Implementation of refraction from the WFI filters as a photon operator. The photon operator lives in the file filter_refraction.py and contains the relevant functions used in the photon operator. Modifications to __init__.py are simply to add the new file and functions within it. Modifications to config/default.yaml are to add refraction prior to charge diffusion. I believe this is the correct order, as this effect occurs before photons reach the detector.

A small breakdown of the functions in filter_refraction.py:

• class RomanFilterRefraction: This class contains the GalSim photon operator. To initialize the operator, you need to specify the bandpass, SCA and SCA position, pixel scale, and callable function to get the index of refraction as a function of wavelength. The applyTo gets the lateral chromatic shifts induced by filter refraction and applies it directly to the photons based on their wavelength. The default for the pixel scale is the native roman scale (0.11 arcsec/pixel), and the default index of refraction comes from the Roman WFI filter substrate (Suprasil 3001)
• class RomanFilterRefractionBuilder: This class builds the photon operator from the information in base, which includes the bandpass, SCA and SCA position.
• _get_lateral_shifts: This is the main function called within applyTo that calculates the lateral chromatic shifts. This function calls other helper functions defined throughout the file.
• Constants: At the top of the file, several relevant constants and coefficients are defined. Primarily, the manufacturer fits to the index of refraction and filter/telescope geometry (radius of curvature, distance between optical elements, filter thickness). In addition, I use a dictionary containing the SCA centers in FPA coordinates. This provides a fast and accurate way to estimate the FPA position at any SCA and SCA position without the need of external libraries (e.g. pysiaf)