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Help with backfocus

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#1 PRastro

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Posted 24 June 2022 - 06:45 PM

I recently picked up a AT72EDii and ATR8 0.8x reducer. I'm trying to dial in the back focus distance and getting some wierd results so far. Here are some single 3 min sub frames at various back focus distances. These images have been debayered, stretched and converted to jpegs. 

 

At the 55mm distance suggested by the manufacturer I'm seeing elongated stars indicating that I need to increase the distance.

https://flic.kr/p/2nu9otH

 

I added a 1.4mm spacer for a total backfocus distance of 56.4mm. Slightly better, but still get elongated stars pointing to the center

https://flic.kr/p/2nu9pr6

 

Added a 5mm spacer for a total backfocus distance of 60mm.

https://flic.kr/p/2nu9pok

 

Added another 5mm for a total backfocus of 65mm

https://flic.kr/p/2nubQEf

 

Looking at the stars they are elongated pointing towards the center of the frame. The camera is an Altair 533C, so fairly small sensor. Do I keep increasing the backfocus distance further? Or am I dealing with tilt or some other issue?

Please help steer me in the right direction to resolve this issue. Thank you for your time!



#2 Tapio

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Posted 24 June 2022 - 07:09 PM

Hard to tell.
Have you seen the charts?
https://www.cloudyni.../#entry11583909
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#3 SeymoreStars

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Posted 24 June 2022 - 07:52 PM

You should read this short PDF by the owner of Astro-Physics - https://www.astro-ph...d-flattener.pdf


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#4 PRastro

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Posted 24 June 2022 - 08:44 PM

Hard to tell.
Have you seen the charts?
https://www.cloudyni.../#entry11583909

Yes, I have. I'm seeing the stars radiate outward from the center. But I have already pushed the sesor 10mm over the recommended distance and still see hardly any improvement.



#5 PRastro

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Posted 24 June 2022 - 08:46 PM

You should read this short PDF by the owner of Astro-Physics - https://www.astro-ph...d-flattener.pdf

Thank you for this. I'll try this on my next outing. Atleast I can figure out how far the corners stars are out compared to the center and in which direction with out fiddling with the image train.



#6 michael8554

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Posted 25 June 2022 - 03:48 AM

I see elongated stars in the centre of your images.

 

Which means you're seeing guide errors in those 3 minute exposures.

 

For spacing tests you need to examine a short exposure of a star-rich area, stretched if necessary,  so that guiding errors can be discounted.


Edited by michael8554, 25 June 2022 - 03:49 AM.


#7 pfile

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Posted 25 June 2022 - 11:42 PM

there is a plugin for NINA called Hocus Focus that can actually automatically do what Roland's white paper describes and just tell you which way you need to move the reducer. it can also graphically show you the sensor tilt, if any.

 

even if you don't use it for imaging it might be worth installing and trying this out.

 

rob


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#8 SeymoreStars

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Posted 26 June 2022 - 06:39 AM

there is a plugin for NINA called Hocus Focus that can actually automatically do what Roland's white paper describes and just tell you which way you need to move the reducer. it can also graphically show you the sensor tilt, if any.

 

even if you don't use it for imaging it might be worth installing and trying this out.

 

rob

Thank you. I tried that and received the errors below. First let me say that NINA's Autofocus (stand alone) was successful, but the HocusFocus (Detailed Analysis) threw these errors.

 

I will try to dive into the HocusFocus documentation to figure out what I'm doing wrong. 

Attached Thumbnails

  • nina_last_error.png


#9 pfile

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Posted 26 June 2022 - 10:25 AM

strange. well, asking on the NINA discord is probably the best way to go, though the author is around here as well.


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#10 PRastro

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Posted 26 June 2022 - 01:50 PM

there is a plugin for NINA called Hocus Focus that can actually automatically do what Roland's white paper describes and just tell you which way you need to move the reducer. it can also graphically show you the sensor tilt, if any.

 

even if you don't use it for imaging it might be worth installing and trying this out.

 

rob

I have tried it with a SCT focus motor and gave up because the NINA autofocus routine did much better. So I have it installed. I will run that with my refractor AF to see if it can help with my backfocus. Thank you for the suggestion!



#11 PRastro

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Posted 26 June 2022 - 01:56 PM

I see elongated stars in the centre of your images.

 

Which means you're seeing guide errors in those 3 minute exposures.

 

For spacing tests you need to examine a short exposure of a star-rich area, stretched if necessary,  so that guiding errors can be discounted.

I agree. I used 10 sec exposures to do the testing, but unfortunately did not save any. All my test shots showed round stars at the very center and elongated stars radiating outwards from the center. I used the subs from my imaging session in this post to get input from the forum.




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