GMOS-N data taken between the nights of 31 August 2020 and 8 September 2020 show a new flat field feature from dust in the optics, as marked in the image below. A preliminary cleaning of the accessible GMOS optics performed during daytime on 3 September 2020 was not successful in removing the new dust feature and the overall effects were found to be negligible. A thorough cleaning was performed during daytime on 8 September 2020 which successfully removed the new piece of dust as well as other longer-term dust features. We recommend to not mix flats taken from 31 August 2020 to 8 September 2020 with those taken prior to 31 August 2020 or after 8 September 2020. Science data taken before 31 August 2020, between 31 August 2020 and 8 September 2020, or after 8 September 2020 should be calibrated using matching flats obtained during the same time period.
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