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What is going on with these corners? AT115EDT -> ASI071MC-P -> AT 0.8x FR

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#1 Taylor

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Posted 12 October 2019 - 05:53 PM

I'm using my AT115EDT with the AT flattener/reducer (ATREDT15) designed for this scope with the ASI071MC Pro attached to it.

 

Can anyone give me advice as to what is going on in the corners? (ignore the processing, I'm still learning there)

 

How come the stars in the corners are showing coma? Shouldn't the ATREDT15 prevent this coma? 

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#2 View2

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Posted 12 October 2019 - 07:12 PM

Check fffr for spacing or tilt first? I don't know that scope but looks bad all over.

#3 rkinnett

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Posted 12 October 2019 - 07:15 PM

Star distortion either stretching radially outward (which is what your image shows) or squeezed like pancakes symmetrically in all four corners usually indicates incorrect back focus distance between your flattener/reducer and your imaging sensor.  Check the specs of your flattener for required back focus distance, then measure the distance from the rear flat face of the flattener cell, just forward of the threads, to the front face of your camera, then add 5.4mm which is the remaining distance from the front face of your camera to the sensor.  If the result is not within ~5mm of the required back focus for your flattener, then you need to adjust your adapters/spacers.  I hope your camera came with spacers.  If you're confused by this, read through some threads on here or elsewhere about back focus.  Also maybe post an image of your setup with a ruler as close as you can get it.



#4 Gipht

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Posted 12 October 2019 - 07:27 PM

The ASI071MC has a back space distance of 17.5mm and two included spacers of 21mm  and 16.5mm to provide the 55mm distance required for most flatteners/reducers.  While the combined distance may vary for individual optics, be sure you're using the two provided spacers as a starting point.



#5 Taylor

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Posted 12 October 2019 - 07:49 PM

I assumed that I had the back focus correct.

The ASI017MC-P has 17.5mm back focus built in, I used the included 21mm and 16.5mm spacers along with the ATREDT15, which should be correct, right? 

17.5mm + 21mm + 16.5mm = 55mm back focus. 

Does the Astro-Tech reducer/flattener alter some of the backfocus? 

 

My components are: ASI071-MC -> 21mm spacer -> 16.5mm spacer -> ATREDT15 flattener -> Orion 2” astrophotography filter.

 

From the Astronomics description of the flattener/reducer: "Back focus is 55mm from the shoulder of the reducer to the image plane, the standard spacing for DSLR camera bodies."

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Edited by Taylor, 12 October 2019 - 07:51 PM.


#6 TOMDEY

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Posted 12 October 2019 - 08:01 PM

If all the optical elements are good... then it's garden variety poor alignments... probably compound poor alignments among all the optics. Most of the problem is probably spacings scope to corrector to camera --- and also tilts. Although this stuff is supposed to be plug-n-play ... it rarely is that simple. If you want that, something like the dedicated Tele Vue-NP127fli will perform magnificently! That may cost a wee bit more, but is most certainly worth it!     Tom



#7 Taylor

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Posted 12 October 2019 - 08:03 PM

Is there a way to go about adjusting the spacings? 

The included components all thread together tightly, and I don't see any other way to go about adjusting the backfocus. 



#8 Taylor

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Posted 12 October 2019 - 08:25 PM

Solved. 
One of the sleuths over at the ZWO Facebook group found that the spacer ring that comes from the factory on the ATREDT15 reducer is missing. Thats what I get for buying used. 

 

 

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#9 rkinnett

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Posted 12 October 2019 - 10:42 PM

Cool, glad you figured it out.

 

Basically confirmed in this thread:

https://www.cloudyni...ner-back-focus/



#10 Taylor

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Posted 13 October 2019 - 08:23 AM

Yep, problem solved. Used my generic 0.8x FF/R from Orion for f/7 refractors last night, stars are round in all corners. 



#11 nightfly85

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Posted 14 October 2019 - 12:33 AM

It's just a 5mm spacer in case someone else needs one. I routinely removed the spacer on mine depending upon setup and spacers available.



#12 Taylor

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Posted 14 October 2019 - 02:34 AM

Sounds easy enough, thanks!

 

something like this?

 

 

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