StarTools

Any idea why I'm getting this square box around the star? It made the diffraction spikes fine, but the box shouldn't be there. I made a mask that just covers the bright center of the star.

This is usually caused by not specifying the correct angular size of the image; you cannot just ignore that value - it needs to be in the ballpark of your actual image. The Synth modules does not "paint" on diffraction spike - it physically models based on the characteristics of the virtual scope and image.

The box you are seeing is the boundary of the used point spread function model (you can increase this with the Sample Size parameter when you create your virtual OTA model, making things quite a bit slower, but it's usually not necessary). If it cannot spread its energy fast enough due to the values you give it, then it will run out of samples before it is done spreading its energy, causing a noticeable boundary/"box".