Hi Christian,
Again thank you for this information. Indeed the ASI 1600 is a very interesting camera. And I quickly learned: Cool it down to the lowest possible temperature. This is interesting to note because I used uncooled DSLR for a while being quite happy.
Regarding your comments on non-linearities in the least significant bits, CCD and CMOS have one thing in common: Digitalization errors due to technical limitations in the electronics of the analog sensor electronics and also analog-digital conversion unit (ADC). I see these effects everywhere looking to the low intensities of an image histogram of a digital imaging sensor. It is no surprise. The error appears as a comb-like structure of a single(!) image histogram. As the number of changing bits is large for the least significant bits (due to noise) in astronomical imaging the effect is more obvious when scaling the image to the Gaussian noise level of the low image intensities. But it is also present with the high bits which don't switch often between 0 and 1. The correct source and interpretation often depends on the electronics of the camera device used and sometimes environmental conditions with unknown source (see my report on this below).
The error (non-linearity) occurs because of two major reasons: (1) non-linearities in the ADC unit (non-linear or deviating resistance steps for the bits to be digitized), and (2) coupling between the power supply and number of bits registered at the ADC. The latter seems a bit more tricky for CMOS imagers having all on one single chip. However, it is interesting to note: Even the CCDs in the 1990ies with first class ADCs showed such effects. Since the 1990ies nothing changed. With several DSLRs, that I used, the effects seems to vary with time, the comb in histogram is not fixed to special bits of the digital numbers and also seems to depend on content and width of its noise distribution. In astronomy use case we find many numbers scattered around a mean background value. This is not the case with conventional photography, where the effect is not so obvious.
There is no ideal ADC. If power supply of the amplifier stage and ADC are not well separated and stabilized, the number of bits set to one or zero may affect the analog amplifier part. So it is typical to find a voltage drop, if many digital "1" bits are set by the ADC drawing more current. Drop in power supply affects pixel intensities, which will vary coupled to the digital numbers. In such a way statistical pixel intensities are coupled to the number of 0 and 1 of the ADC, if power supply of the stages are not clearly separated. As CCDs have decoupled ADCs (but not ideally decoupled voltage supply of the stages) the effect might differ with the designs of the ADC, and the capabilities of the camera manufacturer to decouple power supply of the CCD and digital electronics. For typical monolithic CMOS you have it all on a single chip. I'm actually wondering, why this works so well with modern CMOS imagers.
Anyway the effect is common, for both, CMOS and CCD. There is not much difference compared to CCD to overcome the effects of these digital non-linearities. The effect quickly disappears when averaging (or just adding) a few images together. With averaging images the nonlinearities and comb in histogram disappear quickly due to statistical reasons. This is true for both, CCD and CMOS.
From a quick look on the few image histograms of my rough sky tests taken in the last nights, the ASI 1600MM-Cool provides less digitalization errors seen in histogram compared to a Canon DSLR. However, I expect the ASI to have similar problems like any sensor, CCD or CMOS. And the ASI might show this effect a bit later, when photon count and Poisson noise dominates the very low read-out noise of the Panasonic sensor. In other words, if you have strong background with broader statistical deviation of pixel intensities the effect may change as the prominent bits and distribution may change. This is how you can separate a fixed non-linearity (fixed error in the resistance stage of the ADC) from a statistical influence of the number of 1 bits measured (or 0 in case of inverse logic, where 0 means high current).
It is also interesting to note: Almost every effect documented in about 1 TB of imaging data taken in the last 20 years with CMOS (DSLR) or CCD may vary over time, like the non-fixed position of warm or hot pixels, or digitalization errors. I have no good explanation for this - except saying it is "wear" or runout of the silicon structures -, but it is a fact. Therefore, I prefer to take darks every start of the night and end, as I learned it using CCD twenty years ago. Use of calibration frame libraries, as often discussed in amateur astrophotography, is a method to quickly run an auto-guiding camera. Its a no go for measuring. Don't do it when measuring stellar parameters like in spectroscopy.
People not familiar with the behavior of silicon sensors should read the fundamental work of Craig D. Mackay "Charge Coupled Devices in Astronomy" (1986). And, hey! The author already mentioned CMOS in 1986! Wise guy. So, I'm wondering, why it is still not popular in astronomy and we still discuss about differences between sensors. It is like religion.
http://adsabs.harvard.edu/abs/1986ARA%26A..24..255M
Yes, CCD and CMOS are different. And the difference is hidden in its similarities.
Use Mackay's work as reference and follow the guidelines for calibration of the devices. It is fundamental work now applied in many interdisciplinary fields of science, where imaging with silicon detectors is common practice. Then you are on the safe side with both types: CMOS and CCD. Although. a few problems with flat field are left out and still discussed in literature. This is related on how the non-flat field is caused by your optics, however.
I don't know if someone had a similar problem at the OHP star party 2017: I noticed unusual count of hotpixel and also digitalization errors in the digital numbers of my DSLR at the OHP for almost every image taken starting from the second day. First I thought my camera sensor is going to be dead soon. Back home everything was fine again with exactly the same setup. I guess this was a combination of high environmental temperature, irradition from an anonymous power source like RADAR, the many mobile devices on the base, more powerful WLAN - hey, this year it worked like a charm! -, or whatever. At least my bad image quality collected at the OHP 2017 is a fact. I've never seen this before with my DSLR. The first time I saw such problems was in the early 1990ies when we faced problems with our CCD at the Hoher List observatory. In certain nights we could trash 10% of our images taken with a pro-CCD. The reason was beleived to be caused by a strong local airforce base RADAR sometimes firing in direction of the observatory. We didn't know this for sure, but the image artifacts happened with two different CCD cameras used at the 1m dome of the observatory. Mobile phones or wireless computers were not invented at that time. Long cables connected the PC in the office with the CCD of the dome with the bits crawling slowly over parallel lines and carefully buffered by robust TTL logic drawing high current on the lines. Today we read out 12 Million pixels in a few seconds transferred by USB 3 serial lines. Many people walking around the observation site with a pocket computer called a cellular phone. Nobody knows.
I don't think we should discuss this as typical for this or that sensor or camera. It is typical for both kinds of cameras, which I ever used in the past and future, which we also use in astronomy. As such it is how it is. Sometimes it is impossible to clarify the true source of the effects measured. We are talking about photon-counting case (even if it is denied). Hence, we are talking about very low energy amplified and converted to digital numbers. Hey, that's experimental physics. In Germany we have a proverb: "Wer misst, misst Mist." Who starts measuring will also measure garbage.
Would you agree with what I describe having found as these non-linearities with the CMOS?
Best regards,
Thilo