Eigen PSF model

[ConnorStoneAstro]

Is your feature request related to a problem? Please describe. AstroPhot can fit PSF models live, but we need to be able to account for complex features of emperical PSFs. A good compromise is an Eigen PSF model which could build a PSF as a linear combination of some basis PSF images.

Describe the solution you'd like A PSF model which is just a sum of a set of basis PSF images determined by some other code.

Describe alternatives you've considered It's already possible to fit a parametric PSF or a zernike PSF, but these are more restrictive than the Eigen PSF which could be very general (ie diffraction spikes).