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Bizare mean values in dark masters

Posted by PieterVuylsteke on Jan 17, 2022; 11:20am
URL: http://astroimagej.170.s1.nabble.com/Bizare-mean-values-in-dark-masters-tp1743.html

Hi,

I have been making darks for different exposures : 5s,15s,30s,60s,90s,120s,150s and 240s.

I do have a ZWO 2600mm camera, and use gain 0 and offset 10.
That offset 10 generates an offset of 100 ADU in each picture.

Thus, a dark of 30s has mean ADU of 101.4


When I use pixinsight to look at the mean values inside of these darks I do find very bizare values.
For 5s I have a mean adu value of 2480, for 30s 965, for 90s 222 and for 150s 204.

I do not understand that if my darks of 30s have a mean ADU of 101.4 (all 20 of them +/- 0.somethong), how can the mean adu of the dark master then be 965.363 ? Using bias and de-bias do not really change these values.

Looking at a science image with exposure of 30s, uncalibrated, gives a mean ADU of 143.6 ADU, the calibrated one becomes 5144 ADU, with then also quite a lot of stars saturated.

Have been looking in the paper of AstroImageJ, but I do not really find an explanation about the mecanisms of this all.

I would think that a "mean" dark master would have the mean value of the 20 dark photo's ??

Can you please clarify this a bit ?

Kind regards,
Pieter.