American Association of Variable Star Observers (AAVSO)
Thu, 06/11/2020 - 00:02

In the past I've read that some people here use AstoArt7 for stacking images for photometry as part of their workflow, so I tried it and found it generally easy to use and quite fast.

But there is one "feature" that I think is rather inconvenient for photometry and certainly easily overlooked, so I wonder: are people here aware of the following??

Here's the story: if you take a set of raw light- and dark frames as produced by your camera, plus mabye your master-flat frame, and combine them via AstroArt7's stacking, the software will do all the internal per-frame calibration and (depending on your settings) summing or averaging in floating point pixel math (good!!), but will then convert the final stacked image to the same format as your light images, which will be (fresh out of the camera) an integer FITS format! And if you tell AA7 to retain the master-dark image it created, the same will happen: you'll get it as integer FITS master dark.

If you would rather avoid the added quantization noise introduced by the rounding step, you can convert your first light frame (and your first dark frame) to floating point FITS format, and then AstroArt7 will retain the full precision in the combined stacked image and in the produced master dark image. 

Now if you don't want to mess with your input frames, I guess the lesson is that you should do summing rather than averaging in AstroArt to somewhat mitigate the added quantization noise, and for the same reason for 12 and 14 bit sensors, you should NOT use the native ADU range but let the acquisiation software scale the ADUs to a 16 bit range (e.g. SharpCap allows unscaled and scaled ADUs via configuration).