HST Data Handbook for ACS

2.2 Types of ACS Files

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2.2.1 Data Files and Suffixes

The file suffixes used for ACS observations, described in table 2.1, closely mimic the suffixes used by STIS. The input to the calibration pipeline will be the raw (_RAW) files from Generic Conversion and the association (_ASN) table for the complete observation set.

Table 2.1: ACS File Suffixes:

File Suffix	Description
_RAW	Raw data
_ASN	Association file for observation set
_BLV_TMP	Overscan-trimmed individual exposure
_FLT	Calibrated individual exposure (not produced if CR-split)
_CRJ_TMP	Uncalibrated, CR-split-combined image (renamed to _CRJ)
_CRJ	Calibrated, CR-split-combined image.
_SFL	Calibrated, Repeat-Obs-combined image.
_DRZ	Calibrated, geometrically corrected, dither-combined image

 

For CCD images, a temporary file, with the suffix _BLV_TMP, is created once bias levels are subtracted and the overscan regions are trimmed. This file will be renamed with the _FLT extension after the standard calibrations (flatfielding, dark subtraction, etc.) are complete. The _FLT files will serve as input for cosmic ray rejection (if required). For CR-SPLIT exposures, a temporary `cr-combined image' (_CRJ_TMP) is created and then renamed with the extension _CRJ once basic calibrations are complete. MAMA images are given the suffix _FLT once calibrations are complete. By definition, these images do not have an overscan region and are not affected by cosmic rays. The calibrated products of a REPEAT-OBS association will be several individually calibrated exposures (_FLT) and a calibrated, summed image (_SFL).

Since dither combining is NOT a part of the initial version of the calibration pipeline, CALACS will produce an empty _DRZ image containing only dither headers for each extension in the output but no image data. This image (initially created for testing purposes) is deleted when the user runs PyDrizzle and replaced with a dither-combined product.

2.2.2 Association Tables

Association tables are useful for keeping track of the complex set of relationships that can exist between exposures taken with ACS, especially with REPEAT-OBS, CR-SPLIT, and dithered exposures. Images taken at a given dither position may be additionally CR-SPLIT into multiple exposures. In these cases, associations are built to describe how each exposure relates to the desired final product. As a result, ACS association tables can create one or more science products from the input exposures, unlike NICMOS or STIS associations. The relationships defined in the association tables determine how far through the calibration pipeline the exposures are processed and when the calibrated exposures get combined into sub-products for further calibration.

ACS data files are given the following definitions:

• An exposure is a single image, the atomic unit of HST data.
• A dataset is a collection of files having a common root name (first 9 characters).
• A sub-product is a dataset created by combining a subset of the exposures in an association.
• A product is a dataset created by combining sub-products of an association.

ACS association tables were designed to closely resemble the NICMOS association format, with three primary columns: MEMNAME, MEMTYPE, and MEMPRSNT. The column MEMNAME gives the name of each exposure making up the association and output product name(s). The column MEMTYPE specifies the role the file has in the association. A unique set of MEMTYPES specific to ACS were adopted to provide the support for multiple products. These types are summarized in table 2.2.

Table 2.2: Exposure types in ACS associations

MEMTYPE	Description
EXP-CRn	Input CR-SPLIT exposure for CR-combined image n
PROD-CRn	CR-combined exposure n
PROD-CRJ	CR-combined output product
EXP-RPn	Input REPEAT-OBS exposure n
PROD-RPT	REPEAT-OBS combined output product
PROD-DTH	Dither-combined output product (Contains a null array)

 

A sample association table for a two-position dithered observation with CR-SPLIT=2 is presented in table 2.3. This example shows how both MEMNAME and MEMTYPE are used to associate input and output products. The MEMTYPE for each component of the first CR-SPLIT exposure, JxxxxxECM and JxxxxxEGM, are given the type EXP-CR1. The sub-product Jxxxxx011 is designated in the table with a MEMTYPE of PROD-CR1. The last digit of the product filename corresponds to the output product number in the MEMTYPE. A designation of zero for the last digit in the filename is reserved for the dither-combined product.

The column MEMPRSNT indicates whether a given file already exists. For example, if cosmic ray rejection has not yet been performed by CALACS, the PROD-CRn files will have a MEMPRSNT value of "no". The sample association table in table 2.3 shows the values of MEMPRSNT prior to CALACS processing.

Table 2.3: Sample Association Table Jxxxxx010_ASN

MEMNAME	MEMTYPE	MEMPRSNT
JxxxxxECM	EXP-CR1	yes
JxxxxxEGM	EXP-CR1	yes
Jxxxxx011	PROD-CR1	no
JxxxxxEMM	EXP-CR2	yes
JxxxxxEOM	EXP-CR2	yes
Jxxxxx012	PROD-CR2	no
Jxxxxx010	PROD-DTH	no

 

2.2.3 Trailer Files

Each task in the CALACS package creates messages during processing which describe the progress of the calibration and which are sent to STDOUT. In calibration pipelines written for other HST instruments, trailer files were created by simply redirecting the STDOUT to a file. Because multiple output files can be processed in a single run of CALACS, creating trailer files presents a unique challenge. Each task within the CALACS package must decide which trailer file should be appended with comments and automatically open, populate, and close each trailer file.

CALACS will always overwrite information in trailer files from previous runs of CALACS while preserving any comments generated by Generic Conversion. This ensures that the trailer files accurately reflect the most recent processing performed. The string CALACSBEG will mark the first comment added to the trailer file when running CALACS or any task within the CALACS package. If a trailer file already exists, CALACS will search the trailer file for this string to determine where to append processing comments. If it is not found, the string will be written to the end of the file and all comments will follow. Thus any comments from previous processing are overwritten and only the most current calibrations are recorded.

CALACS begins by interpreting the input data as either an association table or an individual exposure. For a CCD association, all input files are initially processed by the same CALACS task. The output files are given the name `_blv_tmp.fits', each with a corresponding `.trl' trailer file. Similarly, associated MAMA data are written to `_flt.fits' files, each with an accompanying `.trl' trailer files. Further image processing with CALACS will concatenate all input trailer files associated with the output product into a single file. Messages from further processing would then be appended to the end of the concatenated file. This duplicates some information across multiple trailer files but ensures that for any product processed within the pipeline, the trailer file will contain processing comments from all the input files. In addition, linking trailer files together can result in multiple occurrences of the CALACSBEG string. Only the first, however, determines where CALACS will begin overwriting comments if an observation is reprocessed.

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