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Flatfield Features (GMOS-N)

New Flatfield Features introduced in GMOS-N images during August 2006 Shutdown

August 30, 2006

GMOS-N was removed from the telescope July 31, 2006 as part of an extensive Gemini North shutdown period. GMOS-N was mounted on the telescope (port 5, same as previously) again on August 9. The first science data was obtained the evening of August 16 (UT August 17).

Unfortunately, at some point during this time a large flatfield feature was introduced into the GMOS-N optical path. A series of tests has determined that this feature:

  • extinguishes ~2.5-3% of the incident light at the darkest point
  • is present in all filters
  • does not move around with either telescope position or cass rotator angle
  • is located within GMOS-N somewhere downstream of the grating/mirror turret
  • is not located on the detector package
  • is most likely located within the GMOS-N camera
  • appears to flatfield out quite well when appropriate twilight flats are used
  • does not flatfield out when domeflats are used

Once we have taken an appropriate number of twilight flats in all filters and binning combinations that have been used for science imaging and have verified that requested spectral twilight flats have been obtained for all spectral data taken since the return to science August 17, we will begin an investigation into the source of the flatfield feature, with the goal being to eliminate and restore the pre-shutdown sensitivity of the GMOS-N. The results of that investigation will be posted when it has been completed. In the meantime, it is very important that GMOS-N users take care to use appropriate baseline twilight flats when reducing their data: All GMOS-N data taken since and including August 17, 2006 UT should be flatfielded with twilight flats also obtained after and including August 17, 2006 UT. After such time when the flatfield feature has been removed this page will be updated to give the latest date for which GMOS-N science and calibration data contain this feature. Note that until such time we also will not be providing domeflats to the GMOS-N users or the Gemini archive.

Further details and images are provided below. Please note that, while we suspect that this feature arises from dust somewhere in the camera, the affectionate term "dustbunny" is not intended to imply that we actually are aware of the nature of the substance causing the new GMOS-N flatfield feature as yet.

Updates regarding the dust bunny will be posted on the Status and Availability page.