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CPC925 focal plane/CMOS spacing question

Celestron astrophotography
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#1 Jim Cauthen

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Posted 21 March 2019 - 01:15 PM

I've got an equipment question that I'm sure has been asked and answered before, I just haven't been able to find the question/answer after a few days of searching.

 

BLUF (bottom line up front): I'm not sure where the focal plane is for my Celestron GPS C9.25 telescope with & without its f/6 reducer (NOT the EdgeHD), to enable setting spacing for my ASI1600MM camera setup.

 

I've had the scope since around 2012, but it's spent most of its life int he garage collecting dust. I did some basic astrophotography with it in the past with DSLR cameras, but nothing too dedicated. I decided this past winter to pull it out and dust it off (so it's probably my fault for all the bad weather this winter, sorry). This spring I was able to stumble across an amazing find (through a friend) on the 1600mm pro and couldn't pass up the deal. After going through the instruction manual, and lots of online digging, I can't seem to grasp where my focal plane is, or how to calculate it. I came across a thread about a c925 HD and its focal plane, but nowhere did they say where they got the info.

 

And how accurate does the spacing need to be? The thread (link to thread) about the EdgeHD talked about being able to adjust the focal plane with the focus knob - which makes some sense to me.. i'm must not sure how close I should try to get once I'm able to figure out the distance..

 

Any help/guidance/assistance would be appreciated!



#2 einarin

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Posted 21 March 2019 - 01:26 PM

Without the reducer the distance is where it's in focus.

With f/6.3 reducer it's about 105 from reducer to camera chip.

When it's a bit closer to reducer you get less reduction.



#3 Jim Cauthen

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Posted 21 March 2019 - 05:56 PM

Without the reducer the distance is where it's in focus.

With f/6.3 reducer it's about 105 from reducer to camera chip.

When it's a bit closer to reducer you get less reduction.

Thanks for the info for the focal reducer being 105mm. I was able to hit it pretty close using my original Celestron T-Adapter, the 16.5mm spacer, the un-numbered spacer (thread adapter?), and the smaller thread converter all stacked together. They snugged down nice and my calipers put it right at 105.494mm. I'm assuming the distance with the focal reducer isn't as critical, since as you said, it's only going to affect the amount of reduction achieved. But it made the former machinist in my happy when everything lined up and i realized I didn't need to order something else.

 

Still interested in finding the distance for the standard scope without the reducer. I don't see myself imaging at f/10... but I'm still curious



#4 carolinaskies

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Posted 21 March 2019 - 09:35 PM

Thanks for the info for the focal reducer being 105mm. I was able to hit it pretty close using my original Celestron T-Adapter, the 16.5mm spacer, the un-numbered spacer (thread adapter?), and the smaller thread converter all stacked together. They snugged down nice and my calipers put it right at 105.494mm. I'm assuming the distance with the focal reducer isn't as critical, since as you said, it's only going to affect the amount of reduction achieved. But it made the former machinist in my happy when everything lined up and i realized I didn't need to order something else.

 

Still interested in finding the distance for the standard scope without the reducer. I don't see myself imaging at f/10... but I'm still curious

Going back 20 years, the standard visual back and T-ring attached to it -  from Celestron (and Meade) placed most 35mm film cameras(and their commesurate depth to film plane at the prime focus point with the native focal ratio of the SCT.   

Focal reducers will each have a manufacturers specified back spacing for optimal performance.  When fitting together a focal reducer, camera, OAG, filter wheel this back spacing is important to not induce coma. The further the spacing is away from the optimal the more coma is introduced.  



 




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