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Old Thursday 8th September 2016, 01:38   #1
think2100
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Join Date: Sep 2016
Location: Woodinville, WA
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Afocal with 35mm compact?

I have a Sony rx1r II full frame compact camera, which produces lovely photos, particularly in low light. It has a very nice Zeiss 35mm lens and 42mp sensor. I'd like to use it for afocal astro and terrestrial photography, but I've read here that because the afocal method places a lot of glass between the sensor and object, the image quality is not great. I'm wondering to what extent using very good glass would overcome that problem or whether it would be smarter to buy a Sony A7 dslr instead and use the prime focus method. The two options I'm thinking of are as follows:

Option 1: Rx1r II camera + 49mm to 48mm step down ring to connect to T mount, which connects to Adaptaview adapter which connects to 2" 21mm televue ethos eyepiece + Explore Scientific CF 80mm telescope. The 21mm eyepiece has a field stop of 36.2mm, so my hope would be that this would minimize vignetting. Cost - around $1900 (I already own the camera).

Option 2: A7 camera (not quite the quality of the Rx1r II) + T mount + CF 80 telescope. Cost - around $2100.

Which of these do you think would produce better images? How much vignetting could I expect with option 1? Other thoughts? Thanks much.
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Old Thursday 15th September 2016, 09:25   #2
Paul Corfield
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I can't say how much vignetting you would get. Can you get access to a telescope just to try it out? If you don't know anyone with a scope then most shops will allow you to test.

Image quality with either method would be quite similar probably but option 2 is usually slightly better. With option 1 you would need a variety of eyepieces to get a range of magnification. You would also need a good quality 45 glass prism to get the image the right way up and also orientated correctly left to right. An erecting mirror will only get the image the right way up but will be back to front on the left to right orientation. Just putting an eyepiece into the scope will mean the image is upside down for the camera. With option 2 you don't need a prism because the image is already the correct way up when no eyepiece is used.

The eyepiece you mentioned is very expensive. Plenty of eyepieces for a fraction of the cost which are still very good.

If you are using the same scope with either set up then I would try option 1 first and then go to option 2.

Get hold a cheap plossl eyepiece and try it with the camera, no need for a prism when just trying it out for quality. A cheap plossl wont be a million miles away from what you would get with the expensive eyepiece, especially at the low end like the 21mm you mention. The better eyepieces have a better field of view, better coatings etc but the difference for photography is small compared to a big difference when used for visual viewing. Explore Scientific make good eyepieces too, basically copying the Televue design. Baader Hyperions have a good reputation also. A basic plossl eyepiece will let you know if there's any merit in pursuing this method and then you can get a better eyepiece and a prism.

Benefits of option 2 are image quality will be very good, only need one or two teleconverters, start with a 1.4X and that's it. Only thing some people struggle with is manually focusing through the viewfinder. If your eyesight is good then this shouldn't be a problem.

Paul.
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Old Wednesday 5th October 2016, 04:20   #3
think2100
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Thanks, Paul. Decided to sell the rx1r and go with option 2, although I won't be able to get the telescope for several months.
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