ASI1600MC-C Offset Question

rjurevicius

Hi All,

If one were to take a set of one second darks with unity gain and an offset of say 10, should the mean of the integrated stack be at least 10x16=160 (using Pixinsight)? I always get a lower number than 16xoffset and am wondering if I understand offset and or gain correctly for stacks of short darks.

Ray J

Support@ZWO

I'm not quite understand your question. offset of dark and bias and light and flat should be same.

rjurevicius

Thanks for the reply as I have been looking for an explanation for some time.

After stacking dark frames in Pixinsight, the mean of the integrated image as reported by Pixinsight is always less than 16 times the offset. This is with unity gain. I would expect that 16 times the offset would be the minimum.

For example: At unity gain and an offset of 22, a stack of 100 x 1 second darks stack up with a mean of 329, why is the result not greater than 352.

At 0 seconds the mean is 315, not greater than 352. (There is a drop off below .1 seconds)

The trend is the same for the the 50 or so combinations of offsets and integration times I have looked at. (All less than 1 second)

I really like the camera and am not complaining about anything. I'm just trying to understand my results and all I can come up with is that the gain is not unity at a setting of 139.

Ray J

Support@ZWO

Thanks for your explaination. One thing I don't know is why the median value must be 16x than offset? does it have a formula?

rjurevicius

While stacking, the result is in 16 bit space where the resolution can be increased thru stacking and recovering bits. The 16 comes from the difference between 12 bit and 16 bit.

I think the mean should be signal plus any noise plus offset, or, for a 16 bit data, 16x(signal plus any noise plus offset). So, at a minimum, 16x(0+0+offset) for a bias or dark frames at unity gain (139) when read as a 16 bit number.

Support@ZWO

Oh, it's because offset value is not equals 1ADU.

rjurevicius

I just want to be clear.

At unity gain (139), an offset of is not equal to 1 ADU at 12 bit. Correct?

Ray J

Support@ZWO

No, it's not 1 Adu per 1 offset.