HSC imaging is known to exhibit subtle ~1-2 count amp-to-amp offsets. Following on from DM-23794, this ticket implements an empirical amp-to-amp offset correction technique within ISR processing, removing these offset artifacts. An empty placeholder task is placed into ip_isr, and is redirected to the primary run task in obs_subaru. This task may be switched on or off using the doAmpOffset flag, which currently defaults to False.

In summary, this process generates an estimate of the amp offset from amp to amp within an HSC CCD, looping over each amp boundary (HSC CCDs having four amps arranged linearly, and therefore three amp boundaries). Once an estimate of each amp offset value is obtained, a solution is derived to determine the singular value which must be added or subtracted from each amp in order to negate these amp offsets. We refer to these additive/subtractive values as pedestals in the exposure metadata.

The method for generating an amp offset value is thus: first, we perform a temporary coarse background subtraction across the entire CCD. As this amp-offset correction occurs in ISR before primary background subtraction has taken place, a general background subtraction is required in order to remove any strong image gradients in the frame. Second, we perform a temporary source detection. This adds temporary DETECTION footprints to the mask plane, allowing those pixels to be ignored in all subsequent analyses if so directed. All mask planes identified in background.ignoredPixelMask are not used in determining amp offsets (defaulting to BAD, SAT, INTRP, CR, EDGE, DETECTED, DETECTED_NEGATIVE, SUSPECT and NO_DATA). For each amp pair we define a thin strip of pixels along the trailing edge of the first amp (amp "A"), and another along the leading edge of the second amp (amp "B"). In order to avoid any image artifact peculiarities which may occur along an amp edge, these two strips are inset from the amp edge by ampEdgeInset (default 5 pixels). The width of the strip is given by ampEdgeWidth (default 64 pixels). The median value is determined for each row in strip A and strip B, resulting in two one-dimensional arrays of length equal to the height of the CCD. These edge arrays are subtracted from one another, giving an estimate of the amp edge difference from A to B as a function of position along the length of the amp. In order to mitigate any remaining flux contamination from singular bright sources (e.g., the wings of bright stars which spill out beyond the source detection footprint), we calculate the rolling average of this amp edge difference array using a window of size ampEdgeWindow (default 512 pixels). Finally, an estimate of the amp A to amp B offset is given by calculating the 3-sigma clipped mean of this rolling average data.

During testing, the procedures outlined above tend to only ever improve the fidelity of the output data, rarely (if ever) causing 'harm'. However, as additional safety checks, we have also added two failure checks to catch extreme cases. First, if the fraction of available and unmasked datapoints in the amp A to amp B difference array is lower than ampEdgeMinFrac (default 0.5) then no amp offset estimate is generated for that amp boundary. This catches those corner cases such as HSC CCD 101, where vignetting if addition to detected source footprints can significantly remove large portions of the field of view, resulting in only a small number of remaining pixels available for amp offset estimation. It also catches those cases where, for whatever reason, a large number of pixels are masked and therefore unsuitable for amp offset analysis (e.g., a strip of dead pixels caused by hardware failure along an amp edge, as is the case with HSC CCD 33). Second, testing has shown that typical amp offsets fall in the range ~1-2 counts, with larger calculated amp offset values increasingly likely to be caused by bright object contamination rather than a true amp offset signature. As such, any amp offset estimate larger than ampEdgeMaxOffset (default 5 counts) will also be ignored, and no amp offset estimate will be generated for that amp boundary.

All added/subtracted pedestals are entered into the exposure metadata as PEDESTALX (where X is the number of the amp: PEDESTAL1, PEDESTAL2...). The net sum of all pedestal corrections for a single CCD is zero.

Full outputs for a test run on 200 visits from DM-23243 (PDR2) wide can be found on lsst-devl at /project/lskelvin/patConCorr/delta-v1 or at this link. Comments below provide examples and links to movies showing the before (vanilla) and after (amp-offset corrected) impact of amp offset correction on these visits. Additional information and discussion on the impact these corrections have on coadd level data can also be found in the comments below.