Crandall Research

Brant Robertson bio photo By Brant Robertson

Source – Image Registration

Sara has been conducting some tests to see how well SEP (pythonified Sextractor) recovers the input mock catalog properties. She has created a python notebook for this study at:

Source Recovery Jupyter Notebook

When overlaying the sources as a function of RA and dec on top of the image, there appears to be an unusual shift or distortion in the image.

RA-Dec Registration

However, the pixel location of the sources appears to be correct:

Pixel Registration

I reviewed what Sara did and noticed that she assumed RA and Dec are aligned with the pixel values:

x_test = (0,0,nx-1,nx-1)
y_test = (0,ny-1,0,ny-1)
ra_test, dec_test = world(fname_img, x_test, y_test)
print ra_test, dec_test

Which gives

[ 53.10148765  53.10187856  53.1288215   53.12920629] [-27.8135469  -27.7892198  -27.81388789 -27.78956071]

This means the x and y pixels are not aligned with RA and Dec. Using world min and max determined from the pixel extent will lead to a distortion.

The corresponding notebook is here:

JWST Source Registration Test Jupyter Notebook

JWST PSF Aperture Corrections

We would also like to measure the JWST PSF aperture corrections for point sources. We suspect that some of the offset between the source magnitudes found by SEP and those in the original mock catalog originate solely from the SEP aperture photometry. This effect should be easy to measure.

Here are the JWST PSFs:

JWST PSFs (tarball)

Using the F090W PSF, I computed the following aperture corrections:

F090W aperture correction

The notebook I used to compute the aperture correction is here.