PhoSim Tutorial 10: How to Generate Sequences of Images

In addition to generating single images or groups of images for multiple chip focal planes, PhoSim can also naturally generate sequences of images in time with different observing conditions. A variety of physics including the weather/clouds will correctly follow time correlations and patterns. 

A powerful aspect of the stars and galaxy module generator is that it is repeatable. This means that you can generate multiple catalogs and get the same set of stars & galaxies appearing at the same right ascension and declination. Thus, a simple catalog like the following would generate stars and galaxies covering a 0.1 degree diameter circle.

rightascension 0.0

declination 0.0

seed 100

id 100

stars 0.0 30.0 0.1

galaxies 0.0 30.0 0.1

Then, if this was followed by a second catalog like the following with the seed changed and the id different (so the filenames are distinct), the same stars and galaxies would appear. However, the observing conditions (e.g. weather, airmass, filter etc.) would be different, so you would get a different looking image for the same scene.

rightascension 0.0

declination 0.0

seed 101

id 101

stars 0.0 30.0 0.1

galaxies 0.0 30.0 0.1

Note, there are many more complex uses of this. For instance, you could specify the exact time of the observations or the filter, if you were simulating a pre-defined cadence. These additional parameters are optional though. You could also specify an individual object along with the star and galaxy commands. The individual object could then appear only in some of the catalogs in order to simulate variability.

The image from the first catalog is:

The image from the second catalog is:

Notice they are visibly different, but the stars and galaxies are largely in the same relative position.