Description

The photom step loads - and in some cases applies - 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.

Upon successful completion of this step, the status keyword S_PHOTOM will be set to COMPLETE.

Imaging and non-IFU Spectroscopy

Photom Data

For these instrument modes the photom reference file contains a table of exposure parameters that define various instrument configurations and the flux conversion data for each of those configurations. 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. Note that for NIRSpec fixed-slit mode, the step will search the table for each slit in use in the exposure, using the table row that corresponds to each slit.

For these table-based reference files, the calibration information in each row includes a scalar flux 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.

None of the conversion factors are actually applied to the data for these observing modes. They are simply attached to the science product.

Pixel Area Data

For imaging modes, the photom step loads data from a pixel area map reference file and appends it to the science data product. The 2D data array from the pixel area map is copied into an image extension called “AREA” in the science data product.

The process of attaching the pixel area data also populates the keywords PIXAR_SR and PIXAR_A2 in the primary header of the science product, which give the average pixel area in units of steradians and square arcseconds, respectively. Both the photom and pixel area reference files contain the average pixel area values in their primary headers. The photom step copies the values from the pixel area reference file to populate the PIXAR_SR and PIXAR_A2 keywords in the science data. It will issue a warning if the values of those keywords in the two reference files differ by more than 0.1%.

NIRSpec IFU

The photom step uses the same type of tabular reference file for NIRSpec IFU exposures as discussed above for other modes, where there is a single table row that corresponds to a given exposure’s filter and grating settings. It retreives the scalar conversion constant, as well as the 1D wavelength and relative response arrays, from that row. It also loads the IFU pixel area data from the pixel area reference file.

It then uses the scalar conversion constant, the 1D wavelength and relative response, and pixel area data to compute a 2D sensitivity map (pixel-by-pixel) for the entire 2D science image. The 2D SCI and ERR arrays in the science exposure are divided by the 2D sensitivity map, which converts the science pixels from units of DN/sec to mJy/arcsec². Furthermore, the 2D sensitivity array is stored in a new extension of the science exposure called “RELSENS2D”. The BUNIT keyword value in the SCI and ERR extension headers of the science product are updated to reflect the change in units.

MIRI MRS

For the MIRI MRS mode, the photom reference file contains 2D arrays of sensitivity factors and pixel sizes that are loaded into the step. As with NIRSpec IFU, the sensitivity and pixel size data are used to compute a 2D sensitivity map (pixel-by-pixel) for the entire science image. This is divided into both the SCI and ERR arrays of the science exposure, which converts the pixel values from units of DN/sec to mJy/arcsec². The 2D sensitivity array is also stored in a “RELSENS2D” extension of the science exposure. The BUNIT keyword value in the SCI and ERR extension headers of the science product are updated to reflect the change in units.