PhoSim Tutorial 7: How to simulate sky background
In the previous tutorials, we have deliberately turned off sky background. This is a very common use. This because that there are overwhelmingly more photons from sky background than there are from astrophysical sources. The default is to keep sky background on, since the default settings for PhoSim are to have the most realistic settings. However, there are many applications and studies where no background is desired.
There are approximations for background photons to make them simulate a couple of orders of magnitude faster, but there are still billions of photons in a single image. There are essentially three types of background approximations that are described in the background approximation chart.
The standard approximation called "normal approximation" uses all of the same physics on background photons that are on astrophysical photons, except it uses bunches of photons with smoothing prior to hitting the detector. This essentially averages over the physics prior to the detector on a small scale and is a near perfect approximation since the response of telescope varies over large spatial scales.
An example with normal background applied is to simply use no command file as:
The normal background image is: