Description

Algorithm

The photom step adds flux (photometric) calibrations to the metadata of a data product. The calibration information is read from a photometric reference file, and the exact nature of the calibration information loaded from the reference file is described below. This step does not affect the pixel values of the data product.

Upon successful completion of the photometric correction, the “photom” keyword in “cal_step” in the metadata is set to “COMPLETE”.

Photom and Pixel Area Data

The photom reference file contains a table of exposure parameters that define the flux conversion and pixel area data for each optical element. The table contains one row for each optical_element, and the photom step searches the table for the row that matches the parameters of the science exposure and then loads the calibration information from that row of the table.

For these table-based PHOTOM reference files, the calibration information in each row includes a scalar flux conversion constant, the conversion uncertainty, and the nominal pixel area.

The scalar conversion constant is copied to meta.photometry.conversion_megajanskys, which gives the conversion from DN/s to megaJy/steradian, and the uncertainty is copied to meta.photometry.conversion_megajanskys_uncertainty.

The step also populates meta.photometry.pixel_area in the science data product, which gives the average pixel area over the detector in steradians. This is a single nominal value and does not describe how the pixel area varies across the detector; code that needs the solid angle of a particular pixel should compute it from the WCS instead.