GHRS FAQ on Calibration

1. Why don't my file names look like what is in the Data Handbook?
2. Are the data calibrated?
3. How do I know what calibrations were done to the data?
4. How do I know which reference files were used to calibrate my observations?
5. Are the calibration reference files included with the data I will receive?
6. How else might I obtain the calibration files?
7. How do I know if there are better calibrations available for my observations?
8. How do I re-calibrate my data?
9. How do I determine the orientation of the GHRS aperture?
10. How do I determine the start and stop times of my GHRS ACCUM observations?
11. How do I determine the start and stop times of my GHRS RAPID observations?
12. What do I do about sensitivity drops under 1200Å?
13. Are GHRS spectra background subtracted?

Why don't my file names look like what is in the Data Handbook?

The HST Archive stores data in FITS format. STScI uses IRAF/STSDAS to work with HST data. We do not support any other software for working with HST data. You should use the IRAF STSDAS task strfits to "de-fits" the files and then the file names on your computer disk should look like those in the GHRS chapter of the Data Handbook. Chapter 3, especially pp. 82-84, of the Dec 1995 (Version 2) HST Data Handbook covers this information in more detail.

Are the data calibrated?

Yes. The data that you will receive have been flux and wavelength calibrated with the best currently available calibration reference files.

How do I know what calibrations were done to the data?

The "Calibration Switches" section in the science header files will indicate which calibration steps were performed on your data.

How do I know which reference files were used to calibrate my observations?

The reference files used are listed in the "Calibration Reference Files" section of the science header file.

Are the calibration reference files included with the data I will receive?

Data tapes with FOS or GHRS spectra include the reference files. Or, if you retrieve your data from the Archive, you are given the opportunity to also retrieve either (or both) the used and "best" calibration files.

How else might I obtain the calibration files?

All the reference files are public and can be retrieved from the HST Archive. When retrieving data from the HST Archive, you are now given the opportunity to also retrieve either (or both) the used and "best" calibration files. When the files are retrieved from the HST Archive, they are in FITS format and can be converted to GEIS images or STSDAS tables (usable by the calibration software) with the STSDAS task 'strfits'.

How do I know if there are better calibrations available for my observations?

More recent calibration files may have been obtained that were not installed in the pipeline when your data were calibrated. You can check the HST Archive or refer to the Calibration Products and Tools section for GHRS. There you will find updates and a pointer to a document which lists the GHRS Reference Files and Tables with USE_AFTER dates and comments that will help you decide if you need to re-calibrate or not.

How do I re-calibrate my data?

Retrieve new reference files as described above. Follow the steps out lined in the GHRS chapter(s) of the HST Data Handbook.

How do I determine the orientation of the GHRS aperture?

To determine the orientation of the GHRS apertures on the sky you need the value of the PA_APER keyword in the shh header. This number is the position angle of the +Y axis of GHRS measured from north through east. The +Y axis is the direction from the LSA to the SSA. The +X axis is the direction of increasing wavelength.

The HST Data Handbook V.2 (Dec 1995), figure 21.1 on p 320 shows the orientation of the GHRS apertures with V2V3. This is also available in Chapter 6 of the GHRS Instrument Handbook V.6.0, if you have a paper copy.

The trick is to figure out how to display your image so that it has the same directional sense as Figure 6-3 in the instrument handbook. The problem is that tasks typically display +y up and +x to the right but what is needed is +y down and +x to the right. In IRAF this would be something like display test.hhh[*,-*].

Once you have this orientation then you apply the offset angle (PA_APER) to get N-E lined up.

You should also know that RA_APER1 and DECAPER1 are the predicted RA and DEC of the center of the aperture used, for the beginning of the observation. GHRS PEAKUPs effectively re-zero the coordinate system so one would need to use the OBS logs (jitter files) to get the actual pointing.

How do I determine the start and stop times of my GHRS ACCUM observations?

Please look in Chapter 26, p. 381, (Timing of GHRS Observations) of the Dec 1995 V2.0 HST Data Handbook.

How do I determine the start and stop times of my GHRS RAPID observations?

RAPID mode behaves a little different from ACCUM mode, as mentioned under the Data Handbook's section "When did my observations end?" We will have to add more words to explain the timing of RAPID mode observations.

With RAPIDs the first two groups and the last group of raw data are thrown away, as described on page 342 in the Data Handbook. Therefore, the PKTTIME of the 1st c1h group is the same as the PKTTIME of the 3rd d0h group. Since the first 2 groups of raw data don't contain any science, that makes the PKTTIME of the 3rd group more closely related to the actual start time of the observation. The calibrated data simply inherits the PKTTIMEs of the raw science data.

Recall that PKTTIMEs are the times the data is dumped and that is why they are only estimates of start times for science observations. Each individual RAPID exposure is dumped after it is taken. Most ACCUM data is not dumped until the entire exposure is complete and the 2nd UDL (Unique Data Log) has been dumped.

The best estimate of the start time for RAPID data is the PKTTIME of the first group of the c1h file MINUS the integration time (SAMPLE-TIME in your proposal or STEPTIME in the data headers). This time will have an uncertainty of 1/2 of a space craft clock tick (0.5 * 0.125s).

Are GHRS spectra background subtracted?

Two sources of background are relevant for the GHRS: detector background and sky background. Detector background is caused primarily by Cerenkov radiation bursts induced by cosmic rays. Detector background is monitored simultaneously with science observations and is subtracted during normal post-observation pipeline processing. Unless an object is much fainter than the detector background (typically 10^-2 counts/sec/diode), observers usually need not worry about detector background subtraction.

Sky background, in contrast, can often affect science observations. The GHRS does not have spatial 2-dimensional capabilities. Therefore measurements of the sky background are not normally performed, unless additional time for such a measurement is spent at an off-set position. The sky background is determined by earth-shine and by zodiacal light. A detailed quantitative discussion is in the FOC Handbook Version 7.0 (page 82). A condensed version of this discussion is in Version 1.0 of the STIS Handbook (page 74). Figure 6.1 of the STIS Handbook will be used for the following discussion.

The continuous sky background is strongly wavelength dependent. It drops by orders of magnitude from the optical to the ultraviolet below 2000 A. Therefore it is generally negligible for GHRS observations. Geocoronal Lya (and to a lesser extent OI at 1304 A and 1356 A), however, can be significant. Peak flux values of Lya during high background observations can reach 10^-13 erg/cm/cm/s/A/arcsec/arcsec. If observed through the GHRS LSA, a broad (about 8 diodes FWHM) geocoronal emission is often observed even in moderately faint objects. An easy way to suppress geocoronal Lya by about a factor of 100 is to use the SSA, which has a size of 0.22" by 0.22". The width of a geocoronal line with the SSA is only 1 diode.