Calibration

Previously, data requested from MAST were generated using the "on the fly reprocessing" (OTFR) approach. However, with the growing size of the archive and the increasing complexity of calibration software, the archive has switched to become a static one. This means that archived data are no longer reprocessed each time users submit requests to MAST. Instead, the data are only reprocessed when significant changes are made to either the calibration software (e.g. CALACS) or or new/improved reference files (e.g. DARKFILE, BIASFILE, etc..) are delivered to the Calibration Reference Data System (CRDS). As soon as these changes are implemented, the affected datasets are reprocessed and users can download the freshly calibrated data products.

For users who wish to reprocess their data locally, the ACS Team has created an "ACS Reduction" Jupyter notebook that outlines this process starting from RAWs and ending with FLTs/FLCs. We strongly recommend following this guided example closely. You can access this notebook on the ACS Analysis Tools webpage. 

In general, the calibration of ACS data can be summarized in three main steps:

1.   Conversion of POD files sent from the Observatory into RAW files.

2.   The processing of RAW files using CALACS.

• For the HRC and WFC, this corrects for the bias offset, dark current, flat-fielding, an imperfect charge transfer efficiency (CTE), and if the data are CR-SPLIT, rejects cosmic rays.
  • Resulting files have an FLT, FLC, CRJ, or CRC suffix depending on the inputs. Note: CALACS does not correct for geometric distortion and so all of these data products are still distorted.
• For the SBC, this corrects for flat-fielding, and if the data are repeated sub-exposures, it sums together the sub-exposures. SBC data are not dark-corrected by CALACS as the dark current is negligible in most cases. However, due to the spatially variable and temperature dependent growth of dark current there do exist cases where dark subtraction is necessary (ACS ISR 2017-04 ). To account for this, the ACS Team has implemented a new aperture for use with the SBC instrument that is optimized to provide users with a section of the detector that has a constant dark rate at all temperatures (see ACS ISR 2018-07). 
  • Resulting files have an FLT or SFL suffix.

3.   The processing of the products produced in step 2 with AstroDrizzle.

• For dithered data, this will combine the individual images from each filter set while rejecting cosmic rays, performing sky subtraction (if appropriate) and correcting for the geometric distortion. 
  •  This will transform multiple FLTs into one DRZ, or multiple FLCs into one DRC.
• For undithered data (e.g. CR-SPLIT), the cosmic rays have already been rejected and so this will perform the sky subtraction (again, if appropriate) and correct for the geometric distortion.
  • This will transform a CRJ into a DRZ, or a CRC into a DRC.

Each year the ACS Team devises a list of calibration proposals designed to generate the best quality reference files possible. These proposals are combined to form a calibration plan that is executed over the course of the given HST Cycle. These calibration plans includes proposals for generating the ACS superdarks, superbiases, and flat-field reference files, for monitoring the CTE of the ACS/WFC, for annealing the ACS/WFC detector to cure pixels damaged in the harsh radiation environment of space, and so on. A complete list of all calibration plans executed over the lifetime of the ACS is available below.

The ACS/WFC undergoes a monthly annealing process as part of the CCD Hot Pixel Annealing calibration program. During the anneal, the CCD detector is heated from its operating temperature of -81^oC to 20^oC for a period of 12 hours. This process has been shown to temporarily reduce the dark current and population of stable hot pixels that increase linearly over time (ACS ISR 2020-05). A list of the upcoming and completed anneal dates for each HST Cycle, along with the associated calibration proposals for each anneal, can be accessed below.