November 07, 2023S. Hasselquist et al.
The Optics Select Mechanism 1 (OSM1) in the COS instrument is responsible for placing the spectrum on the FUV XDL detector along the dispersion direction in a repeatable way, to a required accuracy of better than half an FP-POS, or ~125 detector pixels. In May 2022, the COS team detected two separate changes in OSM1 motions. First, within the G130M grating positions at the 1291 central wavelength, small OSM1 motions (1 motor step) to the FP-POS = 3 and FP-POS = 4 locations resulted in spectral placement that was offset (~120 pixels) and less precise (𝜎 = 25 pixels to 𝜎 = 40 pixels) than large OSM1 motions (>200 motor steps) to these locations. This behavior began at the start of LP4 operations, and impacts the gain sag holes on the detector due to geocoronal Lyman alpha emission falling on different locations of the FUVB segment. Second, at the 1222 central wavelength, the FP-POS=2 and 4 locations showed a significant positive trend with time in spectrum positioning at a rate of nearly 0.5 pixels/day for close to six months before flattening out. Since this 1222 behavior was directly correlated with a change in OSM1 home position, the COS team hypothesized this change may be the cause of a significant fraction of the behavior. Aside from slightly larger gain sag holes, these changes in OSM1 motions have minimal science impacts on COS data, although users who are concerned with precise spectral placements on COS FUV might want to consider mitigation strategies.