Ahoy Comrades!
I have been pondering about the real gain in using a SuperBias as opposed to a MasterBias in calibrating Lights. It only takes a click of a button to create a SuperBias in PI, but does it really make any difference to the final image? One would need to calibrate data with MasterBias and SuperBias and then process both sets of data in exactly the same way, and then measure final result for noise...
I may do that one day, but I thought that using PixelMath and Statistics tools might give me some indication.
Here are the results:
Single Bias: St.dev 14.7 ADU
Master Bias (150 frames): St.dev 2.143 ADU
Super Bias: St.dev 1.756 ADU
All frames were captured at -15C.
But here is my problem - SuperBias is meant to be noise free, so is there a better way than using the Statistics tool on the entire frame? I wanted to avoid using a small area in a bias frame, because of fixed patterns across the frame - it could also give me a skewed indication of noise.
Then I thought - why not subtract my MasterBias from the SuperBias? I added a fixed value to SuperBias to avoid clipping data. (the first screenshot shows a MasterBias, SuperBias and a difference between the two plus a fixed value).
Now the Statistics tool gave St.dev of 1.267 ADU for the difference between SuperBias and MasterBias.
Would it be correct then to assume that when calibrating with this MasterBias I would be adding 1.267 ADU of noise to all Light frames, while if using the SuperBias I would add no noise to Light frames?
On a slightly different topic, since I have just recollected a full set of Bias frames for the first time since I sent my camera to QSI for a WSG-8 upgrade and for removal of the infamous amp glow, I thought of re-measuring my camera too. Until now I have been using a mix of old and new bias frames...I know, I am lazy...
Interestingly, it looks like gain after the fix has changed from 0.16e/ADU to 0.26e/ADU, resulting in a change in well depth from about 10,000 to 16,500 electrons. Dynamic range remains about the same at 4265 steps (from memory), and read noise measured with three different methods gave values from 3.775e to 3.887e (see second screenshot for reference).
Thank you for reading, I know I can waffle on and on...
Suavi