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Flattener introduces coma-like aberrations on the corners. Backfocus?

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#1 Blueslikehail

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Posted 08 May 2021 - 05:12 PM

Hi,

 

I have taken this picture with an ES ED80, a Starfield 0.8x reducer/flattener with a Canon 100D (APS-C size sensor).

 

The flattener corrected the field curvature but introduced some coma-like aberrations on the corners. The specs of ED80 are within those recommended by the manufacturer, and the backfocus is 55mm, which is what I have with standard T-ring.

 

So is this to be expected (although the description says should correct the field on aps-c sized sensors). Can it be corrected by further fine tuning the backfocus? 

 

Would love to know your experienced opinion, Thanks!

 

W.



#2 Blueslikehail

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Posted 08 May 2021 - 05:25 PM

Forgot the file. This is a left crop of the picture.

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  • NA_Half.jpg


#3 Blueslikehail

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Posted 08 May 2021 - 05:26 PM

Center of FOV is in the middle of the right edge of the picture



#4 michael8554

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Posted 09 May 2021 - 07:03 AM

If the stars are rotated about the centre of frame in all corners, rather than pointing away from the centre, this suggests the CC is too far from the sensor.



#5 Northrim

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Posted 09 May 2021 - 01:01 PM

I agree with Michael 8554 in post #4.  It's very subtle but it does look like the stars in the corners are slightly elongated around the center rather than away from the center.  I think your camera is just a tiny bit  too far away from the reducer/flattener.  

 

The 55 mm is not necessarily  precise.  I have an ES ED 80 with an ES flattener  and I'm at about 66mm and still need to move out farther.  Some on the forum have reported needing 70mm + for a stated 55 mm backfocus.  Seems you have the  opposite  issue.




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