2.1 Types of ACS Files
2.1.1 Data Files and Suffixes
The file suffixes given to ACS data products are described in Table 2.1 and closely mimic the suffixes used by STIS. The initial input files to the calibration pipeline are the raw (RAW) files from Generic Conversion and the association (ASN) table, if applicable, for the complete observation set.
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 is renamed with the FLT extension after the standard calibrations (flat fielding, 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 CRJ extension once basic calibrations are complete. Single MAMA images are given the FLT suffix 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 FLT exposures and a summed flat-fielded (but not cosmic-ray cleaned) SFL image.
Table 2.1: ACS File Suffixes:
| File Suffix |
Description |
Units |
|
Raw data |
|
|
Association file for observation set |
|
|
Telemetry and engineering data |
|
|
Trailer File with processing comments |
|
|
Overscan-trimmed individual exposure (renamed to _FLT) |
|
|
Uncalibrated, CR-rejected Combined image (renamed to _CRJ) |
|
|
Calibrated, Flat fielded individual exposure |
|
|
Calibrated, CR-rejected, Combined image |
|
|
Calibrated, Repeat-Obs, Combined image |
|
|
Calibrated, Geometrically Corrected, Dither-Combined image |
|
2.1.2 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 associations will create one or more science products from the input exposures, unlike NICMOS or STIS associations. The relationships defined in the association table determine how far through the calibration pipeline the exposures are processed and when the calibrated exposures are 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. The suffix "n" is appended to the MEMTYPE when multiple sets are present within a single association.
|
|
Input CR-SPLIT exposure (single set) |
|
Input CR-SPLIT exposure for CR-combined image n (multiple sets) |
|
CR-combined output product (single set) |
|
CR-combined output product n (multiple sets) |
|
Input REPEAT-OBS exposure (single set) |
|
Input REPEAT-OBS exposure for repeated image n (multiple sets) |
|
REPEAT-OBS combined output product (single set) |
|
REPEAT-OBS combined output product n (multiple sets) |
|
|
|
Dither-combined output product |
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
|
| 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 |
2.1.3 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 the pipeline processing 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. If a trailer file already exists, CALACS will search 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.
As each image is processed, an accompanying trailer file with the '.trl' ending will be created. Further processing will concatenate all trailer files associated with the output product into a single file. Additional messages will then be appended to this concatenated file. Thus, some information is duplicated across multiple trailer files, but for any product processed within the pipeline, the trailer file is ensured to contain processing comments from each input file.
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.
