HST Data Handbook for ACS

3.6 Manual Recalibration of ACS Data

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When is OTFR not appropriate?

• Running CALACS with personal versions of reference files
• Running CALACS with non-default calibration switch values

OTFR will always use the latest ACS calibration reference files available by default. In order to use non-default reference files, manual re-calibration is required. The reference file keywords will need to be updated manually in the uncalibrated data with the non-default filenames before running CALACS. The selection criteria in table 3.8 are used to set the values for the calibration switch header keywords.

Table 3.8: Calibration Switch Selection Criteria

Switch	Description	Criteria
DQICORR	Data Quality Array Initialization	DEFAULT = "PERFORM" If OBSMODE = ACQ then "OMIT"
ATODCORR	Analog to Digital Conversion	DEFAULT = "OMIT"
BLEVCORR	Overscan Region Subtraction	DEFAULT = "PERFORM"
BIASCORR	Bias Subtraction	DEFAULT = "PERFORM"
FLSHCORR	Post Flash Subtraction	DEFAULT = "OMIT"
CRCORR	Cosmic Ray Rejection	If CRSPLIT >= 2 then "PERFORM" If CRSPLIT < 2 then "OMIT"
DARKCORR	Dark Subtraction	DEFAULT = "PERFORM"
FLATCORR	Flatfield Division	DEFAULT = "PERFORM" If FILTER = G800L then "OMIT"
SHADCORR	Shutter Shading Correction	DEFAULT = "OMIT"
PHOTCORR	Photometric Processing	DEFAULT = "PERFORM"
RPTCORR	Repeat Processing	DEFAULT = "OMIT" If NRPTEXP > 1 then "PERFORM"
EXPCORR	Full calibration of individual exposures in an association	DEFAULT = "OMIT"
DITHCORR	Dither Processing	DEFAULT = "PERFORM"
GLINCORR	Global Non-Linearity Correction	SBC DEFAULT = "PERFORM"
LFLGCORR	Local and Global Non-Linearity Flagging in DQ Array	SBC DEFAULT = "PERFORM"

 

Requirements for Manual Recalibration

If the observer decides to re-calibrate their ACS data, the following must be available on their system:

• CALACS pipeline software within STSDAS
• PyDrizzle task from STSDAS dither package
• PyRAF, to run PyDrizzle, obtained from STScI
• Reference files obtained from STScI or personal versions
• Uncalibrated data (*_raw.fits) from OTFR
• Association table, if needed

Uncalibrated data can be obtained from OTFR via StarView or the web-based archive request form: http://archive.stsci.edu/cgi-bin/hst. StarView is available for users to download from: http://starview.stsci.edu/html. The pages include instructions on installing and using StarView and on how to become a registered archive user. The ACS ISR 99-03 "CALACS Operation and Implementation" is available on the ACS web site and should be consulted before manually re-processing ACS data.

3.6.1 Speed of Pipeline Processing

Re-processing HRC or SBC data will not put a serious burden on most computing systems since the data sizes are relatively small, both for the science data and the reference files. Processing WFC observations, on the other hand, can severely test most computing platforms.

Great care has been taken to minimize the memory requirements of the pipeline software. Line-by-line I/O used during pipeline processing is particularly useful when more than one image is operated on at a time, for example during flatfield application or for summing images. Even so, CALACS requires up to 130MB memory to process WFC data. Unfortunately, this places an extra burden on the I/O capabilities of the computer.

Timing tests for processing ACS datasets using CALACS are given in table 3.9. Geometric correction or dither-combining using PyDrizzle will take extra time, since these are performed separately. The CPU usage column reports the amount of time the CPU was active and reflects the amount of time waiting for disk I/O. ACS observers should keep these requirements in mind when securing computing resources for data processing.

Table 3.9: Timing Tests for running CALACS alone (no drizzling)

Dataset	Run-time	CPU Usage
Ultra-10: 300Mhz CPU, 256MB memory, Ultra-SCSI HD
WFC Assoc. CR-SPLIT=2	12m 54s	71%
Single WFC	4m 34s	79%
Single HRC	0m 19s	67%

 

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