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Prime Focus Imaging!

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#1 jdoparnico

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Posted 26 January 2013 - 03:10 AM

Hey Everyone!

So, I guess I don't really understand how Prime Focusing works. I have been using my 6" Newtonian Reflector Scope with my Cannon 100d, T-Ring, and my 1.25" Orion Universal Camera Adapter. I also have a new Sirius EQ-G too.

I have a AT65EDQ on the way which people is a great scope to take images on

I can take decent pictures with eye piece projection, but I don't seem to understand how everybody takes such great pictures with just prime focus? I know part of it is the scope I have.

Any help would be great!

Thanks,
-Josh

#2 Mike7Mak

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Posted 26 January 2013 - 04:23 AM

In 'prime focus' imaging the telescope alone IS the camera lens, an eyepiece isn't necessary. If you point your telescope at a bright object, like the moon for instance, with no eyepiece in the focuser, and just hold a piece of paper up to the focuser, move it in or out, at some point a focused image of the moon will be projected on the paper. That is the 'prime focus' of the scope. You can put a camera chip there for 'prime focus' imaging, or an eyepiece, which magnifies the 'prime focus' image, essentially increasing the focal length you're imaging at.

#3 Alex McConahay

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Posted 26 January 2013 - 10:28 AM

Eyepiece projection will get you greater magnification than "Prime Focus." But it is usually harder than prime focus, partly for this very reason.

As Mike has pointed out, just putting your camera on the end of the focuser, without eyepiece, is what most people do, and it is really pretty easy. You are just making your telescope and tube into one large telephoto lens.

What difficulty are you having with "Prime Focus" imaging? It could be your problem is that the scope does not come to focus. This is not a problem with prime focus imaging, but with your focuser/tube setup. Cameras have a different "back focus" requirement than do eyepieces. So, while your newt may come to focus with an eyepiece, it does not with a camera. This is a mechanical problem fixed by changing the focuser height or moving the main mirror. Or it could be that you are not getting the magnification you are used to with projection photography. This will be true. But projection photography should be reserved for extremely small and usually bright objects (like planets) while "Prime focus" is reserved for larger deep space objects.

I have a little something to say about the term "Prime Focus." Rather than hijack this thread, though, I think I will start a new one.

Alex

#4 jdoparnico

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Posted 26 January 2013 - 04:22 PM

I can take pictures with just hooking my camera up to the scope, but the objects are so tiny. I'm just confused how people can take such large pictures of objects by hooking it up by ”prime focus.”

Do theu actually use eye piece projection and just not mention the other equipment they use when taking the picture?

#5 jrcrilly

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Posted 26 January 2013 - 05:30 PM

Do theu actually use eye piece projection and just not mention the other equipment they use when taking the picture?


No. Medium and widefield images are usually captured with medium and short focal length instruments (refractors or Newt variants). Smaller fields are usually captured with longer focal instruments, often Cassegrain variants (SCT, RC, DK, etc.).

#6 WarmWeatherGuy

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Posted 26 January 2013 - 05:41 PM

Watch this YouTube video to learn about the field of view with different lenses and cameras, all at prime focus.

#7 jdoparnico

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Posted 08 March 2013 - 07:43 PM

Thanks for the video and your all's help! It helped out a lot.
I guess I just got my info mixed up somewhere here and there. haha ;)

-Josh






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