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.
The 5 March 2025 lens-interface refill has had a noticeable effect on the GMOS-N flat field, meaning that data taken prior to this intervention have experienced increasing levels of air bubbles over time. The following plot shows the ratio of a z'-band 2x2 twilight flat taken after the refill to a flat taken before the 5 March 2025 refill. Each input flat consists of 6-7 raw frames, reduced and normalized using DRAGONS.
To achieve the best flat fielding results, users with GMOS imaging data observed on or after the night of 6 March 2025 UT should flat field their data with matching twilight flats taken on or after that date. More details on how the air bubbles might manifest themselves in affected GMOS data and possible data reduction strategies are discussed on this webpage.