Author Topic: Calibration Issues  (Read 545 times)

Offline jgeiger

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Calibration Issues
« on: 2018 January 17 08:46:30 »
I had my first experience capturing and processing images with a Lumicon Deep Sky Filter to help cut down the light pollution. In the same evening I captured Flats*20, Lights*10, Darks*13 with the filter in place in place.  I also captured Bias*100 and then created a Superbias from them.  I then calibrated my darks with the Superbias master and stacked them to create a MasterDark file and calibrated and stacked the Flats to create a MasterFlat.  At this point I calibrated my Lights with all my master files only to discover that the calibrated images had lots of hot pixels in them that seem to be added by the Master Flat. (see attached JPEG). Why would adding the calibrated flat cause this problem? Any insight would be greatly appreciated.

Offline jkmorse

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Re: Calibration Issues
« Reply #1 on: 2018 January 17 15:24:30 »
I actually think your calibration turned out just fine.  Clearly the calibrated is far superior.  I think the calibration simply brought out hot pixels that were there all along.  To the extent hot pixels were added, it is much more likely to be coming from your masterdark, especially if your temp wasn't the same on both the dark and the main image.  Another issue may be that you used both the masterdark and the bias when calibrating.  Since your dark already has the bias built in, you do not need to include it when calibrating your main image.

Finally, even with the hot pixels you see, all of those can easily be dealt with using cosmic calibration and, ultimately, ImageIntegration.  And to repeat, I don't see much wrong with the calibrated image in the first place.

Hope that helps.

Jim
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Offline jgeiger

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Re: Calibration Issues
« Reply #2 on: 2018 January 19 12:50:15 »
Thanks for your response Jim.  I did take the darks about a half hour before shooting the lights and the temp did not really change more than a degree.  I did use both the master dark and master bias when calibrating the lights but did not click the calibrate option for each master type since they were already calibrated.  I was following Kayron Mercieca's tutorial on image calibration and it had me include the master bias, master dark and master flat when calibrating.  I did try running CosmeticCorrection but it didn't seem to remove all the hot pixels. I'll try calibrating my lights without the master bias file to see if that makes a difference.  Thanks again for your help.

Offline jgeiger

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Re: Calibration Issues
« Reply #3 on: 2018 January 22 09:57:21 »
I think I may have found the problem with all the hot pixels being introduced after I calibrated with my flats.  My flats were underexposed. 

I have created a reproducible way to get properly exposed flats using a measured string, some photocopy paper and the off-camera flash from my DSLR.  Basically I attach the white paper and the end of string (about 24" long) marked every inch to the front edge of the dew shield. Next I measured the distance from the fully extended dew shield on my scope to the flash using the string and took several 2.5 sec exposures firing the flash at different distances until I got an image with a reproducible exposure range of the about 50%-70% of the maximum Full Well Capacity of the sensor.  I then repeated this for my Deep Sky Filter and OIII filter. Now when I do an imaging session I have a way to consistently take flats with good exposure levels. For no filter, the flash fired at 18"-20" gives me properly exposed flats.  For the OIII filter it's 6"-8" and for the Lumicon Deep Sky Filter it's about 16"-18".