|COS Data Handbook v.4|
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 the astropy.io.fits and astropy.table modules in the AstroConda environment, you will find detailed information about your data including:
• 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: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 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.
Observation type (IMAGING or SPECTROSCOPIC). Operating mode (ACCUM, TIME-TAG). Exposure type (EXTERNAL/SCI, WAVECAL, PHA, DARK, FLAT, ACQ/IMAGE, ACQ/SEARCH, ACQ/PEAKD, ACQ/PEAKXD, ENG DIAG, or MEM DUMP). Detector in use (NUV or FUV). FUV detector segment in use (FUVA, FUVB, BOTH, or N/A). FUV detector high voltage state (NomAB, NomA, NomB, Off, Low). Data from a subarray (T) or full frame (F). Lamp status, NONE or name of lamp which is on (P1, D1, P2, or D2) Type of flashed exposures in TIME-TAG (NONE, AUTO, or uniformly-spaced). Is the target extended (Yes or No). Number of repeat exposures in dataset: Default = 1. External shutter position (OPEN or CLOSED). Corrected exposure time (seconds). For FUV exposures, the largest of EXPTIMEA and EXPTIMEB Cross-dispersion location at which calcos extracted the FUV segment A or NUV stripe A spectrum. Cross-dispersion location at which calcos extracted the FUV segment B or NUV stripe B spectrum. Cross-dispersion location at which calcos extracted the NUV stripe C spectrum. Offset from SP_NOM_A at which the spectrum was found. Offset from SP_NOM_B at which the spectrum was found. Offset from SP_NOM_C at which the spectrum was found. NUV Deadtime correction method (DATA, DEVENTS, or MEVENTS)FUVA Deadtime correction method (DATA, DEVENTS, or MEVENTS)FUVB Deadtime correction method (DATA, DEVENTS, or MEVENTS) TIME-TAG Events Orientation Keywords (in Extension header 1 or greater)2
XWLKCORR1 XTRACTALG2 XWLKCORR and DGEOCORR are lifetime dependent, and are set to OMIT as of April 2018. They will be set to PERFORM as marked in the table with the release of new reference files planned for the end of 2018.FUV data taken at LP1 and LP2 will still use BOXCAR extraction. ALGNCORR and TRCECORR will be set to OMIT.
Table 2.16: Imaging Calibration Switch Keywords
Table 2.17: Reference File Keywords