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The Goddard High Resolution Spectrograph (GHRS) is capable of obtaining data with a wavelength accuracy of 1 km/sec in the echelle modes. Both proper observing and data reduction techniques are required to achieve this accuracy. We will discuss all known sources of errors in the assignment of wavelengths to GHRS observations and describe methods for minimizing these errors. Wavelength errors that can be minimized include: centering errors of the object in the aperture during target acquistion, non-repeatabilty of the grating carrousel, and thermal, geomagnetically induced, and temporal motion of the spectrum.
Shifts of the spectral format at the GHRS diode array can be as large as 300 microns (six diode widths). We have modelled this motion as a function of temperature, time, and the component of the Earth's magnetic field in the direction of dispersion. In the absence of calibration observations of the on-board spectral calibration lamp, this model can be used to reduce the errors from spectral motion to approximately one diode width or 3 km/sec in the echelle modes.
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