Arun H:
John Hayes:
The two main reasons that you might need dark-flats are:
1) Your flat exposures are very long. Remember that dark current is linear with exposure times. Flats taken with an exposure under say 5 seconds generate very little (essentially zero) dark current with a cooled camera. In that case, the primary advantage of dark flats is to prove the bias offset. On the other hand, if your flats are made with say 600s, you need to subtract the dark signal.
2) If you do not use bias offsets in your calibration process, dark-flats will provide the offset needed to properly calibrate your subs.
I personally never use dark-flat and because I include the bias offset, the calibration process works perfectly.
Amp-glow is an RBI effect that comes from NIR light emitted by an amplifier on the sensor chip. It will be the same for light, darks, and (I believe) flats. Amp-glow should be removed by calibration and I don't think that you need dark-flat to make that happen; but, I have to admit that I haven't worked it out to be totally sure of myself on this point.
John
John, at least for the 294MM, the magnitude of the amp glow depends on temperature and you will over or under correct it if you use the incorrect temperature. From this, I think it depends on time as well. That’s why proper flat calibration requires dark flats - though if the time is short enough, it may not make a huge difference.
Arun,
Thanks for that information. As you know, the calibration process requires three things: 1) That the sensor response is linear, 2) That the temperature is held constant between the light data and the calibration data, and 3) That the camera characteristics are stable with time. That’s because dark current, which includes RBI current is temperature dependent. I’m aware that some CMOS sensors have non-linearities that become apparent when using very short exposure times so you have to be careful about calibration in that situation. Amp glow will also vary with temperature and some sensors may indeed have a bigger response to temperature than others. Either way, it’s important to control the temperature when calibrating. If calibrating data from the 294MM works more reliably using dark-flats then by all means take dark flats. My general comments were aimed at the sub-set of sensors like the IMX455, IMX461 and KAI-16803 that are very linear and well temperature controlled. BTW, the FLI-16803 CCD sensor had amp-glow as well and it didn’t require dark-flats for high quality calibration. In fact, that camera had the ability to do pre-flash, which resulted in a HUGE amount of RBI and it still calibrated perfectly—without dark flats.
John
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John Hayes:
Here are the charts that I computed a while back
At 500 calibration frames, you only gain maybe around 5% for calibrating a 50 sub stack and 10% when calibrating a 500 frame stack compared to taking 50 calibration frames, which is 10x less frames! For most stacks of 100 frames or less, 16-20 frames is plenty. Taking more data won't hurt you but you are wasting your time.
John
So while it has a positive effect on the output, the exponential increase amount of frames taken does not justify the outcome.
So, I think I'll keep doing it because I have a lack of common sense  at least on the bias only.
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V.M Legary:
John Hayes:
Here are the charts that I computed a while back
At 500 calibration frames, you only gain maybe around 5% for calibrating a 50 sub stack and 10% when calibrating a 500 frame stack compared to taking 50 calibration frames, which is 10x less frames! For most stacks of 100 frames or less, 16-20 frames is plenty. Taking more data won't hurt you but you are wasting your time.
John
So while it has a positive effect on the output, the exponential increase amount of frames taken does not justify the outcome.
So, I think I'll keep doing it because I have a lack of common sense at least on the bias only.
That's right. Just for fun process one image using 50 darks and one with 500 darks and show us the difference.
John
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John Hayes:
V.M Legary:
John Hayes:
Here are the charts that I computed a while back
At 500 calibration frames, you only gain maybe around 5% for calibrating a 50 sub stack and 10% when calibrating a 500 frame stack compared to taking 50 calibration frames, which is 10x less frames! For most stacks of 100 frames or less, 16-20 frames is plenty. Taking more data won't hurt you but you are wasting your time.
John
So while it has a positive effect on the output, the exponential increase amount of frames taken does not justify the outcome.
So, I think I'll keep doing it because I have a lack of common sense at least on the bias only.
That's right. Just for fun process one image using 50 darks and one with 500 darks and show us the difference.
John
Honestly that does sound like a good educational material for everybody here, so I think I'm going to do that. 500 600" darks is gonna take so long though 😅
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For CMOS is it necessary to take dark-flats? Can I just take darks, flats, bias as for CCDs?
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Depends on the CMOS and the length of the flats. I only take dark-flats and never bias.
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AdrianC.:
For CMOS is it necessary to take dark-flats? Can I just take darks, flats, bias as for CCDs?
As far as I know the only cameras that require flat-darks are the ASI 1600 and 294 (and their equivalents from other manufacturers).
All other recent CMOS cameras work perfectly well using bias. Of course you can use flats-darks as well but it is much faster and more efficient to use bias, there is no difference in the final result.
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