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DSLR Back focus issue or something else?

DSLR Imaging Astrophotography Beginner
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#1 poshchiken

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Posted 21 June 2021 - 04:51 PM

First time poster - please take it easy on me!

 

Recently purchased my first set of AP equipment and took a few initial pics to make sure I am going down the correct path...I'm pretty sure I have an issue based on the two attached photos.

 

Do I have a back focus issue?  The star image is displaying stars that are elongated in the corners - they appear to radiate away from the center.  I also attached a picture that I took of the moon with the same focus setting on the same night. The outline has a haze while you can see that the craters are generally in focus.  Not sure if the two issues are related of if the moon photo is pointing me to another separate issue (bad seeing conditions, telescope optics, etc)? 

I used a Bahtinov mask to focus prior to taking the photos. 

 

Scope: Astronomics Astro-Tech AT80EDT w/Astro-Tech 0.8x Reducer/Field Flattener     

Mount: Sky-Watcher EQ6-R Pro

Main Camera: Canon EOS 60D

Guide Camera: Zwo Asi120mm mini

Guide Scope: Astromania 50mm

ASAIR Pro 

 

Screenshot_20210620-231143_ASIAIR_test.jpg

Screenshot_20210620-224042_ASIAIR_test.jpg



#2 Trenor

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Posted 21 June 2021 - 08:34 PM

Judging by that first image I'd say you had the distance off some where. Check the specs for the distance from the flattener and the camera sensor. Most of the time that is 55mm but it can vary. Once you are sure that is correct, I would see if I could fine someone else with this scope flattener pair and see what spacers they use.

 

 

If you can't find anyone with that information then you can add or remove spacers from between the scope and the flattener and see if it improves the star flares you have on the edges. If adding improves keep going until they are as sharp as you can make them. If it makes it worse remove some and see if that improves it. 

 

It would be better if you have the specs but if you can't find them trail and error should get you closer.

 

Good luck.



#3 Jimmy462

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Posted 21 June 2021 - 08:35 PM

Hi poshchiken, and welcome to CN!

 

Looks like severe coma in your image. In looking at your equipment list I am not seeing mention for any spacer to attain the 55mm backfocus requirement for the Astrotech Field Flattener. Posting an image of your OTA's imaging setup might help diagnose the issue.

 

Best, :)

Jimmy G



#4 asanmax

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Posted 22 June 2021 - 12:00 AM

Welcome to the forum!

 

It does look like a spacing issue but for some reason the coma is too pronounced. Did you buy the flattener second-hand? Is it possible that the lens block is flipped in it?



#5 poshchiken

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Posted 22 June 2021 - 12:03 AM

Thank you for the suggestions.

The Astronomics website states the following for the Flattener: "Back focus is 55mm from the shoulder of the reducer to the image plane, the standard spacing for DSLR camera bodies."  It looks like Canon EF-S mounts have a flange focal length of 44mm.

 

I have a T-ring adapter and a thread adapter that came with the Flattener; in between the Flattener body and my camera.  The T-ring is 11mm and the thread adapter is ~10mm.  If I add those to the flange focal length of 44mm I am up to ~65mm of spacing. Is there anything I can do to reduce the distance if needed?  It looks like I need to find a way to remove the thread adapter component.

I attached a few pics of my setup.

 

I don't think it matters but I did have to remove the camera angle adjuster and attach the reducer/flattener directly to the 2" focuser tube.  This would have shortened the distance between the scope's optics and the reducer/flattener's scope-side optical element.

 

20210621_225748.jpg

20210621_225815.jpg

20210621_225521.jpg

20210621_230106.jpg

 

 

Thanks!

Ian


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#6 Tapio

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Posted 22 June 2021 - 12:23 AM

Don't know why you have 2 adapters for FR/FF.

Only T adapter for DSLR should be enough and that adapter should fit FR/FF.



#7 Jimmy462

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Posted 22 June 2021 - 04:51 AM

The coma would be produced by too-close spacing between the FF and sensor (link provided for instructional purposes only)...

 

Back_Focus_Spacing_Guide_1024x1024.jpg (JPEG Image, 576 × 1024 pixels) - Scaled (91%)

https://cdn.shopify....e_1024x1024.jpg

 

Just a question to help eliminate other possible causes...all optical items (telescope + FF) were purchased brand-new by you or are they second-hand?



#8 unimatrix0

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Posted 22 June 2021 - 07:22 AM

Is the thread adapter a M42 to M48? If it's so, then what you need is a T ring for you camera, but with the M48 thread, so you won't need the thread adapter as it's adding you an additional 10mm you don't want. 


Edited by unimatrix0, 22 June 2021 - 07:23 AM.


#9 poshchiken

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Posted 22 June 2021 - 09:39 AM

Confirmed that the scope and FF were purchased new.

 

I just realized that both ends of the FF have 62mm diameter threads....I'll ask the dumb question, is it possible that I assembled it in the reverse orientation?  

 

The T-Ring adapter has 48mm threads that fit the reduced end of the FF - although per my comment above I could reverse the extra FF adapter ring and put it on the other end.

 

Unfortunately I can't swap the FF orientation until next week.  I will report back with my findings.

 

20210622_091102.jpg



#10 asanmax

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Posted 22 June 2021 - 10:08 AM

Confirmed that the scope and FF were purchased new.

 

I just realized that both ends of the FF have 62mm diameter threads....I'll ask the dumb question, is it possible that I assembled it in the reverse orientation?  

 

The T-Ring adapter has 48mm threads that fit the reduced end of the FF - although per my comment above I could reverse the extra FF adapter ring and put it on the other end.

 

Unfortunately I can't swap the FF orientation until next week.  I will report back with my findings.

 

attachicon.gif20210622_091102.jpg

I don't know about the others, but that's exactly what I asked. And the stars look like the flattener is flipped 180 degrees.


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#11 RedLionNJ

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Posted 22 June 2021 - 10:16 AM

The spacing from the rear of the flattener to the sensor is obviously correct, else the stars near the center would not be in focus.

 

I'm with asanmax - is it possible the flattener was assembled or installed incorrecty?



#12 unimatrix0

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Posted 22 June 2021 - 10:21 AM

Looking at the writing on the flattener on the picture with the camera attached, I think the writing should be facing the telescope not the camera. What I mean is, when you stand behind your camera, the text should not be upside down to you.  I think you put the FF on backwards. 


Edited by unimatrix0, 22 June 2021 - 10:21 AM.


#13 poshchiken

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Posted 30 June 2021 - 03:08 PM

I was able to reverse the orientation of the FF and run a few test shots - all of the image is now in focus.

 

Thank you all for the recommendations!


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