DSLR Photometry Experiments. Visualizing what stacking does
When light frames are stacked, the signal to noise ratio (SNR) goes up as the noise drops. The more lights you use, the more the SNR increases; the stars have smoother profiles and the level of the noise drops and is smoother.
Figure 1 shows an X-Y-intensity plot of a star (HD 121228, V=7.83) taken from a single RAW image (left panel) and from a stack of 17 RAWs (right panel), nicely illustrating this effect.
Figure 1. X-Y-intensity plot of HD 121228 from one light frame (left) and from 17 light frames (right).
Figure 2 graphs the increase in SNR of two stars as additional frames are added to the first light. For HD 121228 and the equipment used (details here) the SNR increased from 29 to 95. A brighter star (HD 120042, V=7.32) was also measured, and it's SNR went from 45 to a quite respectable 180.
Figure 2. Signal to noise ratio as a function of the number of light frames stacked, for HD 120042 (top curve) and HD 121228 (lower curve).
Figure 3 shows, on the left, a series of histograms, and on the right, the X-Y-intensity plots, of HD 121228. The histograms were generated by APT ("Aperture Photometry Tool - User-Friendly Software for Aperture Photometry") and the X-Y-pixel count plots were generated by a script written in SciLab (v 5.4.1) from the original RAW CR2 files.
The top row is for a single light frame, followed by a two-frame stack, three-frame stack, 10-frame stack and finally a 17-frame stack.
A single light frame (no stacking)
A two-frame stack
A three-frame stack
A 10-frame stack
A 17-frame stack
Figure 3. Histograms and X-Y-intensity plots for 1, 2, 3, 10 and 17 light frames of HD 121228.
nothing more to see. please move along.