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37.6 About Wavelength Calibrations

37.6.1 Aperture Offsets

By convention all GHRS wavelengths are referred to the SSA. Corrections to apparent wavelengths from sources in other apertures is necessary because light from those other apertures enters the GHRS at angles slightly different from that of the SSA. These corrections are called aperture offsets.

During calibration, the zero-points of the wavelength scale are adjusted for the difference in incidence angle of apertures LSA, SC1, and SC2 from the SSA. Incident angle correction (IAC_CORR) coefficients are found in the ccr8 table. Currently, an average value for the zero-point wavelength correction is applied to all first order grating LSA observations regardless of the carrousel position. The recently-calculated and old offsets for GHRS gratings are listed in Table 37.6, which is taken from GHRS ISR 080. These coefficients are used to compute the offset using the following formula:





Previous A






















-0.0076 ¥ m




-0.0078 ¥ m



Incident Angle Correction for GHRS Gratings

37.6.2 Doppler Compensation

Since HST orbits the Earth with a velocity of 7.5 km s-1, spectra obtained with GHRS could see a Doppler shift of up to 15 km s-1. The effect of the spacecraft velocity was corrected for in real time for ACCUM mode observations by deflecting the image of the spectrum an amount equal to the Doppler shift so that the spectrum appears fixed with respect to the diode array which is recording the spectrum. RAPID mode observations are not corrected for this effect. Unfortunately, it was discovered that GHRS spectra obtained prior to the end of March 1993 suffered from incorrect Doppler compensation.

The problem became visible in a set of high dispersion spectra obtained with short exposure times, where one could actually see a doubling of spectral features corresponding to the different Doppler shifts applied. At the maximum required correction, the flight software was mistakenly applying zero correction. Affected data will be obvious only in extreme cases, but the problem may degrade your data even when the effect is not obvious.

An on-board fix to the first problem was implemented in the flight software as of April 1993. Observations made after the update should not suffer from the Doppler compensation error. However a cumulative error in the onboard Doppler compensation still existed, which caused the accuracy of the Doppler compensation to be reduced for long exposures.

The obsum task in the STSDAS calhrs package can be used to identify GHRS spectra which were potentially corrupted by the first Doppler compensation problem. This task identifies periods when the Doppler compensation should have been maximal and provides information to allow you to estimate the fraction of the data in each group that is contaminated by incorrect Doppler compensation. If a substantial fraction of the data are corrupted for a given period of time, the only recourse is to discard the affected groups and reduce the remaining good data. See the help file of obsum for a detailed description of its use.

Also, all observations of moving targets made before July, 1994, were compensated incorrectly. The fix to this problem was implemented in July 1994 and moving target observations since then do not exhibit that problem. For help identifying and correcting moving target observations, please contact the STScI Help Desk via E-mail to help@stsci.edu.

37.6.3 Geomagnetically Induced Motion

The displacement of the image relative to the diodes due to the earth's magnetic field, Geomagnetic Image Motion (GIM) problem, may affect a GHRS observation depending upon the length of the exposure and the orbit during the observation. The rate of drift of an image across the diodes was small enough that there is no significant smearing of data on time scales of five minutes or less. Very long exposures may exhibit GIM related symptoms and should be investigated by the user. No correction for GIM has been incorporated into either the operations of the GHRS or in the data reduction procedures.

37.6.4 Carrousel Properties

The carrousel was rotated to engage the desired dispersive element or mirror and to place the requested wavelength at the center of the diode array. In the spring of 1991, the Side 1 carrousel control electronics developed an intermittent failure and the carrousel function was modified to operate the Side 1 carrousel from the Side 2 electronics. Any Side 1 observation obtained after April 1991 will have different carrousel function coefficients for specific wavelengths.

During normal operation, the carrousel was commanded to rotate to a pre-selected position depending upon the carrousel function. The carrousel could oscillate before achieving the desired position and take an appreciable amount of time to lock at that position. This may take longer than the normal amount of allocated time and thereby, following observations are affected. Before Spring 1995, if the carrousel took a significant time to lock, the following observations would not occur and be lost. After spring 1995, the flight software was updated to time out a set of GHRS observations if the end time of the observations was reached. The time-out was not enforced for GHRS target acquisitions. The header keyword FINCODE (see Table 37.7) will be set to 106 to indicate to the observer that a time-out occurred. The affected GHRS observation may be several observations down stream of the observation for which the carrousel took too long to lock into position. The -STSDAS task obsum in the calhrs package can be used to display to the screen the carrousel position and the FINCODE values.

st> obsum z2bd010c






X-null balance failure during coarse locate

Usually benign


X-null balance failure during fine locate

Usually benign


Number of slews to center exceeded max

Usually benign


Normal beginning of observation



Normal end of observation



Observation ended, over-exposure

Exposure may have been shortened


Observation ended, too many bad data

Exposure may have been shortened


Observation ended, time out

Exposure may have been shortened

37.6.5 Wavelength Data Quality

The pipeline data reduction system (PODPS) automatically assigns a wavelength scale to your GHRS ACCUMs1 and RAPIDs when they are reduced. Note that this default wavelength scale is the one appearing in the .c0h file that goes with the .c1h file containing fluxes, even if there is a wavelength calibration exposure (wavecal) available for that program. If you have a wavecal, you must analyze it and apply the results yourself; this is explained in the section "Recalibrating GHRS Data" on page 36-13.

This default wavelength scale is calculated using terms that depend primarily on the carrousel position (i.e., the orientation of the grating that was used), but there are also terms for the temperature within the GHRS (recorded in the engineering data stream). There is also a weak time-dependent term.

The default wavelength scale is good to approximately one diode rms, with contributions from various effects as enumerated in Table 37.8 (which is taken from Heap et al., 1995, PASP, 107, 871). Not listed is the error in wavelength that occurs for observations made in the Large Science Aperture (LSA), which was discussed in "Aperture Offsets" on page 37-23.

Wavelength Error Sources

Source of Error

Maximum Error (diodes)

Quality of dispersion coefficients


Incident angle correction, SC2 to SSA


Uncertainty in thermal and time models


Short-term thermal motions

0.4 hour-1

Carrousel repeatability

0.5 (0.17 typical)

Onboard Doppler compensation effects

0.15 typical

Geomagnetic image motion


Uncertainty in centering target in SSA


The largest sources of uncertainty in wavelength are obviously due to the geomagnetic image motion (GIM) and the model used to correct for thermal and time effects. We cautioned observers to break down long exposures into units lasting no more than five to ten minutes, in order to reduce the effects of GIM below significant levels. Thus you should not ordinarily find GIM leading to loss of resolution in the final spectrum.

Thermal effects, however, include a significant component that appears to be unpredictable. These thermal effects are best removed through use of a wavecal or SPYBAL. Thermal motions are just that: a motion of the overall image of the spectrum. The changes in image scale-dispersion-that occur are very small and can safely be ignored in most instances. For example, for grating G270M, the centers of our calibration spectra deviate from the default wavelength scale by no more than 100 mÅ (and typically about 70). The slope of a fit through the measured positions for the comparison lines relative to the default wavelength scale deviates in the center by no more than 100 mÅ (and typically about 70 mÅ), and has a slope of about 3 x 10-4 (in dimensionless units of Å per Å), so that the ends deviate from the center by, typically, about 3 mÅ across a 40 Å wide spectrum. The rms scatter of the fit is typically about 0.2 km s-1. Not all the gratings are this good, and some other values for the quality of fit are provided in Table 37.9. The data in that table are taken from GHRS ISR 081, which summarizes wavelength calibrations for the first-order gratings in Cycles 4, 5, and 6 (i.e., post-COSTAR). Other ISRs with wavelength analyses include:

Quality of Default Wavelength Scale for Side 2 First-Order Gratings


rms deviation of fit(mÅ)

Deviation at center (mÅ)

Slope of fit (times 104)


7 to 11




30 to 50

20 to 80



7 to 16

50 to 100

0.3 to 0.5

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1 Here and elsewhere in this chapter we will consider OSCANs and WSCANs as equivalent to ACCUMs. Both OSCANs and WSCANs are macros that generate a series of ACCUMs when the program is executed on the telescope.
