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Mask Preparation: Introduction

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The GMOS MOS masks must be designed either from images acquired with GMOS, or from object catalogs with very accurate relative astrometry. MOS PIs are encouraged to submit their mask designs as early as possible in order to increase the chance that the MOS observations will be completed. Since all MOS masks are currently cut at Gemini South, different deadlines apply to the mask design submission for GMOS-S and GMOS-N masks. Detailed information on the corresponding deadlines can be found in the Phase II instructions for the current semester.

Mask design from new GMOS pre-imaging

The pixel positions of the spectroscopic targets are obtained from a GMOS image. Due to distortions over the GMOS field of view, this image has to be taken with the same pointing and orientation on sky as planned for the MOS observations. If no prior GMOS image is available, PIs can request pre-imaging in queue mode prior to the classical or queue mode MOS spectroscopic observations. Observing time for pre-imaging will be charged against the program's allocated time and must be requested during Phase I. The observations should be defined as part of the Phase II process as early as possible during the semester, so that the images can be obtained once the target field becomes accessible. PIs may want to access carefully the observing conditions needed for the pre-imaging. Requesting very good observing conditions may lower the chance of getting pre-imaging data early during a semester, and therefore negatively impact the overall chance of getting the MOS observations completed within a given semester.

Mask design from previous GMOS images

Imaging obtained with one GMOS in a prior semester may be used for the mask design for the same instrument if the observations were taken with exactly the same pointing and orientation on the sky as is planned for the MOS observations. For a given pointing and orientation, the location of the OIWFS patrol field on the sky depends on whether GMOS is on the up-looking port or one of the side-looking ports on the telescope. Imaging data obtained with GMOS on the up- or side-looking port may be used for a mask design for the other type of port only if a suitable guide star is available without changing the pointing and orientation of the field on the sky. GMOS North was on the up-looking port in 2001B, and has since been on the side-looking port. GMOS South is on the side-looking port. If you are in doubt about whether existing pre-imaging can be used for your mask design, please contact the Gemini HelpDesk.

Existing images from other telescopes or the other GMOS instrument

GMOS-N images cannot directly be used for GMOS-S mask designs and vice versa. If a PI has prior imaging from the respective other GMOS instrument or other telescopes, pre-imaging with the GMOS instrument used for the MOS observations is required. However, if there are existing images of the field, it may be possible to base the mask design on new pre-images obtained with quite short exposure times and/or in worse seeing conditions than needed for the spectroscopy. If the GMOS images contain a sufficient number of objects (> 50) with good signal-to-noise then these objects may be used to "boot-strap" the pixel positions of the science targets. This would be useful if, for example, the relative astrometry provided by the WCS is not accurate enough to construct a reliable object catalog.

Mask design from a catalog

The mask is designed from accurate relative coordinates (RA and Dec) for the spectroscopic targets from a catalog. In this case, known transformations are used to calculate the GMOS pixel positions from these coordinates. The PI must have an object catalog with accurate relative coordinates for spectroscopic targets and alignment stars. It is crucial that all coordinates in the object catalog be given in the same astrometric system, as systematic offsets between science targets and acquisition targets for example will produce a fatally flawed MOS mask. In most cases these coordinates should be known with a relative accuracy < 0.1 arcsec. Gemini recommends that masks with slit widths narrower than 1 arcsec be designed from GMOS direct imaging rather than solely from object catalogs. The PI must also create a "pseudo-GMOS" image of the field, using the gmskcreate task available as part of the Gemini IRAF package. This pseudo-GMOS image must be submitted via the OT along with the mask design as it is required during the mask checking process. This image is created from a PI-supplied image taken with another telescope and the gmskcreate task transforms that image onto GMOS pixels when you specify fl_getim=yes. As with MOS masks designed from GMOS direct imaging, the PI designs the mask for a specific field center and position angle on the sky. Once the mask design is submitted these cannot be changed, therefore the PI must verify that a suitable guide star exists for that field center and position angle. The OT can be used for this. The MOS observations defined in the OT must use the same coordinates and position angle as those used for the mask design.

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