rfdesigner said:
I hate the transatlantic time gap.. I keep missing the middle of conversations. I see nothing that Jrista has said that I really disagree with.
When we were doing the AviStack development our team of three had the lead in Germany, me in the US and the third fellow in Australia. Somehow we managed to get everyone together for regular discussions. It was great fun.
rfdesigner said:
It also sounds like we both have to concern ourselves with noise professionally, in my case almost a quarter century of radio development. The astronomy is pure hobby, but parts do occasionally spill over.
Radio is quite a different beast from simply collecting and estimating the number of photon that fall into a bucket. Imagers do not have to worry about phase jitter/recovery, bandwidth, or proper temporal sampling of the signal. These days RF work is a mix of analog front ends and digital (de)modulation. With some of the new direct synthesis chips out there, the analog bits may soon disappear altogether.
rfdesigner said:
Given your background I'd be interested in fully understanding why you think 1e/ADU is important in the noisy systems we have today (I agree in a noiseless system 1e/ADU is optimal). My understanding is that the readout noise alone will disturb the measurement sufficiently not to have to concern ourselves, as Jrista has already stated we normally adjust length of shots to maximise overall performance and that usually means skyglow>K*RN^2, where K is an arbitrary constant, I use K=2 as an absolute minimum.
A noiseless system is never optimal unless the detection system is analog (continuous, not quantized). Digital systems require noise to work properly (specifically when combining samples or doing any sort of statistical manipulations). While we can measure and characterize the various sources of noise, in the end noise is noise. Knowing their origins does allow us to take corrective action and/or design the system properly. At the point of digitization, noise should never be less than 0.5ADU (1-sigma). Lower than that, quantization artifacts emerge and can be quite annoying. Maximizing performance is very subjective and necessarily implies a pre-selected course of action for processing the signal to extract the desired information. As I stated originally, 1e/ADU maximizes both the DR and the amount of information (available levels of signal if you will). Alter that gain and you will give up one or the other. While it has been a while since I've gone shopping for imager HW, back when I bought my FLI camera the standard was to build HW that operated very near 1e/ADU. I believe that is still standard practice. So, don't just take my word on this approach.
Depending upon what your measurement requirements are, it is certainly reasonable to set the gain lower so that it takes more electrons per ADU. Although there are practical limits relating to pixel size and the associated well capacity. The SAGE III detector (developed in the mid 1990's) has a gain of 75e/ADU, but we have a reasonably bright source and a high SNR requirement.
rfdesigner said:
On the subject of 5D rn have you seen: http://www.astrosurf.com/buil/50d/test.htm Buil does seem to know his stuff, but then he started with CCDs in the 80s, so he ought to.
Interesting, the 5D and 5D2 numbers look very familiar. I can't find my original characterization data and results, so I suspect that my recollection was that the read noise was a relatively constant ADU - not electrons. His data do show that. It is a shame that he did not report the full well capacity at ISO 100. Characterizing these sensors is not difficult. It takes only a handful of exposures and some simple code. I am a bit surprised that he did not specify the values for the R, G, and B channels separately. Perhaps he does, but I did not dig into the details on his site.
I have to run and will edit this later if needed. Cheers!