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Backfocus for WO60mm ED with their latest 0.8x reducer/corrector & the ASI183MM Pro

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#1 cheesified

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Posted 31 May 2020 - 10:15 AM

Hi all,

quick backfocus question,

Should the backfocus be 55mm?

 

On their website it states backfocus should be 56mm for DSLRs but it does not specify for astrocams.

Asking as I am still having difficulty achieving backfocus.



#2 sg6

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Posted 31 May 2020 - 11:00 AM

The same, 55mm is where the flat field should be located from some flat surface of the reducer/flattener. Whether you stick a DSLR sensor there or a astro camera sensor is immaterial.

 

For an astro camera you may need a spacer or two.

 

A DSLR flange distance is often around 45mm whereas the ZWO's are 6.5mm.



#3 cheesified

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Posted 31 May 2020 - 12:10 PM

At 55mm with the 0.8x reducer, I'm still getting curved stars. On the ordinary 1.0x flattener I'm getting good stars at 55mm though.

 

Backfocus: 55mm

https://www.cloudyni...405_1886099.jpg

 

Same goes for 56 & 57mm

Backfocus: 57mm

https://www.cloudyni...405_5756123.jpg



#4 YAOG

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Posted 31 May 2020 - 08:49 PM

Chessified,

 

You have multiple issues, tilt and spacing are both out. Something is tilting or sagging in the imaging train. The stars are pointing the same direction, this indicates a tilt or tracking problem. Some stars point to the center this indicates a spacing problem. In your case I think the tracking is okay based on the two corners with comatic star shapes pointed to the center.  




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