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32.2 Mechanism Stability

This section discusses non-repeatable motions of moving parts in the FOS optics-these motions would position the image of the aperture at different physical locations on the photocathode. These positional uncertainties affected the following:

Wavelength calibration as the x-position of the spectrum may not duplicate that obtained for the STScI wavelength calibration spectra.
Flatfield calibration as the position of the spectrum may not sample the same photocathode granularity as the STScI calibration.

32.2.1 Filter-Grating Wheel Non-Repeatability

Although the positions of the filter-grating wheel (FGW) in the beam were stabilized by so-called detents or notches, there was still some mechanical variability of the wheel position, even at the same detent, from observation to observation. The x-component of any FGW residual positioning uncertainty caused an offset in the observed wavelengths from the calibrated dispersion curve. Additionally, any offset in either x or y added a small uncertainty to the flatfield calibration.

The amplitude of the FGW mechanism positioning uncertainty is discussed thoroughly in FOS ISRs 145 and 142. The 1 FGW non-repeatability determined in those ISRs was 0.12 diodes or approximately 0.47 quarter-stepped pixels. Recent analysis of a larger set of FGW motions indicates a peak-to-peak maximum displacement of 0.5 diode width (+/- 125 km/sec at high dispersion) with no changes over the lifetime of the FOS.

Summary of FGW Motion

x-motion:
- pre-COSTAR: 1 motion = 0.12 diode = 0.5 pixel ~ 0.035" = 30 km/sec
- post-COSTAR: 1 motion = 0.12 diode = 0.5 pixel ~ 0.03" = 30 km/sec
y-motion:
- pre-COSTAR: 1 motion = 8-9 Y-bases = 0.050"
- post-COSTAR: 1 motion = 8-9 Y-bases = 0.045"

32.2.2 Aperture Wheel Non-Repeatability

There was no reason to believe that aperture wheel non-repeatability contributed significantly to errors in FOS spectra. Pre-launch measurements indicate the variability was very small. Attempts to measure these on orbit are discussed in FOS ISR 131 and were limited by residual GIM motion (0.02 diodes 1

). Pre-launch measurements indicated a non-repeatability an order of magnitude smaller than on-orbit GIM motion residuals.

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