GMOS

GMOS-N B480 unavailable from July 3 (HST) for ~3 weeks

July 1st, 2025

The spring and bearings securing the GMOS-N B480 grating within its frame have not been functioning properly and need replacement. The net effects have been possible wavelength zeropoint shifts depending on the gravity vector. These shifts have not been a major concern for science quality, since they can typically be calibrated out via the on-sky arcs or via skylines, per these instructions.

Update on air bubbles in the GMOS-N lens interfaces

March 14th, 2025

Seven lens interfaces in the GMOS-N collimator were refilled with index-matching oil on 5 March 2025 HST. This work was performed to address the known recurring issue of air bubbles developing in the GMOS lens interfaces.

The air bubbles move with changes in telescope elevation and cass rotator angle and can cause a shifting partial obscuration in the lower part of the GMOS frames. This results in a limited flat fielding accuracy, particularly impacting imaging data that use the entire GMOS field of view.

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