Description

Saturation Checking

The flag_saturated_pixels() routine flags pixels at or below the A/D floor or above the saturation threshold. Pixel 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 or less.

The method loops over all integrations within an exposure, examining each one group-by-group, comparing the pixel values in the data 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 (as defined by the input dqflags dictionary) in the corresponding location of the input gdq array. 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 input gdq array. 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 sat_dq array 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 pixel data quality (pdq) array output to indicate which pixels fall into this category.

If the optional read_pattern input is provided, this method will use information about the read pattern to find pixels that saturated in the middle of grouped data. This can be particularly important for flagging data that saturated during the second group but did not trigger the normal saturation threshold due to the grouped data averaging. To trigger second group saturation in a pixel all three of the following criteria must be met:

  1. The count rate estimated from the first group is not expected to saturate by the third group (as estimated by the difference between the first group counts and the superbias if available), which may occur for bright sources.

  2. The difference in counts between the first and second group is larger than the remaining counts needed to saturate divided by the number of frames in the second group, i.e., the expected frame-averaged counts of a saturating signal that occurs in the last frame of the group.

  3. The third group is saturated.

Charge Migration

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.

The flag_saturated_pixels() routine 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 input parameter n_pix_grow_sat. The default value is 1, resulting in a 3x3 box of neighboring pixels that will be flagged.