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Instructions for Designing a MOS Mask

This page gives step-by-step instructions for designing masks for MOS observations with GMOS. The required steps are:

The National Gemini Offices (NGOs) check the mask design. The mask checks are performed following the NGO mask design check instructions. PIs are welcome to read these instructions to check and improve their mask designs.

Please note that there are procedural changes for submitting your Object Definition Table  (see below).

Creating an Object Table

The first step is for the user to create an Object Table, which is then input to the Gemini MOS Mask Preparation Sofware. There are two options for doing this:

Please note that masks designed using object catalogs is a feature added late in 2007 due to popular demand. PIs are reminded that use of this mode carries some risk due to inherent inaccuracies in object catalog astrometry and the transformation of those coordinates onto the GMOS focal plane. PIs should read the recommendations to ensure a good mask design when using object catalogs. For those programs requiring the most precise slit placement Gemini recommends designing the mask based on direct GMOS imaging.

A Guide to the Gemini MOS Mask Preparation Software (GMMPS)

The Mask Design is done using the Gemini MOS Mask Preparation Software (GMMPS) that is available pre-compiled for Sun/Solaris, Linux/Redhat/Fedora Core and Mac OS X. To use GMMPS the users will need a current version of Skycat as well as the the latest version of the Gemini IRAF package installed.

The basic steps in the use of the Gemini MOS Mask Preparation Software are as follows:

  1. Start the software using the command gmmps. This loads the GMMPS plugin within Skycat.
  2. Load the image.
  3. Load the Object Table. The Object Table should be prepared as described above.
  4. The objects may now be edited and prioritized as required, including selecting acquisition objects.
  5. There are now two options. Either; create a regular mask, or, create a Nod & Shuffle mask.
  6. The output from the mask creation is a FITS table - a minimal Object Definition File (ODF). You may want to read the detailed information on how the slit-positioning algorithm works.
  7. Plot the slits by loading the minimal ODF.
  8. Once the mask design is perfect, confirm that your mask complies with the Mask design checklist.
  9. Follow the instructions to submit the ODF(s) (FITS files) to Gemini.

Information about troubleshooting, tips, bugs and additional features.

Mask design checklist

When you are done designing your mask, use this checklist to make sure everything is in order before you submit your mask design to Gemini.

  1. Does the field of view drawn by GMMPS match your image? If not, then you most likely have a problem with a missing PIXSCALE in the header of your Object Table.
  2. Does your design have a minimum of two acquisition objects (three recommended for masks made from object catalogs)? They should be sufficiently spaced out over the field - ideally at least one per CCD.
  3. Are your acquisition objects away from the gaps between the CCDs? You can see the gaps as areas of higher noise in the co-added reduced image supplied from your pre-imaging. GMMPS can also plot the gaps for you.
  4. Are the acquisition objects bright enough to give good alignment from 30sec acquisition images? A good rule of thumb is for them to have magnitudes in the range of 14-20. They should also be of the same brightness (within about 2 mag), if possible.
  5. Are the acquisition objects unsaturated in your pre-imaging data?
  6. Do the RA and DEC values in the Object Table (and therefore in the ODF) match the WCS of the GMOS image?
  7. If using tilted slits, the slit angles must be within +- 45 deg.
  8. If you boot-strapped from previous image data, have you double checked that the x_ccd and y_ccd values correspond to the correct pixel values on the GMOS image? The GMOS masks are cut using (x_ccd,y_ccd).
  9. No manual editing of the output ODF FITS file should be done before submission.
  10. ODF *cat files are not accepted as mask design submissions.

Submission procedure for the minimal ODF

The minimal ODF (the FITS table) produced by the Gemini MOS Mask Preparation Software (GMMPS) has to be submitted to Gemini in order for the mask to be cut. For a mask designed from an object catalog the pseudo-GMOS image also must be submitted (please gzip it to save space). For programs designing masks from direct GMOS imaging, information on how to retrieve the co-added images of their targets from the Gemini Science Archive will be e-mailed to the PI once the imaging data has been taken. The minimal ODF files and (if applicable) pseudo-GMOS images should be submitted using the OT facility for storing and fetching attachments files.

We ask PIs to use a standard naming convention when submitting mask designs (ODFs). This is to avoid confusion as to which mask name (as defined in the OT Phase II MOS observations) is associated with the submitted Object Definition Table. If you wish to submit an Object Definition Table from which mask G(N/S)YYYYSQPPP-XX (N/S indicates North or South) should be designed then that Object Definition Table should have the name G(N/SYYYYSQPPP-XX_ODF.fits. Here YYYY is the year, S is the semester, PPP is the program number and XX is the mask number. This will enable us to ensure that the masks that are cut for the program have the mask name intended by the PI when the MOS observations were defined. For Fast Turnaround programs using masks please replace Q with FT and include a leading 0 for the program number to ensure three digits. 

When attaching ODFs to the OT, be sure to include in the Description field the name of the image associated with the mask design, whether this image is a GMOS direct image or a pseudo-image.

Note that the National Gemini Offices (NGOs) check the mask design.  The GMOS mask design check document contains all necessary procedures for NGOs mask checking. The PIs can use this document to check and improve the mask design.

From ODF to mask - done by Gemini staff

Gemini staff will convert the minimal ODF to a Mask Definition File (MDF), taking into account the transformation between the detector plane and the mask plane, as well as the distortions introduced by the laser cutting machine.

The MDF is then converted to instructions to the laser cutting machine and the mask is cut. The mask is outfitted with a barcode, mounted in a frame and transported to the summit, where it is mounted in GMOS. The mask is checked in the instrument by imaging it with a flat field source. The image is used as the final quality check of the mask before it is ready for use.