FOS Filter-Grating Wheel Repeatability


Anuradha Koratkar and Stephan Martin
Space Telescope Science Institute


Instrument Science Report CAL/FOS-145
July 1995



Abstract

The post-COSTAR FOS Filter-Grating Wheel (FGW) repeatability was tested for both
 the red and blue detectors. The test consisted of obtaining repeated FOS images
 in an uninterrupted sequence through three different gratings on each detector.
 The observations consisted of exposures using G190H, G400H and G570H with
 FOS/RD, and G130H, G190H and G270H with FOS/BL. All other observing parameters
 were unchanged during the test.


The standard deviations of the positions of spectra with respect to each other
 are found to be independent of the chosen grating. The rms deviations of the
 position of the spectra in the FOS X and Y directions are of the order of 0.1
 diode and 9 ybases, respectively. Deviations as large as 0.36 diodes and 18
 ybases are also seen. These values are similar to those obtained from
 pre-launch calibration activities. The non-repeatability of the FGW could be a
 major source of instrumental inaccuracy in the wavelength calibration of
 spectra obtained without contemporaneous calibration lamp spectra. It should be
 noted that the absolute wavelength calibration for a typical FOS spectrum is
 not only affected by internal instrumental effects, such as, FGW position and
 aperture location, but is also affected by the target acquisition accuracy.