A variation on the question. Could someone describe in detail, how to determine whatever frames are needed, and compute the sky background flux in electrons per second? Please? I've looked at a lot of discussions on this, spent hours searching the Net, and I'm still baffled. Bias surely is important. Flats? Darks?
You can assume we set the PI mode (would using zero to one automatically work?) to whatever bits our camera produces. And know how to determine camera gain in electrons per RN (ADU). Those things I understand. Sky flux in electrons per second remains elusive.
It is best to use a calibrated sub for this, however it is not absolutely necessary. If you know generally what your bias offset is, say 21 (i.e. 12-bit, unity gain on ASI1600) or 512 (14-bit, bias offset of all Canon DSLRs), you can simply subtract that from your measurements.
To properly calculate skyfog flux, you need to know the background level in electron counts, as well as the exposure time. The flux would then simply be e-/tsub.
To determine your background level, you need to know your gain. If you measure 120 ADU for your background level (and this can often be roughly estimated just as the median of the image, or more accurately imaged by drawing previews around regions of true background sky, running PreviewAggregator to generate an image from those previews, then running statistics on that generated aggregate preview image), and your gain is 0.63e-/ADU, then your true background sky signal is 75.6e-. If your exposures were 300 seconds, then your flux is 75.6/60, or 0.252e-/s, or about 15.12e-/min. That is about the level of a good dark site, give or take (depends a bit on your f-ratio and pixel size, of course.)
You can figure out what your gain is by using the BasicCCDParameters script.
And, as always, remember to do your measurements in the proper bit depth for your camera as it was configured during readout. (i.e. an ASI1600, when using hardware binning mode, is 10-bit...otherwise it is 12-bit. Always use the proper bit depth!)
In formula form:
SkyFlux = ((Background16bit / (216/2ADC) - Offset) * Gain) / ExposureTime
Background = Measured background level in 16-bit
ADC = The bit depth of the camera ADC (Camera specification)
Offset = The conversion bias offset (configured in the camera driver, if it is configurable at all)
Gain = The conversion gain (measured with BasicCCDParameters script)
ExposureTime = The length of the sub exposure
If you are configuring your measurements for the proper bit depth initially, the formula can be simplified to:
SkyFlux = ((BackgroundADC - Offset) * Gain) / ExposureTime
Edited by Jon Rista, 27 December 2016 - 01:40 PM.