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Stars with 'holes'

ZWO ASI294MC Pro ZWO ASIAIR Plus Acquisition Deep Sky Artifacts
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Niels_L avatar
Niels_L avatar
I took two nights photos of the Dolphin Nebula and stacked using WBPP.  It seems that a lot of the stars, anywhere in the image, have a 'hole' in the middle... What could be the cause of this? 

Even though the FRA300 is petzval, stars show some aberration and elongation in the corners.  Didn't notice it before being this bad and now also the stars have holes...

Screenprint of the centre of the photo (no processing other than WBPP and STF):


Equipment:
ASIAIR+
ASI294MC Pro (T=-10C, gain=120)
AltAir HO Dualband 4nm (I think)
Askar FRA300
131 x 300s light frames

Thanks!
Niels.
Engaging
andrea tasselli avatar
andrea tasselli avatar
Some settings can created that effect for undersampled stars. What the unprocessed frames look like? Just autostretched and debayered. If they do NOT have holes in the middle then it might be the settings in WBPP, specifically the CosmeticCorrection ones, if active.
Niels_L avatar
Niels_L avatar
andrea tasselli:
settings in WBPP, specifically the CosmeticCorrection ones


hmmm, now that I think of it, I did change the setting of cosmetic correction! 

I randomly checked a few single frames and no holes.  Let me redo the WBPP with different CC settings. I will revert back tomorrow (after running WBPP and a night sleep )
andrea tasselli avatar
andrea tasselli avatar
Just disable auto-detection.
Alan Brunelle avatar
Alan Brunelle avatar
I can confirm that CC caused this for me.  Trouble shooting as Andrea is always a good approach.  Hence the value of the intermediate files saved by WBPP…
Niels_L avatar
Niels_L avatar
I redid the WBPP, all same settings except the cosmetic correction. Holes are now gone!

I used the below autoCC setting that then gave the holes.  I got the numbers from someone on Youtube, cannot remember who it was.  Perhaps I had the setting 3 wrong... Anyhow, I am using my own dark master setting, which work well (as far as I can understand, I am far from the PI expert).

Thanks for the quick a@andrea tasselli !!

Mau_Bard avatar
Mau_Bard avatar
I redid the WBPP, all same settings except the cosmetic correction. Holes are now gone!

I used the below autoCC setting that then gave the holes.  I got the numbers from someone on Youtube, cannot remember who it was.  Perhaps I had the setting 3 wrong... Anyhow, I am using my own dark master setting, which work well (as far as I can understand, I am far from the PI expert).

Thanks for the quick a@andrea tasselli !!


Hi Niels, if you do cosmetic correction by creating the  dedicated process template (that is not in auto mode in wbpp) you have to set the sigma parameters each time, specially the auto detect>hot sigma, by selecting a preview on an unprocessed subexposure and checking a value that is not over correcting. Do not rely always on the default value 3 that sometimes is too low and kills star kernels. If you have multiple filters, you may set different templates one for each filter ( i.e. one for rgb and one for narrowband if working with color camera).

Hope this helps!
Helpful
Niels_L avatar
Niels_L avatar
selecting a preview on an unprocessed subexposure and checking a value that is not over correcting


Thanks @Mau_Bard, this is helpful!  Previously I used the option for masterdark but actually just used a number... I had an old computer that could not always do these previews, now I upgraded to a new computer that runs PI smooth! (Finally). 

Would you advise to use the masterdark or automated option for CC?
andrea tasselli avatar
andrea tasselli avatar
Use the master dark always, with a setting of 3 sigma.
Mau_Bard avatar
Mau_Bard avatar
selecting a preview on an unprocessed subexposure and checking a value that is not over correcting


Thanks @Mau_Bard, this is helpful!  Previously I used the option for masterdark but actually just used a number... I had an old computer that could not always do these previews, now I upgraded to a new computer that runs PI smooth! (Finally). 

Would you advise to use the masterdark or automated option for CC?

Yes use the master dark and use the slider to find the right value. This cleans most of hot pixels. After that tune the sigma to eliminate any residuals. As said 3 was in the past discarding star cores, look specially for narrowband images the right value, not too aggressive. 
I used the auto option once an I had problems, since then I avoided it. It might be good but I have no extensive experience to share.