Internal/External Offsets in the FOS Wavelength Calibration


G. A. Kriss, W. P. Blair, and A. F. Davidsen
The Johns Hopkins University

Instrument Science Report CAL/FOS-070
February 1992


Abstract

In September 1991 the M1.6Ve star AU Microscopii was observed with
gratings G13OH, G19OH, and 0160L on the blue side. and gratings GL9OH,
0270H, G400H, and G570H on the red side to evaluate offsets in the FOS
wavelength calibration between the internal Pt-CrNe calibration lamps
and an extemal source of light. Observatioris of the intemal lamps
were obtained with the identical gratings either before or after the
observation of AU Mic without changing the position of the
fllter/grating wheel. After correction for the geomagnetically induced
motion problem on the red side, we find a rnean offset (Xj~~~3j  ~ of
±0.176 diodes with a dispersion of 0.105 diodes. On the blue side, the
offset (Xmtern~i  Xe~«ernai) i~0.102 diodes with a dispersion of 0.100
diodes. Allowing for the scatter. in the in-flight 9 measurements, the
offsets arc similar in size and character to those observed in
pre-flight tests. Given the observed dispersion of —0.1 diode in our
measured offsets, velocity measurements based on single lines in FOS
spectra have a limiting accuracy of roughly 20 kms