Science Operations

Mask Preparation

These pages serve to guide the users through generating a MOS mask for MODS.  Masks are prepared using MODS Mask Design Software (MMS), which is based on the LUCI Mask design Software (LMS).

A New Version of MMS is available for use

The following focusing on the functions and features used in the new Version 2.0+ of MMS, although many of the tips and tricks will apply to masks designed with V160 as well.

The major updates to MMS_2.0+ include:

  • Auto-slit available at all position angles
  • Updated menus to help guide the user through the optimal mask configuration
  • Updated features and functions such as:
    • “Move Pointing”
    • “Save Revision”
    • “Alignment Box Setup” and “Alignment Box Verification”
    • “Verify Slits”

    Details of these functions to follow

  • Fully backwards compatible with MMS_v160 releases

Users are encouraged to contact with any issues or questions they may have with the updated version, or mask generation in general.

MMS is written in Tcl/Tk and has been tested on both linux and MacOS operating systems. The version available here includes a linux binary for version 2.7.3 of ESO’s SkyCat Tool. This version is outdated and will not be able to load some images from DSS or star catalogs.  Users of MacOS and other operating systems will need to download the SkyCat binary appropriate to their operating system. MMS has been tested with skycat version 3.1.4 for MacOS. The main web page for SkyCat at ESO is here:  The ESO SkyCat Tool

Steps for Successful Mask Generation

The following pages will walk the user the steps for successful mask generation:

The linked pages will guide you through each step of the process.  Many features described in the linked pages are specific to version 2.0+ so we recommend you upgrade to this before proceeding with mask development.

Additional tips and troubleshooting hints can be found on the MMS Troubleshooting tips page.

Mask Preparation with MMS

The Gerber and mms files that are output by the MMS software should be sent to your Partner Coordinator by the mask submission deadline. If you any questions about creating or submitting a MOS mask, please contact

For all submissions remember the following:

  • Send the Gerber (.gbr) and mms (.mms) files to your Partner Coordinator. The Partner Coordinator will then ship all of these files to
  • Please use as many alignment stars as possible.
    • For MODS masks, alignment stars should lie within 2.5′ from the mask center (2.8′ at most); at distances greater than 2.8′, aberrations are sufficiently severe that centroids may not be well determined. 
    • Please insure that proper motions are taken into account.
  1. Please check if your reference stars have proper motions
    1. If you are using your own fits images to design masks, it may be useful to compare them with DSS or older images to check if stars have moved.
    2. One could load an image or the field center coordinates into DS9.  Then search catalogs (e.g. GAIA, GSC, UCAC, etc) for stars in the field and if there are known proper motion values for the stars.
    3. LMS and MMS do not accept proper motion updates, you will have to manually update your coordinates with proper motions
  2. Slit Separations: We routinely receive mask designs with slits packed way too close together, either in the vertical direction, or sometimes in both vertical and horizontal directions. As a rule of thumb, please keep any space between slits (and alignment stars) to about 1.8 millimeters (about 3”). If slits are too close, you risk losing the bridge between them and creating a very strangely shaped slit. This also poses a risk to the instrument.  If you think it looks too close, it probably is too close. If you are not sure, please contact us (or have your Partner Coordinator contact us) before the deadline so that we can check the masks.
  3. Please make sure not to rename the LMS and MMS files to something other than what LMS or MMS produces. Both softwares allow unique names to be created (i.e. mods.42.M81Targ1.mms) , there is no need to rename the files. This issue becomes a problem when using the LMS file to create scripts and can lead to downtime at the telescope when LMS names do no match what is in the script and/or provided to observers.
  4. Unless otherwise instructed, we will fabricate TWO copies of each mask submitted.

As before, if you or your PIs have questions or concerns, don’t hesitate to drop us a line at