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Unable to focus DSLR with OAG Set-up

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#1 AstroStar50

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Posted 20 March 2017 - 10:19 PM

I am unable to achieve focus on my DSLR with the following set up:

Celestron C-11 SCT, f6.3 focal reducer/field flattener, Orion Deluxe OAG, Canon T5i DSLR.

In the live view, I could see the star out of focus but there did not appear to be enough back focus to bring the star into focus. I tried removing the 30 mm spacer between the OAG and the DSLR but that did not appear to be enough to bring focus and then my camera body was right up against the OAG body.

I had no problem focusing when I removed the focal reducer from the back of the SCT, but I really would like to have the focal reducer attached to my scope when I do astrophotography. And, I would like to keep using the Zhumell Crawford style 10:1 focuser.

I am sure others have made this system work so I'm looking forward to a solution. Thanks in advance.

#2 Jon Rista

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Posted 20 March 2017 - 10:31 PM

Can you spell out your entire optical train in detail? You need the camera sensor to be, IIRC, 146mm from the back of the scope (I guess I am assuming a Celestron 1100 EdgeHD? Not one of the older 11" SCTs they have produced? The older ones might have even more backfocus...), or in the case of using the reducer, from the back of the reducer (note that depending on the exact reducer you are using, the backfocus may change). 

 

You've mentioned the scope, the reducer, the Orion Deluxe OAG, and the Canon T5i. You also mentioned another 30mm spacer. Assuming you need 146mm of backfocus, the camera will consume 44mm, the OAG will consume 30mm itself (IIRC, for that particular OAG), and you had 30mm of additional spacing. That gets you up to 104mm of backfocus, which would leave you 42mm short of the requirement. With that much of a difference, it is not surprising you couldn't reach focus. You would need MORE spacing, not less. :p 

 

Now, if your f/6.3 reducer has different backfocus requirements, then that will certainly change the story. If it has less than 146mm, you would need less additional spacing. If you are actually using one of the older Celestron 11" SCTs, a true "C11" (never liked it when the new EdgeHDs were called that, as it's just confusing), then that, too, will certainly change the story. I think the older C11s had 154mm of backfocus, so you would be 50mm short instead of 42mm. 



#3 AstroStar50

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Posted 20 March 2017 - 11:06 PM

Jon, thank you for your reply. I have a standard Celestron C-11 I bought new 3 1/2 years ago. It is not the new HD so it's focus apparatus should be more traditional.

 

I initially left the 30mm spacer between the Orion Deluxe OAG and the camera body/T-ring because the OAG instructions suggested starting with that configuration with a DSLR. 

 

I have not attempted to measure any of the components but to make sure I have covered everything, here is the best description of the set-up I attempted:

 

An f6.3 focal reducer threaded to the back of my standard Celestron C-11. A Zhumell 10:1 Crayford style focuser threaded to the focal reducer, the OAG inserted into the Zhumell focuser, and the Canon T5i/T-adapter attached to the OAG 30mm spacer. 

 

From what you posted, I may need to go with a longer than 30mm spacer between the OAG and T-adapter. That is good, as you can always find a way to add components, but shortening them is another issue:) Based on your measurement estimates of the extension I likely need, are you aware of where I could get such an extension? I imagine I am not the first to try this set-up so there are probably a number of commercial options. 

 

Feel free to fire away additional questions. I am becoming fairly comfortable with astrophotography, but the guiding aspect is all new to me. Thanks again. 



#4 james7ca

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Posted 20 March 2017 - 11:11 PM

I suspect that you need to allow for 55mm of spacing with that Canon DSLR, since the OP is probably using a standard t-adapter which almost always take a full 55mm. I believe that the standard Celestron f/6.3 reducer for the non-EdgeHD scopes requires 105mm, so take away the 55mm for the DSLR and you have 50mm left for the OAG and any other threading and spacers.

 

However, even at an incorrect spacing for the reducer (which may or may not be the case) I'm surprised that the OP can't focus his scope. With a moving-mirror type focus there should be plenty of range, at least when not using the reducer. But the OP says he only has this problem with the reducer, so maybe the reducer is altering the effective spacings so much that it can't come to focus unless the reducer is placed right where it needs to be in the optical path. That would include a recommended spacing between the rear of the scope and the reducer itself. But, the OP also says something about wanting to keep the "Zhumell Crawford style 10:1 focuser" which suggests to me that the Crawford focuser may be somewhere in the optical path and that could be the problem. If so (the Crawford focuser is attached to the scope) is the reducer in front of or behind the focuser?

 

Okay, the OP just added this:


An f6.3 focal reducer threaded to the back of my standard Celestron C-11. A Zhumell 10:1 Crayford style focuser threaded to the focal reducer, the OAG inserted into the Zhumell focuser, and the Canon T5i/T-adapter attached to the OAG 30mm spacer.

 

So, I suspect as I mentioned just above that the problem is the Zhumell focuser, that's obviously adding a lot of spacing between the reducer and the camera and that may not work.


Edited by james7ca, 20 March 2017 - 11:15 PM.


#5 darkstar3d

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Posted 20 March 2017 - 11:41 PM

I had this same issue with a Zhumell Crayford. The distance from the reducer to the sensor was too far. There are Crayfords, Moonlight I believe, where the FR sits inside of the focuser and provides the correct spacing. My solution was to sell the Zhumell to someone not doing AP.


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#6 leveye

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Posted 20 March 2017 - 11:51 PM

105mm using a 6.3 reducer corrector from the glass of the R/C to the camera sensor gives me the best results so adjust as needed with the OAG in between. This is why I like using the older Meade version because the spacing is perfect. No fussing with spacers.


Edited by leveye, 20 March 2017 - 11:51 PM.


#7 baron555

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Posted 21 March 2017 - 09:37 AM

During the day focus on something then physically hold and move the camera away from its present attachment point, you should be able to roughly see how much extension you need.

#8 Bloated Star

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Posted 21 March 2017 - 08:53 PM

I have the C11 with the Celestron OAG setup.  Might I ask what kind of guide camera you are using? The only reason I ask is that once you get the spacing issue solved (which I recently achieved) and can achieve focus, the next issue will likely be finding guide stars.  I'd suggest stepping up from an Orion SSAG which seems to often be the initial guide camera a lot of use when imaging with C11). 


Edited by Bloated Star, 21 March 2017 - 08:55 PM.



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