STScI Logo

Hubble Space Telescope
CALACS Updates

HST Data Processing (HSTDP) Pipeline Releases

All of the packages required for HST data processing are released as HSTDP builds. Each build contains all of the normal tools utilized in the calibration and analysis of HST data for the various instruments. The different calibration software (e.g. CALACS, CALCOS, CALSTIS, and CALWF3) used by each of the instrument teams are combined into a single software package called HSTCAL.

Three times a year the instrument teams are given the opportunity to make updates to their software packages which are then incorporated into a new version of HSTCAL and released as a new HSTDP build. CALACS is written entirely in C and the complied executables may be run from the command line. The ACS Team also provides a convenient and easy to use python wrapper around the CALACS called ACSTOOLS. Hence, whenever a new version of CALACS is released it results in a new version of both HSTCAL and ACSTOOLS.

For an in-depth description of CALACS, we refer users to the ACS Data Handbook. For a description of the python wrapper, ACSTOOLS, we refer users to the online documentation hosted here

HSTDP 2018.3

CALACS updates to version 10.1.0 and HSTCAL updates to version 2.2.0.


  • CALACS updates to Version 10.1.0 due to updates in the ACSCCD and ACS2D algorithms.
  • The dark correction, DARKCORR step of the ACS2D algorithm, has been modified to work directly on the full 2D data rather than on a line-by-line basis.
  • The bias shift correction, BLEVCORR step of the ACSCCD algorithm, has been upgraded to accommodate the new supported subarrays which have both physical and virtual overscan. This is the same algorithm which applies to full frame data.
  • Forward model functionality was implemented which exploits the existing CTE code and usage (Generation 2 only). Instead of correcting the CTE trails, the forward model functionality simulates (adds) the trails. This functionality should be accessed via the new ACSTOOLS function,
  • Updated ACS library routine, getacskeys, updated to call the new HSTIO function which robustly determines the number of IMSETS in a FITS file.

HSTDP 2018.1

CALACS updates to version 10.0.0 and HSTCAL updates to version 2.0.0


  • An updated ACSREJ algorithm was implemented where all calculations are now done in electrons versus DN. The threshold computation uses the ERR arrays to account for post-flash noise and contributions from readout dark.
  • CALACS now uses the amp-dependent read noise as the threshold to determine if a pixel has enough signal-to-noise ratio such that a reasonable correction can be applied.
  • A bug fix has been implemented which is only applicable for RAWs with PCTECORR=PERFORM for ASNs used for cosmic ray rejection. In this case the CR rejected output (CRJ) previously used the wrong dark; it used the CTE-corrected dark. This has been addressed, and the CRJ product is now correct utilizing the non-CTE corrected dark PR#273.
  • All BIAS files are now processed by CALACS to use only read noise (vs Poisson) for the computation of the ERR array as the date discriminant (pre- and post-SM4) has been removed PR#288.
  • The C interface to the ACSREJ standalone component of CALACS was modified for clarity. The "newbias" option was renamed "readnoise_only". The functionality associated with this option remains the same.

HSTDP 2017.2A

CALACS updates to version 9.2.0 and HSTCAL updates to version 1.3.0


  • New BLEVCORR algorithm that uses new OSCNTAB, which now correctly calculates bias level for AMP B and D in subarrays.
  • New SINKCORR step to apply new SNKCFILE in order to flag sink pixels. See ACS ISR 2017-01.
  • New pixel based PCTECORR algorithm implemented (CTE_NAME= 'PixelCTE 2017').
    • The code defaults to this new algorithm, however, the old one can still be executed using the command line option --ctegen <1|2>. Note: The correct accompanying PCTETAB file must also be used.
    • The user can now override the PCTETAB file used (normally specified in the image header) with the command line option --pctetab <file path>.
    • The number of threads used for computing the PCTECORR can now be specified with the command line option --nthreads <N>. Note: OPENMP is required for N > 1.
  • Fixed uninitialized variable in prinfo.c.
  • Corrected --version reporting and added -r to report version string that was previously reported by --version.

Last modified on May 16, 2012.