Description

The tso_photometry step does aperture photometry with a circular aperture for the target. Background is computed as the mean within a circular annulus. The output is a catalog, a table (ecsv format) containing the time at the midpoint of each integration and the photometry values.

Assumptions

This step is intended to be used for aperture photometry with time-series exposures. Only direct images should be used, not spectra.

The location of the target is assumed to be given by the CRPIX1 and CRPIX2 FITS keywords (note that these are one-based).

Algorithm

The Astropy affiliated package photutils does the work.

If the input file was not averaged over integrations, and if the file contains an INT_TIMES table, the times shown in the output table will be extracted from column ‘int_mid_MJD_UTC’ of the INT_TIMES table. Otherwise, the times will be computed from the exposure start time, the group time, and the number of groups in an integration. In either case, the times are Modified Julian Date, time scale UTC.

The output catalog will contain these fields:

• MJD
• aperture_sum
• aperture_sum_err
• annulus_sum
• annulus_sum_err
• annulus_mean
• annulus_mean_err
• aperture_bkg
• aperture_bkg_err
• net_aperture_sum
• net_aperture_sum_err

Subarrays

If a subarray is used that is so small that the target aperture extends beyond the limits of the subarray, the entire area of the subarray will be used for the target aperture, and no background subtraction will be done. A specific example is SUB64 with NIRCam, using PUPIL = WLP8.