Quote:
Originally Posted by Merlin66
Ray,
Why not median combine?
It's only a scaling factor and doesn't change the SNR?
How do you measure your SNR?
I use average combine for my spectroscope lights and median for the darks...I haven't found flats very useful with the ATiK314 in this application.
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Hi Ken.
As I understand it, median combine will pull out the single value that is the median of all of the inputs - it cannot have a fractional ADU value, so the scaling factor will be subject to quantisation noise.
For SNR, the basic question is "of what". To fix a consistent reference for the modelling and measurements, I measured the average signal of a dim part of the galaxy to be 1/10 the sky background on one of the lights. Since the sky and the galaxy did not change much during the imaging run, I thereafter used 1/10 sky as the Signal and so the SNR values refer to the chosen part of the galaxy. I could choose some other reference point and all of the SNR results would be scaled, but the same picture would emerge when comparing model and test data. With a dim target, the noise is totally dominated by the sky, so I measured the RMS variability of a featureless part of the sky to find the noise. I turned off all of the smart bits while stacking in PI (to get an undoctored result) and used the pixel stats function of Nebulosity to do the measurements.
Interesting observation on the 314. I would guess that the Sony chip in the Atik has very low fixed pattern variability, so flats will not help unless you expose for very long periods of time.
Regards Ray