PSF Fitting

A few PSF fitting utilities are included to interface between observations within Roman datamodels and methods within dependencies that generate and fit PSF models to observations.

Create PSF models

create_gridded_psf_model computes a gridded PSF model for a given detector using the reference files in CRDS.

These PSF models account for jitter by deconvolving the amount of jitter indicated to be present in the PSF reference file, and reconvolving with the amount of jitter present in the individual image. Because the amount of jitter present in the reference file is small (~8 mas), this deconvolution does not introduce significant noise. The convolution process works in the Fourier domain using the approach of Lang (2020) because the jitter kernel would otherwise be badly undersampled.

Fit model PSFs to an ImageModel

Once PSF models are generated, you can fit those models to observations in an ImageModel with fit_psf_to_image_model using photutils. By default the fits are done with PSFPhotometry, and crowded fields may benefit from using IterativePSFPhotometry. For neighboring sources that are near one another on the detector, grouping the sources and fitting their PSFs simultaneously improves the fit quality. Initial guesses for target centroids can be given or source detection can be performed with, e.g., DAOStarFinder.