much tests as it is very stressful and hard (no mounting or tripod of sort, yet with patience I got something nice). $\endgroup$ – Alchimista Aug 6 '20 at 9:37

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