# 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 table (ASCII 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 target is assumed to have been placed at the aperture reference location, which is stored in the CRPIX1 and CRPIX2 FITS keywords (note that these are 1-indexed values). Hence the step uses those keyword values as the target location within the image.

## Algorithm¶

The Astropy affiliated package photutils does the work.

If the input file was not averaged over integrations (i.e. a _calints product), and if the file contains an INT_TIMES table extension, the times listed in the output table from this step will be extracted from column ‘int_mid_MJD_UTC’ of the INT_TIMES extension. 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 table contains 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.