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#1 ryansmonacles20

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Posted 11 August 2020 - 01:09 AM

So I see some of those DSLR image/eyepieces. They are clearly used specially for telescopes etc..  Other cameras etc. can be mounted using a T adapter and can be an everyday use camera used for telescopes. So, my main question is how will I know if my camera will produce images with my telescope. Someone had pointed out to me something I didn’t understand and said some cameras won’t produce images. Shouldn’t this kind of issue only arise if your going to use some random camera you use for everything rather than the imager/eyepiece? I mean the DSLR eyepiece would be made specifically for a telescope. Thanks for any reply. 



#2 sg6

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Posted 11 August 2020 - 02:01 AM

I cannot work out the question. I think you are mixing up imaging, eyepiece, prime focus and afocal terms.

 

Visual is simple: Scope-Focuser-diagonal-eyepiece. The diagonal is part of the optical path length and so should(not always) position the eyepiece a little beyond the focal plane of the main objective - lens or mirror. The key here is that the eyepiee sits a little beyound the focal plane.

 

Prime focus imaging: Pull out the eyepiece and people think you swap to a camera and magically everything is right. Right being the camera sensor is exactly on the focal plane - it cannot be.

You either have to wind the focuser in a fair amount, or remove the diagonal, and then likely have to wind the focuser out. Called "playtime". Focusers these days seem to have minimal travel so expect to remove diagonal then add in say half the diagonal length for the focuser travel to be in the right region. Basically you have to stick the focal plane and so the image bang on the camera sensor.

 

Afocal imaging: A bit of a mix and relies on what a scope actually does. When you use a scope and look through it you do not get an "image". The optics is: "Collimated light in, collimated light out". Collimated light being light coming from a source at "infinity" (a long way off). Which is what goes into a camera and the camera lens then forms an image at the focal plane of the camera lens which should be on the sensor.

So eyepiece in standard scope, use your 20/20 eyeball to get a good image, take eyeball away, put camera and lens exactly over the eyepiece and take an image. With all the bits and the angles required Afocal tends to produce not so good image. It is a percentage game.

 

If your camera cannot be positioned with the sensor on the focal plane then it will not obtain images.

What happened here was Skywatcher said was:

2" focuser to attach a DSLR, on their general reflector scopes, especially the 200P.

What was "omitted" was the focuser never had sufficent travel to get the sensor and focal plane to coincide. So no image on camera sensor, so no chance of any image. But the DSLR was "attached", which was all Skywatcher actually said.

 

Often an assumtion is that imaging uses a camera lens, usually it doesn't, just a DSLR body.

If you use a DSLR for Afocal whatever holds the DSLR in stable position needs to be fairly big and solid. So a chunky framework and a heavy DSLR really throws balance out.



#3 ryansmonacles20

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Posted 11 August 2020 - 02:52 AM

You answered my question lol. And I was thinking basically: have the camera snug up against the eyepiece as much as possible, not just threaded into the eyepiece. 


Edited by ryansmonacles20, 11 August 2020 - 02:53 AM.


#4 ryansmonacles20

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Posted 11 August 2020 - 03:40 AM

So I just did some more reading. And now I get what you mean by focal plane but I have no idea how to determine focal plane. I guess by using the lens equation but...if my eyepiece was located on the side of the telescope not behind the focal plane and not the back of the telescope, then shouldn’t it be ok. 




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