PhoSim Tutorial 17: How to Make a Grid of Point Sources

A grid of point sources can be useful in evaluating the performance of a telescope system. This can be useful to identify astrometric distortion patterns by measuring the deviation of source centroids across the field. It can also be used to understand the point spread function (PSF) properties. Both the PSF size and shape can shift across the field and can be measured using a grid of sources. Similarly, the photometric variation can also be measured from a grid of sources.

You can create a grid of sources by simply listing the source positions in a catalog. However, PhoSim has a simpler way of doing this by a single command. This feature correctly places the sources correctly with a constant angular shift. This is then mapped using the correct spherical geometry. The following catalog is an example that makes a grid of stars.

rightascension 0

declination 0

stargrid 0.01 0.2 20

The first argument sets the angular separation in degrees (0.01), the second argument sets the field angle of the grid (0.2), and the third argument sets the magnitude of the sources (20). The corresponding image using the generic telescope is:

PhoSim Grid of Sources

Then the astrometric, photometric, and PSF size/shape properties can be measured for every source using images such as these.