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Dithering or Sigma Clipping not working?

Posted: Wed Mar 13, 2019 7:07 pm
by Astrogate
Hi Robin

Last night I was out doing some live stacking and noticed issues with either dithering, sigma clipping or both? I enclosed two images of M 64 with the white point lowered a lot in order to show the background flaws. One image is 26 x 30 secs and the other is 40 x 30 secs both at gain 350 with a matching master dark and flat under sever light pollution and brightness level at 10. I was using a C9.25 sct at f 7.2 and guiding. As you can clearly see in the images the hot pixels are not being removed and grouping in the bottom right corners. Also with the longer stack of 40 sub-images a satellite streak has developed on right side and at about 7 o'clock below M 64 a line of warm pixels has developed. I have dithering set to dither at 12 pixels for my imaging camera according to the image camera scale and guide camera scale. Should either using sigma clipping with ample sub-images, dithering or both combine remove at least these hot/warm pixels, cosmic rays and possibly the satellite streak when pixel scale is set to dither high enough?

Thank you!

Chris A

Re: Dithering or Sigma Clipping not working?

Posted: Wed Mar 13, 2019 8:55 pm
by admin
Hi Chris,

I'm not aware of any bugs in this area, but that's not to say that there couldn't be new bags that I haven't found yet.

You should be able to watch the progress of dithering if you keep an eye on the PHD2 screen while SharpCap is working – that will let you confirm whether the dithering commands are getting through to the guiding application or not.

The Sigma clipping algorithm is not active for about the first 10 frames that are captured in a stack as SharpCap has to have stacked a good number of frames in order to work out the statistics of variation at each pixel to be able to apply the Sigma clipping algorithm. So it is possible that the satellite streak came through early in the stack before the Sigma clipping algorithm could become active. If you're sure that's not the case please let me know. If you happen to have saved the raw frames during that stack then it might be interesting if you shared with me the one with the satellite streak in and about 15 to 20 other frames and I can see if I can reproduce the issue and work out where the bug is.

Cheers, Robin

Re: Dithering or Sigma Clipping not working?

Posted: Thu Mar 14, 2019 1:05 am
by Astrogate
Hi Robin

Thank you for the fast reply! I was watching PHD2 on and off and did notice the dithering commands coming from SharpCap to PHD2. Very good point about the satellite streak, but unfortunately I did not capture any raw's last night. Perhaps it was added to the 40 stacked towards the end of the live stacking. I am surprised though that in both images (26 and 40 stacked sub-images at 30 secs each sub) there is a blue and green group of hot pixels forming bottom right corner which I thought the dithering would have eliminated. Thank you for your help!

Chris A

Re: Dithering or Sigma Clipping not working?

Posted: Thu Mar 14, 2019 9:15 pm
by admin
Hi Chris,

Dithering will help spread around the effect of a hot pixel, but if you only dither a small number of times during the stack then you will still see the effects of hot pixels although they will not be as bright. I would usually imagine using dark subtraction to remove the worst effects of hot pixels and then dithering would help remove any pattern or raining nois by the dark subtraction.

Cheers, Robin

Re: Dithering or Sigma Clipping not working?

Posted: Fri Mar 15, 2019 12:59 am
by Astrogate
Thank you Robin for the tips and I will try it out. I was using 30 second subs and dithering set to every 90 secs. I was using a matching dark master but it is a month old, and perhaps I need to take a new set.

Cheers,

Chris