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A wavelength calibration exposure made with the star in the SSA may be contaminated by the stellar spectrum. This is because the SSA has no shutter and the fact that the SC2 aperture is in line with the SSA. This contamination is rarely a serious problem, however, because it is possible to subtract the stellar component. Also, wavelength calibration exposures are generally used to just confirm the zero-point of the spectrum and not to obtain a full wavelength solution.
For a comprehensive listing of the platinum lines, see Reader et al. (1990).
The GHRS wavelength monitoring program runs four times per year for both the echelles and first-order gratings to confirm that anomalies are not occurring and also to build up the database used to establish how motions depend on temperature, time, and magnetic field.
Aperture Offsets
The light from the spectrum calibration lamps does not enter the spectrograph along the same path that starlight takes. This introduces a wavelength shift that must be compensated for in determining the wavelength solution. The data reduction software incorporates corrections that were determined during pre-flight ground testing (new values are being measured in Cycle 5). Thermal Effects
The image formed by the Digicons is affected by the thermal environment within the GHRS, which in turn is influenced by whatever electronics happen to be on or off in the GHRS and the other instruments. The temperature inside the GHRS can be monitored and the image motion calibrated. This correction is also provided for in the data reduction software. However, this correction is applied only once to a given exposure line. We recommend that the exposure times for individual exposure lines be kept shorter than about one hour as long as you do not encounter problems with using too much on-board memory (see Section on page 52). Geomagnetic Effects
As for overall image stability, geomagnetic effects influence wavelength stability. Long exposures should be divided into units of about 5 minutes each, the time over which the wavelength scale does not change measurably. Calibrating the Wavelength
The precision and accuracy of the wavelength scale of a GHRS spectrum depend on:
The quality of the wavelength scale is primarily an issue with ACCUM mode observations; RAPID mode observations cannot be interrupted and are usually made to look for short-time-scale variability, not wavelength shifts. Our requirement for routine wavelength calibrations is to have them good to about one diode rms. We achieve much better than this in practice. In particular, the default wavelength scale (i.e., the dispersion) is reliable to about 2%, and the zero-point can be established to within about 0.2 diode by obtaining a wavelength calibration exposure just before your ACCUM.