Calibration Product Review for the GHRS

Now available as a PostScript [200 Kb] file and Adobe PDF [76 Kb] file.

• 1. Introduction
• 2. Calibration Process
• Figure 1: Calibration Flowchart
• Figure 2: Calibration Flowchart (Continued)
• Figure 3: Example Science Data Header File Showing Calibration Reference Files and Switches
• 3. Summary of Reference Files
• 4. File Descriptions
• 4.1 Selecting the "Right" Images and Tables
• 5. Individual Reference Files
• 5.1 Diode Response Files (DIOHFILE).
• 5.2 Photocathode Response Files (PHCHFILE).
• 5.3 "Vignetting" (VIGHFILE).
• 5.4 Absolute Sensitivity Files (ABSHFILE, NETHFILE).
• 5.5 Data Quality Initialization (DQIHFILE).
• 5.6 SAA Contour Mask Header (SAAHFILE)
• 5.7 Line and Sample Mapping Parameters (CCR1, CCR2).
• 5.8 Detector Parameters (CCR3).
• 5.9 Wavelength Ranges (CCR4).
• 5.10 Spectrum Constants (CCR5).
• 5.11 Dispersion Constants (CCR6).
• 5.12 Thermal Constants (CCR7).
• 5.13 Incident Angle Correction, or Aperture Offsets (CCR8).
• 5.14 Echelle Blaze Function Interpolation and Non-Interpolation (CCR9 and CCRA).
• 5.15 Scale Factors for Scattered Light Subtraction (CCRB).
• 5.16 Global Wavelength Coefficients (CCRC).
• 5.17 Photocathode Blemish Locations (CCRD).
• 5.18 Background Count Rate Model (CCRE).
• 5.19 Paired-Pulse Corrections (CCG2).
• 6.Tables stored in CDBS but not used by CALHRS
• 6.1 Platinum-Neon Lamp Line List (*.czy).
• 6.2 Small Science Aperture Wavelength Offsets (*.czz).

(GHRS Instrument Science Report 067, Official Update)

This Report summarizes the status of calibrations and calibration reference files for the Goddard High Resolution Spectrograph. Each calibration file that is stored in the Calibration Database System (CDBS) is briefly described as well.

1. Introduction

The pipeline data reduction program, calhrs, determines how to calibrate a particular dataset by checking "switches" that are set to PERFORM or OMIT and by accessing various calibration reference files. These reference files were summarized in GHRS-ISR-067 Calibration Product Review for the GHRS in Early Cycle 4, by Hulbert, et al., but due to changes in calibration, this document needs to be continually updated. Up until now, we have been providing updates via the GHRS Calibration Resource pages on the World Wide Web. This is the latest version of that document, dated 06 Jun 1997, and it will be available on only on the WWW, initially.

This document contains a listing of all GHRS Reference Images and Tables that are presently available in the Calibration Data Base System (CDBS). The tables herein are intended to provide a history of the differences among the files and tables which were used by the pipeline to calibrate GHRS data. This Report provides a description of the files and their usage as of 06 June 1997, for the convenience of Archival HST users. A brief review of the calibration process and control switches is included here. Additional information on the usage of calhrs and the reference files may be found in HST Data Handbook, issued in December, 1995 and later.

2. Calibration Process

Figure 1 shows a flowchart of the calibration process followed by calhrs. In this diagram, the rectangles on the right-hand side contain the name of a calibration switch that "controls" a given calibration step. The performed calibration step is briefly described in the central rectangular box. The rounded boxes on the left contain the names of calibration reference files needed for a particular calibration step to be completed. Figure 2 is an example science data header, listing the header keywords that act as calibration "switches" as well as the list of calibration reference files used.

Figure 1: Calibration Flowchart

Figure 2: Calibration Flowchart (Continued) t

Figure 3: Example Science Data Header File S howing Calibration Reference Files and Switches

                  / CALIBRATION REFERENCE FILES
DIOHFILE = 'zref$cce1504az.r0h' / diode response header file
PHCHFILE = 'zref$bcc11275z.r1h' / photocathode response header file
VIGHFILE = 'zref$e751116dz.r2h' / vignetting response header file
ABSHFILE = 'zref$e5v0936rz.r3h' / absolute flux header file
NETHFILE = 'zref$e5v0936az.r4h' / absolute flux wavelength net header file
DQIHFILE = 'zref$cce1505kz.r5h' / data quality initialization header file
SAAHFILE = 'zref$h5m1337dz.r6h' / SAA Contour Mask header file
CCR1     = 'ztab$aau13518z.cz1' / photocathode line mapping parameters table
CCR2     = 'ztab$ba412190z.cz2' / photocathode sample parameters table
CCR3     = 'ztab$c7f1130oz.cz3' / detector parameters table
CCR4     = 'ztab$a3d1045dz.cz4' / wavelength ranges table
CCR5     = 'ztab$e3u1417oz.cz5' / spectral order constants table
CCR6     = 'ztab$e3t1251bz.cz6' / dispersion constants table
CCR7     = 'ztab$e3t1250tz.cz7' / thermal constants table
CCR8     = 'ztab$e3t1250lz.cz8' / incidence angle coefficients table
CCR9     = 'ztab$e751042hz.cz9' / echelle interpolation constants table
CCRA     = 'ztab$e751041qz.cza' / echelle non-interpolation constants table
CCRB     = 'ztab$d8b1457az.czb' / scattered light correction factors
CCRC     = 'ztab$e3u14301z.czc' / global wavelength coefficients table
CCRD     = 'ztab$e3t1028nz.czd' / photocathode blemish table
CCRE     = 'ztab$h5m13378z.cze' / background count rate model
CCG2     = 'mtab$a3d1145ly.cmg' / paired-pulse correction table

                  / CALIBRATION SWITCHES
DQI_CORR = 'PERFORM           ' / data quality initialization
EXP_CORR = 'PERFORM           ' / division by exposure time
DIO_CORR = 'PERFORM           ' / diode response correction
PPC_CORR = 'PERFORM           ' / paired-pulse correction
MAP_CORR = 'PERFORM           ' / mapping function
DOP_CORR = 'OMIT              ' / doppler compensation
PHC_CORR = 'PERFORM           ' / removal of photocathode nonuniformity
VIG_CORR = 'PERFORM           ' / removal of vignetting nonuniformity
MER_CORR = 'PERFORM           ' / merging of substep bins
GWC_CORR = 'PERFORM           ' / use global wavelength coefficients
ADC_CORR = 'PERFORM           ' / application of dispersion constants
MDF_CORR = 'OMIT              ' / median filter of background spectra
MNF_CORR = 'OMIT              ' / mean filter of background spectra
PLY_CORR = 'PERFORM           ' / polynomial smoothing of background spectra
BMD_CORR = 'OMIT              ' / background model INSTEAD of bkgd spectra
BCK_CORR = 'PERFORM           ' / background removal
IAC_CORR = 'PERFORM           ' / incidence angle correction
ECH_CORR = 'PERFORM           ' / correction for echelle ripple
FLX_CORR = 'PERFORM           ' / absolute flux calibration
HEL_CORR = 'PERFORM           ' / conversion to heliocentric wavelengths
VAC_CORR = 'OMIT              ' / vacuum to air correction

3. Summary of Reference Files

Table 1 lists the 7 types of GHRS reference images and 15 types of GHRS reference tables contained in CDBS as well as 2 reference tables used for post-pipeline processing. Table 1 is organized by file format, the filename extension used to identify a given type of reference image or table, the name of the header keyword in the science data header that contains the actual name of the reference image or table used by the calibration software to calibrate the data, a short description of the reference image or table, and, where appropriate, the selection criterion used to differentiate among reference files of the same type.

4. File Descriptions

The summaries found in Tables 2 through 24 are grouped by the DESCRIPTION column in Table 1. Each existing reference image has listed the detector or grating for which this file is appropriate-reference tables are not differentiated in this manner since they contain data appropriate for all gratings or detectors. The date of its installation into CDBS, the USE_AFTER date, the source of the data (PEDIGREE), and a short description of why the file was updated is also given. The USE_AFTER date is an important feature of CDBS. This keyword allows for the proper time-tagging of reference files and tables. A given file or table with a particular USE_AFTER date should only be used to process data obtained on or after the USE_AFTER date-hence, the name. The PEDIGREE of a reference file or table indicates the source of the data. The permitted values for this parameter are:

GROUND means that pre-launch measurements were used to generate the reference data;

INFLIGHT means that measurements were collected with GHRS in orbit; and, MODEL means that the data are based on a model instead of measurements; DUMMY means that there is no reference data, instead the file or table contains all 1's or 0's and has no effect on calibration.

For internal tracking purposes we are now including from the CDBS-PODPS tracking forms, the initials of those persons who (submitted/verified/approved) the data. The slashes separate the different steps in the process of getting the data into the CDBS; information which is unavailable will be left blank. A list of full names corresponding to the initials used is available below:

4.1 Selecting the "Right" Images and Tables

Over the past several years a number of changes have been made to both the reference images and tables and the calibration software that employs them. In general, software changes are backwards-compatible with earlier versions of reference tables. However, this has not always been the case. Consequently, the current version of the software will not properly run with "old" data and/or "old" reference files. This is most likely to be a problem if one is using data from before 11 Nov 91 in its original form. The simplest work-around to any problem of this sort is to obtain officially processed data and the latest (appropriate) reference files and tables from the HST data archive. There are two methods by which one can identify the "best" reference files and tables to use when recalibrating GHRS data. 1) Use this document and compare it to your headers by hand. 2) Use the HST Data archive -the GHRS calibration file form in StarView provides a simple interface for identifying the "best" files.

5. Individual Reference Files

5.1 Diode Response Files (DIOHFILE).

The diodes within the Digicons of the GHRS do not have identical sensitivities. The diode response correction (controlled by DIO_CORR) divides the input data by diode response values (which are near unity). Applying a correction to the observed count rates is normally done in all cases. This file has changed with time because of threshold adjustment in a few diodes. Each detector has 512 diodes, the middle 500 of which are used for science.

5.2 Photocathode Response Files (PHCHFILE).

The photocathode response correction (controlled by PHC_CORR) is included to allow for the removal of the granularity of the GHRS photocathodes to achieve high signal-to-noise. Presently, however, the GHRS does not use this feature because obtaining the flat-field exposures for all possible grating positions is impracticable for most gratings. Therefore the default pipeline image is a DUMMY file that is populated with ones and zeroes and the only changes made to the default since launch have been to correct mistakes and keep grating names up to date. However, in 1996, G140L flat filed files became available. Please see GHRS-ISR-076 for more details.

5.3 "Vignetting" (VIGHFILE).

The response of the GHRS photocathode drops near its edges, producing an effect similar to optical vignetting, hence the name. This effect is calibrated by comparing similar spectra obtained at the center and edges of the photocathode. Setting VIG_CORR to PERFORM will correct for this phenomena.

5.4 Absolute Sensitivity Files (ABSHFILE, NETHFILE).

The FLX_CORR switch controls the flux calibration on GHRS data. Each sensitivity file is associated with a net wavelength file, NETHFILE. The standard fluxes in the far ultraviolet were changed around the time COSTAR was installed and are now based on the star G191B2B, FOS high-resolution observations, and models by Finley. Note that time variable post-COSTAR sensitivities became available after GHRS was removed from HST during the Second Servicing Mission.

5.5 Data Quality Initialization (DQIHFILE).

This file is used to flag dead diodes in the GHRS Digicons during data quality initialization (DQI_CORR). The dead diodes are "removed" from the calibrated data during the merging of substep bins (controlled by the MER_CORR switch).

5.6 SAA Contour Mask Header (SAAHFILE)

This file is used by the optional background model correction switch (BMD_CORR), made available with calhrs v.1.3.11 (Mar 97), as a basis for the shape of the South Atlantic Anomaly (SAA) based on HST PDB (Project Database) Model 7. The location of HST during the observation is compared to the location of the SAA Model contour. Warning messages are issued for any readout which is found to encroach upon the SAA. Background count rate model results are unreliable in this region.

5.7 Line and Sample Mapping Parameters (CCR1, CCR2).

The mapping parameter tables referred to in the MAP_CORR calibration step contain coefficients that relate the photocathode coordinate system to the diode array coordinate system. Sample position is measured in the dispersion direction while line position is measured perpendicular to the dispersion.