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What is Causing This Star Defect?

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#1 Gary Minder

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Posted 19 July 2013 - 09:32 AM

I am hoping somebody can tell me what might be causing stars to be so defective as seen in this tight crop of M13 from last night.

I have registered and stacked subs in DSS and PixInsight and see the same thing. I saw this same thing once in a set of subs I processed in DSS. I tried PixInsight with the same subs and it was gone.

I assume the culprit is some setting I am using. I just ran PI preprocessing, stopping at registration and there are no defects. I am working with 13 90-second subs. The PI integration setting I used are:

Pixel Rejection Algorithm: Windorized Sigma Clipping
Sigma Low/High: 4.0/2.0
Scale Estimator: Iterative k-sigma/biweight midvariance
Combination: Average
Normalization: Additive with Scaling
Weights: Noise Evaluation

Help! Thanks.

Gary

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#2 Gary Minder

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Posted 19 July 2013 - 09:40 AM

As I look at that picture it almost seems like I am seeing a cross section of each star...like I've cut each star in half. Neat trick but not what I'm after!

#3 shawnhar

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Posted 19 July 2013 - 09:52 AM

I don't know about Pixinsight but I got something resembling this with DSS when my "clean remaining hot pixels" was set too aggressive.

#4 zerro1

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Posted 19 July 2013 - 10:15 AM

I don't have a clue based on the settings that you posted.

have you tried a different rejection algorithm other than windsorized? Like Min/Max or even no rejection to see if it will complete without the "defects"

#5 DaemonGPF

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Posted 19 July 2013 - 10:24 AM

Wait, I have a pic just like that. I know what it is. It's olive oil suspended in water!
Posted Image

:lol:

#6 DaemonGPF

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Posted 19 July 2013 - 10:26 AM

Seriously though - do you have a single, unstacked sub we could look at, and maybe the stacked, unprocessed version as well?

#7 Jeff2011

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Posted 19 July 2013 - 10:57 AM

I find that when I start messing with the DSS setting, I can get some weird stuff happening. In my case I was getting Stars with dark centers. I would set the DSS settings back to the defaults and try again. Also reviewing the DSS documentation is helpful. There may be some settings that you may have to change, but the defaults generally work. That is assuming the problem is with DSS. As Josh said we need to see the results of the stacking to be sure.

#8 lawrie

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Posted 19 July 2013 - 11:22 AM

I used to get that artifact with DSS also.

#9 RedLionNJ

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Posted 19 July 2013 - 03:09 PM

DSS - Stacking Parameters - Cosmetic

Disable (uncheck) "Detect and clean remaining hot pixels"

Grant

#10 David Ault

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Posted 19 July 2013 - 06:53 PM

As Josh said, a single frame, or even better all the individual frames, would be very useful in figuring out what is going on. Both DropBox and Google Drive have free online storage (up to a certain quota) for sharing data, I'm sure there are others.

Honestly, the star shapes look like an out of focus star where the diffraction rings are visible, but since this is a multi-second exposure that seems unlikely.

This is just a guess, but another possibility is that some frames are being included that have no stars, or the stars are in a very different location, which will throw the rejection algorithms off. The edges of the stars are closer to the background level so they have a better chance of passing through the rejection algorithm while the peak of the star is thrown out. I see this same thing with plane trails when stacking with PixInsight, the central part of the plane's light path is thrown out, but as you move away from the center and the light level approaches the background level the pixels from that image are included in the stack leaving a double line effect.

Regards,
David

#11 Gary Minder

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Posted 20 July 2013 - 12:41 AM

DSS - Stacking Parameters - Cosmetic

Disable (uncheck) "Detect and clean remaining hot pixels"

Grant


That was it. I ran DSS again with the "Detect and clean remaining hot pixels" disabled and there were no defects. Thanks.

#12 lawrie

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Posted 20 July 2013 - 10:49 AM

Nice catch Grant.............well done.

#13 RedLionNJ

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Posted 20 July 2013 - 01:49 PM

Thanks, guys. I knew because I'd been there :)

Grant

#14 sullij1

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Posted 22 July 2013 - 09:52 AM

That was very useful. I have had that problem with DSS also.






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