9 January 2018 at 8:00 pm #573918
I have looked at trying different ways of doing the image calibrations following the observation that the difference between bias/offset frames and dark frames (taken at 10 minutes exposure) was small.
Once hot pixels are removed from the master dark and using a bias master I found they differ in mean by only 10 ADU with a mean of the bias level of 2249 ADU (STD 7.49 ADU).
So with such a small difference I process the spectra after doing the image calibration with 1) Dark removal 2) Bias removal 3) Mean Bias removal)
This figure compares the results
and this one the difference between the bias and mean bias spectra and the dark dark spectra.
I conclude it is still best to use Dark subtraction as it does not seem to add noise even though the differences are small.
Attachments:10 January 2018 at 7:05 pm #578946Andy WilsonKeymaster
That is an interesting test Andrew, a great demonstration of just how small the dark current is with these modern CCDs. Can you remind me which camera you use?
Also, have you also looked to see if there is any bias drift? That is the one thing that surprised and horrified me when I ran some tests on my camera last year.
Andy10 January 2018 at 7:15 pm #578947
Hi Andy , camera as title. I have not looked for drift but will do so. It is run at a constant temperature in an enclosure held at a constant temperature so I would hope not! However, I will run some tests and report back.
Regards Andrew11 January 2018 at 9:50 am #578948
Hi Andy looking at 12 x 10 min Darks taken over two hours the mean value was 2664 +/- 1 ADU after hot pixel removal. If there were any drift in the Offset/Bias I would have expected a bigger variation and or trend.
I have observed that if you take a sequence of bias frames the temperature of the camera shifts slightly. In order to counter this I add a delay between taking Bias frames of 10s to let the camera re-stabilise. It may have no impact on the but I did not want to take any chances.
Regards Andrew11 January 2018 at 7:23 pm #578949Andy WilsonKeymaster
Thanks for posting your results. You have the same camera as me, so it is interesting to see your results.
It is a while since I ran my test, and I don’t think I removed the hot pixels, so it is possible they are the cause of what I saw. I really wanted to measure the median pixel value, but the software I was using did not calculate that statistic. So I measured the max, min and average pixel values. Leaving the camera be, and then running a series of bias and dark frames. I must redo it sometime, this time removing the hot pixels.
Here are my results for reference.
Test Run 1
Test Run 2
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