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Hubble Space Telescope
OSM Drift and NUV Wavelength

Long term drift of the COS Optics Select Mechanisms (OSMs) is resulting in changes to the NUV bandpasses. While this drift is different for each grating, in all cases it is in the direction of placing shorter wavelengths on the detector. When combined with the inherent OSM positional uncertainty, this can result in a shift of up to 100 pixels on the detector. Observers are instructed to select NUV modes which place wavelengths of interest at least 100 pixels away from the edges of the detector for all observations. This corresponds to 3.3--4.0 for medium resolution modes, and 39 for G230L. All NUV wavelengths which were previously able to be observed with at least 3 settings are still available, with the exception of the very longest wavelengths of G285M (-3.2 ) and G230L (-39 ).