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NICMOS Data Handbook > Chapter 2: Data Structures > 2.2 Header Keywords

2.2 Header Keywords
Both the primary header and the headers of each image extension in a science data file contain keywords. The keywords store a wide range of information about the observations themselves (e.g., observing mode, integration time, filters or grisms used), the processing of the data by the OPUS pipeline (e.g., calibration steps performed and reference files used), and the properties of the data themselves (e.g., number of image extensions, dimensions and data type of each image, coordinate system information, flux units, and image statistics).
The primary header carries global keywords which are applicable to all extensions. The extension headers carry extension-specific keywords, which contain information relevant to the image in a particular extension. For example, observation parameters, calibration switches and reference file names are contained in the primary header. Exposure time and World Coordinate System information, on the other hand, are contained in the header of each image extension because this information could vary from one set of extensions to another.
Table 2.4 below lists most of the keywords in the primary header of the science data files. The entries in this table are appropriate for data retrieved in 2009 from the HST Archive via on-the-fly reprocessing (OTFR - see Chapter 3). Some header keywords in older NICMOS data may be different. In particular, the observing pattern keywords have been changed for Cycle 11, and the dictionary below describes the new keyword format. The post-SAA dark keywords have been in use since Cycle 11. For data taken in Cycle 7 and 7N, these keywords are not populated. Since April 2007, the SAAclean task has been incorporated into the OTFR to clean SAA persistence from science observations. The related keywords can be found in the SAACLEAN and RUNCALSAA sections of Table 2.4. Keywords associated with the bright earth persistence and the bias-derived temperature were added to the OTFR in march 2008 in OPUS version 2008.1. The STScI Data Processing Team maintains a complete HST FITS header keyword dictionary with descriptions of the keywords used by NICMOS and other HST instruments.
 
Static data quality file pedigree (values: GROUND dd/mm/yyyy - for reference files originated from Thermal Vacuum data; INFLIGHT dd/mm/yyyy - for reference files originated from on-orbit calibration observations)
Identify cosmic ray hits (update of DQ arrays in *_ima.fits output of calnica for MULTIACCUM)
Identify cosmic ray hits (update of DQ arrays in *_ima.fits output of calnica for MULTIACCUM)
Saaclean reference table (with task params)
Pedigree of saaclean reference table
Role of the dataset in the association (e.g., target, first background, second background, etc.; allowed values: EXP_TARG, EXP_BCKn, PROD_TARG, PROD_BCKn, where EXP=input exposure, PROD=output product, TARG=target, BCK=background, n=1-8)

1
The SAACRMAP keyword is no longer being used by the OTFR pipeline.

 
In the SCI image extensions, additional keywords describing the data quality are present. They give the number of pixels which have a flag different from zero in the DQ extension, and a suite of statistical information (mean, standard deviation, minimum and maximum of good pixels in the entire detector and per quadrant) on the image. Table 2.5 lists some of the relevant keywords that are specific to image extensions; they appear in the extension headers, but not in the primary header.
Table 2.5: Image Extension Header Keywords in Science Data Files
When ALL pixels in an image extension have the same value (e.g., the SAMP and TIME arrays in the *_ima.fits file from a MULTIACCUM exposure or the ERR, DQ, SAMP and TIME arrays in the *_raw.fits files from a MULTIACCUM, ACCUM or BRIGHTOBJ exposure) the pixel array of that extension is not generated, and the PIXVALUE keyword is instead populated with the common value of the pixels to save space.
Keywords in the support file, *_spt.fits, report information from the ephemeris and engineering data on the status of the telescope and of the instrument during the observations. Information common to all readouts of a MULTIACCUM sequence are stored in the primary header of the support file, while information which may change from readout to readout (e.g., detector temperature) is stored in the headers for each corresponding extension. Table 2.6 describes some of the relevant primary header keywords from the *_spt.fits file, and Table 2.7 gives some useful keywords from the extension headers.
Table 2.6: NICMOS Primary Header Keywords in the Support Files
Table 2.7: Some Useful NICMOS Extension Header Keywords in the Support Files
Table 2.8: NICMOS and NCS Telemetry Mnemonics
a. Operational limits are a range of values over which the sensor will operate normally.

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