ASI2400MC best GAIN and OFFSET values (Doubt about my masterdark image)

Equipment Dark Frames Hot Pixels
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Thomas LELU avatar
Thomas LELU avatar
Hello,
I have just purchased this superb camera.
Before being able to use it on the sky (while waiting for a fixing ring for my Wynne corrector) I start to make my library.I'm a little surprised at the hot pixels shown on the master image (50x300s at -10 ° C Offset = 40 GAIN = 145)
Attached you will find my master image and I would like to know if this is normal.

With simple STF in PIXINSIGHTLink to MasterDark : [(https://wetransfer.com/downloads/19b7da7e05983bfdd293c0be5ff79b2220210711162222/c7e391e8fc5e2830d78940a892e2736220210711162241/ce8385)]Do you have a master image to provide for comparaison?What values ​​do you use for GAIN and OFFSET?!Thank you very much 
dkamen avatar
dkamen avatar
Hi,

Hot pixels should be close to 100% brightness and you have only a handful of those. The pixels you see as white  are only 15% brighter than your average pixel (2.1% brightness vs 1.9% in the average). You see them as white because that's how STF stretching works, by definition: the brightest pixels (2.1% brightness) are mapped to white. In your case, the difference is grossly exaggerated by stretching.

The correct way to evaluate a dark using PixInsight is with the histogram shown e.g. in HistogramTransformation if you simply select the image and/or with the Statistics process. 

To me this looks like an excellent dark frame for 5 minute subs.
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dkamen avatar
dkamen avatar
Oh, with regards to offset, look at your histogram:



As long as your histogram doesn't "touch" the left edge, it means you are good because no values are clipped to zero. 

Maybe for fewer than 5 minutes you would want to consider a larger offset, but for 5 minutes 40 is obviously okay. 

Cheers,
Dimitris
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Thomas LELU avatar
Thomas LELU avatar
Thank you so much Dimitris for your complet response smile
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