The photom step loads information into a data product that allows for the conversion of count rates to absolute flux units. The flux conversion information is read from the photometric reference file. The exact nature of the information that’s stored in the reference file and loaded into the science data product depends on the instrument mode.
For most instrument modes the photom reference file contains a table of exposure parameters that define various observing modes and the flux conversion data for each of those modes. The table contains one row for each allowed combination of exposure parameters, such as detector, filter, pupil, and grating. The photom step searches the table for the row that matches the parameters of the science exposure and then copies the calibration information from that table row into the science product.
For the MIRI MRS mode, the photom reference file contains arrays of sensitivity factors and pixel sizes that are copied into the science product and also applied to the SCI and ERR arrays of the science product.
For the table-based reference files, the calibration information in each row includes a scalar conversion constant, as well as optional arrays of wavelength and relative response (as a function of wavelength). The scalar conversion constant in a selected table row is copied into the keyword PHOTMJSR in the primary header of the science product. The value of PHOTMJSR can then be used to convert data from units of DN/sec to MJy/steradian. The step also computes, on the fly, the equivalent conversion factor for converting the data to units of microJy/square-arcsecond and stores this value in the header keyword PHOTUJA2.
If the photom step finds that the wavelength and relative response arrays are populated in the selected table row, it copies those arrays to a table extension called “RELSENS” in the science data product. For the MIRI MRS mode, the sensitivity factors and pixel size arrays are multiplied together and copied to an image extension called “RELSENS2D” in the science data product.
For multiple-integration datasets, the photom step can take either of these as input: a dataset containing the slope results for each integration in the exposure, or the dataset containing the single slope image that is the result of averaging over all integrations.
Finally, if the science data are from an imaging mode, which is determined from the value of the EXP_TYPE keyword, the data from a pixel area map reference file will also be loaded into the science data product. The 2D data array from the pixel area map will be copied into an image extension called “AREA” in the science data product. For imaging mode exposures, the values of the PIXAR_SR and PIXAR_A2 keywords in the photom reference table will also be copied into keywords of the same name in the primary header of the science data product.
Note that, except for MIRI MRS exposures, the pixel values of the science data product are not actually changed by the photom step. All of the reference data is simply attached to the science data product in some way.
Upon successful completion of this step, the status keyword S_PHOTOM will be set to COMPLETE.