hrs -- Calibration of Goddard High Resolution Spectrograph (GHRS) observations.
All observations made by the Hubble Space Telescope (HST) undergo what is known as Routine Science Data Processing (RSDP). RSDP essentially consists of two steps implemented by the Post Observation Data Processing System (PODPS). The first step is called "generic conversion", which takes the telemetry from the HST, extracts the relevant science and engineering data, and creates the GEIS (or group-format) files containing the uncalibrated science data. The second step is to run the calibration task appropriate for the instrument involved, to produce the calibrated science data GEIS files.
This package contains the calibration task, calhrs, used by PODPS to calibrate GHRS data. The remaining tasks in this package are used to perform post-calibration processing and to help plan GHRS observations. These tasks are provided to the General Observer (GO) to help facilitate understanding on how the data were calibrated, allow the observer to recalibrate if so desired, and to help prepare the data for use by other spectral reduction tasks provided by the IRAF system, or provided by other software products.
A thorough discussion of observation planning is provided to the GO's in the "Phase II Proposal Instructions" and the "GHRS Instrument Handbook". STSDAS does not provide any software to help directly in preparing Phase II proposals, except for one area, exposure time calculation. The "synphot" package provides GO's with the tools necessary to calculate exposure time and expected count rates. In addition to "synphot", the "hrs" package provides one task, tacount, to help predict expected count rates for GHRS target acquisitions. Please refer to the latest edition of the "GHRS Instrument Handbook" for more information.
When a GHRS observation is processed by RSDP, the result is a number of files. These files are written in GEIS, or group-format, files. The files are named using the following convention:
The individual fields of the name are as follows:
f - FGS k - COSTAR u - WFPC II v - HSP w - WFPC x - FOC y - FOS z - GHRS <-- This is GHRS data.
Each GEIS data file consists of two disk files. An ASCII "header", denoted by the "h" in the extension, contains various parameters of the data, including size and dimensionality. The second file is the "data", denoted by the "d" in the extension, which contains the actual data. In the remaining discussion, data files will be referred by the "header" file only; the "data" file will be implied. For example, when referring to the file z0x20101t.c1h, implicitly the reference also includes the file z0x20101t.c1d. Or, the GEIS data file z0x20101t.c1h is composed of two disk files, z0x20101t.c1h and z0x20101t.c1d.
The main feature of GEIS data is that a single GEIS file can contain multiple groups of data. For example, the file z0x20101t.c1h may contain four groups, each group containing a different spectrum. Each group is identified by a number. In IRAF, the syntax for identifying a group is by appending "[X]" where X is the group number. So, to refer to the fourth group of the file z0x20101t.c1h, the syntax would be z0x20101t.c1h.
For more information about the STSDAS GEIS format, and IRAF, please refer to the "STSDAS Users Guide".
As stated previously, a single GHRS observation actually consists of a number of GEIS files. Referring to the naming convention presented above, each file is identified by the two letter code, XX, in the extension. For GHRS, the files and what they contain are as follows:
0 - Good data 1 - Failed Reed-Solomon error checking 16 - Data contains a fill value.
Besides the above GEIS files, a simple ascii file whose extension is TRL is present. This file is the trailer file and contains a textual log of the RSDP processing of the observation.
For more information about the GHRS data files, refer to the "HST Data Handbook", "STSDAS Users Guide", and the help for calhrs.
All calibration performed on GHRS data by RSDP occurs within the one task, calhrs. The input to calhrs is the raw data files and reference data files and tables. The output is the calibrated science data. A brief description of the calibration is provided below. For detailed descriptions of the calibration performed by calhrs, refer to the help for calhrs and the "HST Data Handbook".
The following steps comprise the calibration process:
1) Removal of diode non-linearities and non-uniformities. 2) Removal of photocathode granularities. 3) Removal of vignetting and low frequency variations across the photocathode. 4) Wavelength assignments. 5) Background subtraction. 6) Echelle ripple correction. 7) Conversion to absolute flux units.
The calibration uses tables and files whose names are contained in keywords in the science data header. The references files/tables are created by the Instrument Scientists for each instrument using various facilities. These tables are archived, just as the observational data is archived. Reference data often changes, either because the instrument is changing with time, or a better understanding of the calibration is developed. Hence, the reference data is "timed-tagged" such that a "best" set of reference files exist for a particular observation. It is not necessarily true that the best references exist at the time of observation, hence necessitating the need to recalibrate.
For GO's, who are located at STScI and have access to the Calibration Data Base System (CDBS), the task getref is available to determine whether the set of reference files used for an observation are the "best" or not. For offsite users, StarView provides a facility similar to getref for making the same determination. For more information, see the help for getref or the "HST Archive Manual". For information concerning the reference files themselves, see the help for calhrs, the "HST Data Handbook", and ICD-47 (see REFERENCES section below).
The following briefly discusses the relationship between the tasks provided by STSDAS/IRAF and GHRS data. As always, for more information about usage of individual tasks, see the help for those tasks.
As stated earlier, STSDAS provides minimal software for preparing Phase II proposal instructions. For determination of count rates and exposure times, tasks in the "synphot" package would be used. "Synphot" uses throughput tables representing the optical path of the HST through to the diodes of GHRS. For target acquisitions, once an expected count rate has been determined, the task tacount in the "hrs" package can be used to convert this expected count rate to what the GHRS will actually see in target acquisition mode. tacount takes into account the current Point Spread Function (PSF) that the acquisition diodes see and the paired-pulse effect of the diode electronics.
For calibration, the calhrs task is predominant, as already discussed. To confirm that the GO has the most update-to-date reference data, getref and upreffile can be used to determine new reference files and update the header keyword parameters in the raw science data headers. To change the calibration switches and reference file keywords manually, the chcalpar task would be used. If the GO has taken wavelength calibration observations along with the science observations, the zwavecal task can be used to produce a new wavelength reference file and determine a zero-point offset to apply to already-calibrated wavelength solutions.
For post-calibration analysis, the following tasks would be useful. For GHRS observations taken in FP-SPLIT mode, the tasks poffsets and specalign would be used. These tasks determine the offset between each split observation and then recombine the individual split observations into a single spectrum. The wavelength solutions are preserved while the spectra are combined.
There are a couple of tasks to handle the fact that the calibrated science data and wavelength solutions reside in different files. The task fwplot can be used to plot wavelength vs. flux. The resample task resamples the data to a linear wavelength dispersion, placing the wavelength solution in the header as a World Coordinate System transformation, allowing tasks to access the wavelength information without the wavelength file.
If the GO does not wish to resample the data, the task mkmultispec allows various functions to be fit to the wavelength solution and place them as header keyword parameters in the science data, again allowing other tasks access to the wavelength information.
The tasks showspiral and spiralmap allows observers to examine a target acquisition image using the IRAF image display.
The reflux performs the flux calibration step of calhrs, but with a new set of sensitivity reference data.
The obsum task allows a quick overview of an observation, summarizing what observations occurred when, summed counts, completion status, etc. The main usage for GO's is the ability to flag observations that may be affected by the Doppler compensation problem. Any data observed before the 1 March, 1993, may be affected. Please refer to the "GHRS Instrument Handbook" for more information.
For more general spectral analysis tasks, the GO is referred to the noao.onedspec package, the stsdas.fitting package, and the stsdas.toolbox.imgtools. Tasks in these package provide many things ranging from basic arithmetic (imcalc) to line profile fitting (nfit1d) and general interactive spectral analysis (splot).
For many issues regarding STSDAS, file formats, etc., please refer to the "STSDAS User's Guide", available from the STSDAS group at STScI.
For calibration related questions and a general discussion of HST data, refer to the "HST Data Handbook".
For specific questions with regard to GHRS, other references are the "GHRS Data Products Guide" and the "GHRS Instrument Handbook", produced by the Science Software Division (SSD) of STScI.
Help for "synphot" is available online in the "synphot" package. Also, the "Synphot User's Guide" may be obtained from the STSDAS group of STScI.
The algorithms used by CALHRS are described in "SOGS Requirements Document", (SE-06-01).
The document describing the contents and form of the reference data is "Post Observation Data Processing System to Calibration Database System Interface Control Document", (ICD-47).
The document describing the keywords is "Post Observation Data Processing System to Space Telescope Science Data Analysis Software Interface Control Document", (ICD-19).
The document describing the GHRS is "SI Systems Description and User's Handbook for the High Resolution Spectrograph (HRS) for the Hubble Space Telescope (HST)", Ball Aerospace, SE-01.