2.6	Headers, Keywords, and Relationship to Phase II

As with previous HST instruments, the FITS header keywords in COS data files store important information characterizing the observations and telemetry received during the observations, and describe the post-observation processing of your dataset. Each keyword follows FITS conventions and is no longer than eight characters. Values of keywords can be integer, real (floating-point), boolean, and character strings. Several keywords are HST and COS specific. Knowledge of the keywords and where to find them is an important first step in understanding your data. By examining your file headers, using either catfits, imhead, hselect, thselect or hedit, in STSDAS you will find detailed information about your data including:

•	Target name, coordinates, proposal ID, and other proposal level information.

•	Observation and exposure time information such as observation start and duration.

•	Instrument configuration information such as detector, grating, central wavelength setting, and aperture.

•	Readout definition parameters such as subarray parameters.

•	Exposure-specific information such as more detailed timing, world coordinate system information, and Fine Guidance Sensor identification.

•	Calibration information such as the calibration switches and reference files used by the pipeline and parameters derived from the calibration, such as image statistics and wavelength shifts.

The keywords relevant for one COS data type will not necessarily be relevant to another. Accordingly, you will find that the header in a particular file type contains a unique combination of keywords appropriate for that type of observation. Long definitions for the keywords can also be accessed from the following Web page, which provides detailed explanations of the contents and algorithm for populating the keywords. This site also provides sample headers for different COS file types:

http://stdatu.stsci.edu/keyword/.

Keywords that deal with a particular topic, such as the instrument configuration, are grouped together logically throughout the headers. Table 2.15 lists a useful subset of these groups of keywords, indicates the name of the grouping, and where applicable, shows their relationship to the corresponding information from the Phase II proposal.

Table 2.16 summarizes the possible calibration switch keywords, and indicates whether they are present for a particular observation; it also indicates the reference file keyword corresponding to the particular calibration step. A calibration switch keyword is initially populated with values of OMIT, PERFORM or N/A in the raw uncalibrated science data (with the exception of XTRACTALG, which has values of ‘BOXCAR’ or ‘TWOZONE’ to select the extraction algorithm to be used). After each calibration step is executed in the COS calibration pipeline, calcos will set the keyword switch to COMPLETE.

Table 2.15:

Selected Header Keywords and Relationship to Phase II Parameters

Header Keyword	Phase II Equivalent	Description
General File Information (Primary Header)
FILENAME		Name of the file
FILETYPE		Type of data found in the file (SCI, ACQ, SPT, ASN_TABLE)
NEXTEND		Number of extensions in the file.
Date		Date file was created
Program Information (Primary Header)
PROPOSID		4 or 5 digit program number.
PR_INV_L PR_INV_F PR_INV_m	PI Last Name PI First Name PI Middle Initial	Last name of principal investigator First name of principal investigator Middle name initial of principal investigator
LINENUM	Visit_Number, Exposure_Number	Indicates the visit and exposure number from the Phase II proposal: Visit_Number, Exposure_Number.
Target Information (Primary Header)
TARGNAME	TargetName	Name of target.
RA_TARG DEC_TARG	RA DEC	Right ascension of the target (deg) (J2000). Declination of the target (deg) (J2000).
POSTARG1 POSTARG2	POSTARG POSTARG	Postarg in axis 1 direction. Postarg in axis 2 direction.
Science Instrument Configuration (Primary Header)
OBSTYPE		Observation type (IMAGING or SPECTROSCOPIC).
OBSMODE	Opmode	Operating mode (ACCUM, TIME-TAG).
EXPTYPE	Opmode	Exposure type (EXTERNAL/SCI, WAVECAL, PHA, DARK, FLAT, ACQ/IMAGE, ACQ/SEARCH, ACQ/PEAKD, ACQ/PEAKXD, ENG DIAG, or MEM DUMP).
DETECTOR	Config	Detector in use (NUV or FUV).
SEGMENT	SEGMENT	FUV detector segment in use (FUVA, FUVB, BOTH, or N/A).
DETECTHV		FUV detector high voltage state (NomAB, NomA, NomB, Off, Low).
SUBARRAY		Data from a subarray (T) or full frame (F).
LAMPUSED