So, please give me a practical explanation of what Pedestal is and how it works, and why I might or might not need to use it in my processing workflow.
I've never seen it either. Is there a set of circumstances that would produce this calibration clipping repeatedly, or is this a somewhat rare occurrence?
I came across a set of circumstances around the beginning of this year where a pedestal is consistently needed for me. (The required pedestal was rather small at ~45 - 50 ADU but real.) I used to have problems with clipping during calibration and sometimes had to band-aid it using the pedestal. (Sometimes, I didn't bother if the clipping was minimal.)
I began to do a detailed analysis of my two CCD cameras. One is an older Orion StarShootPro OSC (Sony ICX413AQ-based) and the other is a StarLight XPress SXVR-H694M mono (Sony ICX694ALG-based). Both showed some similarly odd results as I was building a library of calibration frames (Bias and Dark). What I noticed was that depending on how I gathered my calibration frames, there were sometimes differences between the beginning and ending frames in a sequence.
After going through my Bias frame collection sequentially (for both cameras) and doing some research online, I found that my Bias frames tended to have higher median ADU values at the beginning and end of the run compared to those in the middle. I found that the higher values at the beginning of the run appeared to be due to not letting the TEC "soak" the sensor long enough before starting the run. I would often just start a Bias run after the temperature readout indicated that I had reached my set-point. The slow rise in values for subsequent frames after reaching a minimum was still a bit puzzling.
In my online research, I learned that at least some StarLight Xpress cameras temporarily turn off cooling power to the TEC during image download to reduce noise. I found that if I inserted a delay between Bias frames rather than "letting them rip" when building the library, the results were more consistent. The same was found to be true of the Orion camera even though it simply runs the cooler all out all the time. (The old Orion camera doesn't have set-point cooling -- it's just on at fixed power all the time.)
My extensive testing indicated to me that taking Bias frames back to back as fast as possible seemed to be inducing self-heating of the readout and clocking circuitry on the chip for both cameras. This tended to lead to higher median values in the integrated Master Bias frames than might be expected otherwise. I "fixed" this problem by inserting a one minute delay between frames in SGP when building my library. The delay gives the cameras time to cool back after the download process before taking another frame.
After gathering a new library using the delay between frames, I found that the required ~50 ADU pedestal was no longer routinely required to prevent clipping. Other cameras may not have this problem but I suspect it may be more common than some think. The constant clocking out of data for downloading Bias frames compared to the "exposure time" of the Bias frames does not mirror our normal light frame exposures.
Lights are characterized by long idle time of the clocking circuits on the chip during exposure followed by furious clocking during download. The download time to idle time ratio for a light frame might be something much less than 1% while for library gathered Bias frames, it will be close to 100% download time. That self heating of the readout circuitry during download clocking can contribute to higher Bias signal in the Master Bias frame compared to the Bias signal in the light frame. In such a case, it could be enough that a pedestal is required to compensate.
David Ault and I talked this over one night and he mostly agreed although he had never seen the effect in his camera. I vaguely recall that he later mentioned having done some testing himself and noticed a similar albeit less effect compared to what I found. If he sees this post, maybe he can refresh my memory.
So, in my opinion, a small pedestal may be generally useful depending on how Bias frames are gathered. The same thing might happen for short duration Flats, Flat Darks, or any other frame type where download time of multiple frames becomes a significant percentage of the total imaging cycle.
Edited by jdupton, 07 July 2016 - 02:32 PM.