NAME

calhrs -- Calibrate Goddard High Resolution Spectrograph (GHRS) data.

USAGE

calhrs rootname output

DESCRIPTION

This task performs the routine calibration of GHRS data. This task is different in philosophy from the usual IRAF task in that input is not primarily from the parameters of the task, but a set of files and keywords in those files. A discussion of how to control calhrs through the calibration header keywords is discussed. Then a detailed description of the calibration process is presented.

INPUT

The input for calhrs based on the value of the task parameter "rootname". This parameter specifies the rootname of the files that contain the observational data to be calibrated. The observation files, as produced by the OSS PODPS Unified System (OPUS), will have a rootname of the form

	IPPPSSOOT

Based on the rootname, calhrs will open a number of files, each with a different file name extension. The necessary files are as follows:

	.SHH	- Standard Header Packet (SHP) file
	.ULH	- Unique Data Log (UDL) file
	.D0H	- Science Data file
	.Q0H	- Science Data Quality file
	.X0H	- Special Diode/Engineering Trailer file
	.XQH	- Special Diode Quality file

All these files constitute the raw data of a single observation and are required by calhrs.

CONTROL OF CALIBRATION

The calibration steps carried out by calhrs are determined by the values of certain keywords in the science data header file. This file is identified by the extension .d0h. The science data header file must be edited (with the tasks, chcalpar, hedit, or eheader, for example) to change the values of keywords, thus selecting the desired calibration steps. These keywords may contain either of two values: "OMIT" or "PERFORM".

Some processing steps require the use of reference tables and files. Keywords that contain names of reference files and tables must be edited to refer to the desired reference files (the name must include the directory path).

The following is a list of the calibration keywords and what step of the calibration the keyword controls. Any reference files or tables used by that calibration step are listed. The calibration occurs in the order listed.

DQI_CORR

Apply data quality initialization using the reference file specified by the keyword "DQIHFILE".

EXP_CORR

Divide by the exposure time to convert to count rates.

DIO_CORR

Divide the count value by the diode's response to correct for diode nonuniformity. The diode response file is specified by the keyword "DIOHFILE".

PPC_CORR

Correct the raw count rates for saturation in the detector electronics. The paired-pulse correction table is specified by the keyword "CCG2".

MAP_CORR

Perform the photocathode mapping function. The photocathode line mapping table is specified by the keyword "CCR1" and the sample mapping table is specified by the keyword "CCR2".

DOP_CORR

Correct for on-board Doppler compensation when removing photocathode nonuniformities and vignetting.

PHC_CORR

Remove photocathode nonuniformity by dividing the count value by the photocathode response contained in the file specified by the keyword "PHCHFILE".

VIG_CORR

Remove vignetting and wavelength-dependent photocathode variations by dividing each count value by the vignetting response contained in the file specified by the keyword "VIGHFILE".

MER_CORR

Merge the substep bins.

ADC_CORR

Convert the sample positions on the photocathode to wavelengths by applying the dispersion constants. Requires tables "CCR5", "CCR6", and "CCR7" containing spectral order, dispersion, and thermal constants.

GWC_CORR

Convert the sample positions on the photocathode to wavelengths by applying the global wavelength coefficients (GWC). This is an alternative, and better defined, method of calculating wavelengths for an observation. This step requires tables "CCR5", "CCR7", and "CCRC".

MDF_CORR

Use a median filter on the background. The filter width is contained in table specified by the keyword "CCR3".

MNF_CORR

Use a mean filter on the background. The filter width is contained in table specified by the keyword "CCR3".

PLY_CORR

Use a low-order polynomial filter on the background. The order of polynomial is contained in table specified by the keyword "CCR3".

BCK_CORR

Subtract the background counts from the raw counts of the diode array. Correct for the presence of scattered light using the coefficients found in table specified by the keyword "CCRB".

BMD_CORR

Instead of using acquired background data, use the background count rate model to determine a background spectrum. The background count rate model is stored in a table and is specified by the keyword "CCRE". This option also uses the SAA Model Contour 7 to issue warning messages. The SAA Contour is contained in the file specified by the keyword "SAAHFILE".

IAC_CORR

Adjust the zero-point of the wavelength scale for the large science aperture and the two spectral lamp apertures. The incident angle coefficients are found in the table specified by the keyword "CCR8".

ECH_CORR

If one of the Echelle gratings is used, divide the flux value by the normalized grating efficiency to remove the Echelle ripple. The tables specified in the keywords "CCR9" and "CCRA" contain Echelle ripple constants.

FLX_CORR

Calculate the absolute flux by dividing the flux by the absolute flux coefficients. Requires interpolation in the absolute flux file, specified by the keyword "ABSHFILE", and corresponding wavelength file, specified by the keyword "NETHFILE".

HEL_CORR

Convert wavelengths to the heliocentric coordinate system.

VAC_CORR

Apply vacuum-to-air correction to the wavelengths.

The calibration keywords are reset from "PERFORM" to "COMPLETE" upon successful completion.

PARAMETERS

rootname [string]

The rootname of the input GHRS observation data set.

output [string]

The rootname of the output GHRS data set. If not specified, the output file name will be created using the value of "rootname".

EXAMPLES

1. Calibrate the observation z00xh903r. Output files are to have the same root name as the input files.

  hr> calhrs z00xh903r ""

2. Calibrate the observation z00xh104r found in the directory nite1$ and produce output files with a root name of test001 and put them in the test subdirectory.

  hr> calhrs nite1$z00xh902r test/test001

BUGS

REFERENCES

The following references are available from STScI and describe various aspects of the calibration process, simple cookbook, and general usage of STSDAS:

	"HST Data Handbook"
	"STSDAS Users Guide"
	"GHRS Data Products Guide"
	"GHRS Instrument Handbook"
	"Phase II Proposal Instructions"

The following are technical references meant for internal usage and are not written as "end-user" products. However, these documents can be retrieved if a detailed understanding of the instrument is required.

The document describing the CALHRS algorithms is "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 Goddard High Resolution Spectrograph (GHRS) for the Hubble Space Telescope (HST)", Ball Aerospace, SE-01.

HELP

For assistance using this or any other tasks, please contact help@stsci.edu or call the help desk at 410-338-1082.

SEE ALSO

chcalpar, hedit, eheader

Type "help hrs option=sysdoc" for a higher-level description of the hrs package and the calibration process.