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ACS Data Handbook V7.2
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ACS Data Handbook > Chapter 2: ACS Data Structure > 2.4 Headers and Keywords

2.4
ACS image headers for WFC, HRC, HRC-ACQ, and SBC observations have very similar formats. However, MAMA (SBC) headers require a slightly modified structure from the CCD format, and HRC-ACQ headers require several unique keywords.
ACS header keywords can be viewed in several ways. The STSDAS tasks hedit or hselect tasks can be used to query the images. The examples below show how these tasks are used to get the values for group (or extension) keywords CCDCHIP, CRVAL1, and CRVAL2. The contents of the entire primary or group header can be viewed on-screen, or by directing the output to a text file, using the task imheader.
ACS image header keywords are described in Tables 2.6 and 2.7. When displaying the header using imheader, note that the keywords are grouped together by type, for example, “Target Information,” “Science Instrument Configuration,” “Calibration Switches,” and “Calibration Reference files.”
Keywords in the image’s primary header (FITS extension 0) contain parameter values that are common to the entire image dataset, and are given in Table 2.6.
Extension or group header keywords (FITS extensions > 0), as shown in Table 2.7, contain parameter values that are specific to the image group (however, many primary header keyword values are also accessible when querying an image group header).
Some keywords are no longer in use, but are included for users who may have older data. The text for these keywords are colored in grey.
Most of the keywords described in Tables 2.6 and 2.7 are “user” keywords, i.e., keywords with values directly recorded in the image headers. A few keywords in the tables with the prefix “i_” cannot be accessed by tasks like eheader and imheader, but when queried by hedit and hselect, the values of these keywords are determined on-the-fly by PyRAF.
TARGET INFORMATION (All Detectors)
PROPOSAL INFORMATION (All Detectors)
EXPOSURE INFORMATION (All Detectors)
Number of gyros scheduled, T=3+OBAD. (Number of gyros available for use.)
Exposure duration (seconds)--calculated. (Exposure time as executed.)
POINTING INFORMATION (All Detectors)
DIAGNOSTIC KEYWORDS (All Detectors)
calacs code version.
SCIENCE INSTRUMENT CONFIGURATION (All Detectors, Except Where Noted)
MAMA OFFSETS (SBC Only)
Axis 1 MAMA offset (low-res pixels.).
Axis 2 MAMA offset (low-res pixels.).
CALIBRATION SWITCHES: PERFORM, OMIT (All Detectors, Except Where Noted)
Calculate statistics? (Not used. Statistics are always calculated in calacs.)
Write out error array extension. (Not used. Always true for calacs.)
Correct for A to D conversion errors (CCDs only. Not used by calacs.)
Apply shutter shading correction. (CCDs only. Always set to ‘OMIT for ACS.)
CTE correction. (Always set to “OMIT” for non-WFC exposures.)
CALIBRATION REFERENCE FILES (All Detectors, Except Where Noted)
Analog-to-digital correction file. (CCDs only. Not used by calacs.)
Shutter shading correction file. (CCDs only. Not used for ACS.)
CTE correction table. (Always “N/A/” for non-WFC exposures.)
De-trailed dark reference file. (Always N/A for non-WFC exposures.)
Photometric throughput table (not used by calacs).
HST graph table (not used by calacs).
HST components table (not used by calacs).
Distortion correction image (not used by AstroDrizzle for ACS).
AstroDrizzle parameter table.
Sky value computed by MultiDrizzle (Primary header keyword, SBC only).
PATTERN KEYWORDS (All Detectors)
POST-FLASH PARAMETERS (WFC, HRC, HRC ACQ)
ENGINEERING PARAMETERS (WFC, HRC, HRC ACQ)
Commanded gain of CCD (electrons/DN).
ASSOCIATION KEYWORDS (All Detectors)
 
ASSOCIATION KEYWORDS (All Detectors)
Number of data axes. (For ACS images, value is set to 2.)
Brightness units (counts, electrons, electrons/sec).
Offset in X to subsection start. (x coordinate of the first science pixel in the SCI array.)
Offset in Y to subsection start. (y coordinate of the first science pixel in the SCI array.)
Reciprocal of sampling rate in X. (The reference pixel size in units of the current pixel size.)
Reciprocal of sampling rate in Y. (The reference pixel size in units of the current pixel size)
PHOTOMETRY KEYWORDS (All Detectors)
Number of segments containing fill. (Number of segments for which data was lost in transmission and filled with a fill pattern prior to arriving at STScI.)
PODPS fill present (T/F). (Indicates if there are missing packets in the data stream; in such cases, the OPUS pipeline inserts fill packets so that the image can be processed by analysis software.)
Science telemetry fill data present? (T=1/F=0). (Indicates if errors were found in the science dataset at the Space Telescope Data Capture Facility; if there are missing packets or failed segments, fill data is inserted as placeholders.)
MAMA jMDECODT pwb temp from eng snap 1 (deg. C) (Temperature at the start of observation.)
MAMA JMDECODT pwb temp from eng snap 2 (deg. C) (Temperature at the end of observation.)
REPEATED EXPOSURES INFO (WFC, HRC, HRC ACQ)
IMAGE STATISTICS AND DATA QUALITY FLAGS (All Detectors, Except Where Noted)
Serious data quality flags. (Bad pixels that are not included in calculations for header keyword image statistics, e.g., GOODMEAN, NGOODPIX.)
OCRVAL11
Axis to which the DET2IM correction is applied (1=X, 2=Y). (Which detector column or row requires width corrections.)
Reference file used by updatewcs to create a D2IMARR extension. (The D2IMARR image extension holds the column or row width corrections.)
1
Values before geometric distortion corrections.


ACS Data Handbook > Chapter 2: ACS Data Structure > 2.4 Headers and Keywords

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