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Help me save trial and error time - backfocus is not correct

astrophotography CMOS imaging refractor
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#1 PhotonHunter1

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Posted 01 June 2020 - 05:26 PM

I purchased a Stellarvue focal reducer field flattener (SFFR102-2). I have it to my Baader VariLock 29 T2 Variable Extension Tube - 20 mm - 29 mm, for use with my ES102mm APO Triplet refractor. In reviewing my Ha images of the Pelican from Saturday night I noticed that, for the most part, all the stars in the corners are elongated and pointing towards the center of the image. Stars in the center are pin point. I'm guessing my back focus is off a bit. My current back focus on the Varilock is set to 28.5mm.

 

In an effort to save trail and error time in adjusting I'm hoping the CN community can help me. To correct this issue would I set the Varilock less than, or greater than, the current 28.5mm setting?

 

Thank you.



#2 Aaron_tragle

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Posted 01 June 2020 - 05:46 PM

This image has saved me many hours of pain adjusting spacing, I hope this helps you as much as it did I.

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#3 PhotonHunter1

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Posted 01 June 2020 - 07:52 PM

Too close it is. Thank you - this is a big help!



#4 jmp_

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Posted 01 June 2020 - 08:25 PM

I had similar issues. I found this document to be helpful: https://www.astro-ph...d-flattener.pdf
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#5 GrafikDihzahyn

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Posted 02 June 2020 - 10:25 AM

^^ THIS

 

With a varilock and the AstroPhysics writeup you should get perfect backfocus in just a few minutes. Rarely is it ever the stated 55m. Even then you may still have odd shaped stars due to astigmatism.


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