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Weird flat streaks

ZWO ASI2600MM Pro Processing Calibration Deep Sky Artifacts
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Arny avatar
Arny avatar
My last series of flats all have weird quarter circle streaks across - but only for Ha and Sii, but not for Oiii.
Filters are Zwo 2" Ha, Sii and Oiii filters
in 7x2" ZWO filter wheel
on Asi2600mm pro cooled camera
on Founder Optics Apo86

First I though it was on the camera or lens, but Oiii does not show.
Second I changed the T-Shirt I use to diffuse sky light for the flats, as it is not homgenous. But effect remains despite very homogenous fotographic silk.
Third I cannot see any dirt or spots on the filters, nor does it change with camera rotation.
I get artifacts in individual flats as well as stacked masterflats.

Any idea what I might be facing here?

Arny


Sii sub 
Sii masterflat

Ha subframe

Ha master flat:

Oiii subframe
​​​​​​​
Oiii masterflat
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Dave Rust avatar
Dave Rust avatar
Could these be cleaning streaks on the camera's protective glass cover? Also, streaks on the back surface of a Field Flattener can cause similar artifacts. I suspect it's not surprising that they'd show up at some wavelengths and not others.
Well Written Insightful Respectful Concise
Brian Puhl avatar
Brian Puhl avatar
Have you tried actually stacking your data with those flats?    Its common for some NB flats to look strange.   Don't count them out unless it actually shows up in the final stack.
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Arny avatar
Arny avatar
Brian Puhl:
Have you tried actually stacking your data with those flats?    Its common for some NB flats to look strange.   Don't count them out unless it actually shows up in the final stack.


final stacked images look ok and do not replicate the effect, which adds to the weirdness as the stretching of the flats is the same as the of the stacked images …
Arny avatar
Arny avatar
Dave Rust:
Could these be cleaning streaks on the camera's protective glass cover? Also, streaks on the back surface of a Field Flattener can cause similar artifacts. I suspect it's not surprising that they'd show up at some wavelengths and not others.



Hi Dave,

Agree, I will have to disassemble the image train to find out …

Arny
Brian Puhl avatar
Brian Puhl avatar
Arny:
Brian Puhl:
Have you tried actually stacking your data with those flats?    Its common for some NB flats to look strange.   Don't count them out unless it actually shows up in the final stack.


final stacked images look ok and do not replicate the effect, which adds to the weirdness as the stretching of the flats is the same as the of the stacked images …



Take it from someone who just was just helping another for the past week about it.  He has a very similar issue to you.   His pattern was a diamond tho.  My flats look wierd as well, but they never had a pattern like his, so I assumed something might be wrong.    He chased it for weeks, tearing down his entire imaging train.... it ended up being nothing.   The final stack was fine.  

If the final stacked image looks fine, then don't chase it.   It's doing exactly what it's supposed to do.