The core algorithm for this step is called from the external package
stcal, an STScI
effort to unify common calibration processing algorithms for use by multiple observatories.
saturation step flags pixels at or below the A/D floor or above the
saturation threshold. Pixels values are flagged as saturated if the pixel value
is larger than the defined saturation threshold. Pixel values are flagged as
below the A/D floor if they have a value of zero DN.
This step loops over all integrations within an exposure, examining each one group-by-group, comparing the pixel values in the SCI array with defined saturation thresholds for each pixel. When it finds a pixel value in a given group that is above the saturation threshold (high saturation), it sets the “SATURATED” flag in the corresponding location of the “GROUPDQ” array in the science exposure. When it finds a pixel in a given group that has a zero or negative value (below the A/D floor), it sets the “AD_FLOOR” and “DO_NOT_USE” flags in the corresponding location of the “GROUPDQ” array in the science exposure. For the saturation case, it also flags all subsequent groups for that pixel as saturated. For example, if there are 10 groups in an integration and group 7 is the first one to cross the saturation threshold for a given pixel, then groups 7 through 10 will all be flagged for that pixel.
Pixels with thresholds set to NaN or flagged as “NO_SAT_CHECK” in the saturation reference file have their thresholds set above the 16-bit A-to-D converter limit of 65535 and hence will never be flagged as saturated. The “NO_SAT_CHECK” flag is propagated to the PIXELDQ array in the output science data to indicate which pixels fall into this category.
There is an effect in IR detectors that results in charge migrating (spilling) from a pixel that has “hard” saturation (i.e. where the pixel no longer accumulates charge) into neighboring pixels. This results in non-linearities in the accumulating signal ramp in the neighboring pixels and hence the ramp data following the onset of saturation is not usable.
saturation step accounts for charge migration by flagging - as saturated -
all pixels neighboring a pixel that goes above the saturation threshold. This is
accomplished by first flagging all pixels that cross their saturation thresholds
and then making a second pass through the data to flag neighbors within a specified
region. The region of neighboring pixels is specified as a 2N+1 pixel wide box that
is centered on the saturating pixel and N is set by the step parameter
n_pix_grow_sat. The default value is 1, resulting in a 3x3 box of neighboring
pixels that will be flagged.
NIRSpec IRS2 Readouts¶
NIRSpec data acquired using the “IRS2” readout pattern require special handling in this step, due to the extra reference pixel values that are interleaved within the science data. The saturation reference file data does not contain extra entries for these pixels. The step-by-step process is as follows:
Retrieve and load data from the appropriate “SATURATION” reference file from CRDS
If the input science exposure used the NIRSpec IRS2 readout pattern:
Create a temporary saturation array that is the same size as the IRS2 readout
Copy the saturation threshold values from the original reference data into the larger saturation array, skipping over the interleaved reference pixel locations within the array
If the input science exposure used a subarray readout, extract the matching subarray from the full-frame saturation reference file data
For pixels that contain NaN in the reference file saturation threshold array or are flagged in the reference file with “NO_SAT_CHECK” (no saturation check available), propagate the “NO_SAT_CHECK” flag to the science data PIXELDQ array
For each group in the input science data, set the “SATURATION” flag in the “GROUPDQ” array if the pixel value is greater than or equal to the saturation threshold from the reference file
NIRCam Frame 0¶
If the input contains a frame zero data cube, the frame zero image for each integration is checked for saturation in the same way as the regular science data. This means doing the same comparison of pixel values in the frame zero image to the saturation thresholds defined in the saturation reference file. Because the frame zero does not carry its own Data Quality (DQ) information, pixels found to be above the saturation threshold are simply reset to a value of zero in the frame zero image itself. Subsequent calibration steps are setup to recognize these zero values as indicating that the data were saturated.
saturation step will accept either full-frame or subarray saturation reference files.
If only a full-frame reference file is available, the step will extract a
subarray to match that of the science exposure. Otherwise, subarray-specific
saturation reference files will be used if they are available.