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Mask Preparation: Introduction

The GMOS MOS masks must be designed either from images acquired with GMOS, or from object catalogs with very accurate relative astrometry. In the first case observers use the GMOS imaging mode to record an image of the sky from which the spectroscopic target pixel positions are measured. The required imaging is obtained in queue mode prior to either classical or queue mode MOS spectroscopic observations. In the second case, accurate relative coordinates (RA and Dec) are required for the spectroscopic targets, and known transformations are used to calculate the GMOS pixel positions from these coordinates.

The MOS mode requires that the PI designs the masks and submits the relevant information to Gemini prior to the planned observations. Detailed instructions for the MOS observer as well as information regarding the GMOS mask-making software are available:



GMOS pre-imaging

GMOS imaging of the field is executed in queue mode only (even for Classical programs), and must be obtained before the MOS mask can be designed. Images which are primarily obtained for the purpose of mask design are referred to as pre-imaging in order to distinguish them from regular imaging for queue planning purposes. Normally, this imaging is executed as early in the semester as possible once the field becomes available. MOS masks are cut twice per week at Gemini South, there are deadlines (linked from the Phase II instructions for the current semester) for submission of mask designs in order to have them included in the batch of masks cut at the next cutting date. MOS PIs are encourage to submit their mask designs as early as possible in order to increase the chance that the MOS observations will be completed.

Observing time to obtain the required direct imaging must be requested during Phase I and the observations defined as part of the Phase II process. The observations executed will be charged against the program's total allocated time.

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-looking port may be used for mask design for a side-looking port only if a suitable guide star is available without changing the pointing and orientation of the field on the sky. The same constraint applies to imaging data obtained with GMOS on a side-looking port and used for mask design for the up-looking port. 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.

If a PI has prior imaging from other telescopes, it may be possible to use GMOS images for the mask design 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. This might be the best procedure to use, for example, when designing a GMOS-S MOS mask based on imaging obtained previously with GMOS-N. The procedure for doing this is outlined in the detailed mask design instructions.

PIs may want to access carefully the observing conditions needed for the imaging of the field. Requesting very good observing conditions for the imaging may lower the chance of getting the imaging done early during a semester, and therefore negatively impact the overall chance of getting the MOS observations completed within a given semester.