Science Operations

MMS Alignment Boxes and Guide Stars


Selecting Alignment Stars

Select alignment stars to properly register the mask with the sky. The alignment stars are simply square 4×4 arcsec boxes.  


NOTE: If using mms_v160, the alignment stars should not be selected as reference stars in MMS. The reference star category is used by LUCI for mask alignment, but the MODS alignment software (modsAlign) does not use them. In fact, if the alignment stars are labeled as reference stars, it may be quite difficult to place a box on them.  This has been removed from MMS_v2.0+.


We recommend a minimum of 3-5 alignment stars with r = 15-18 mag. These stars must be within the central 5.6′ diameter, and ideally within the central 5′ diameter since the IQ degrades outside this area. And should have relative large separation between them to result in the most reliable rotational offset measurement (dTheta). Five stars, all separated from each other by a few arcminutes, is ideal, especially if you do not have complete confidence in your astrometric catalog. Extra stars allow the observers freedom to eliminate one or more problematic stars, if necessary. Note that the alignment stars should be selected from catalogs (or images) with the same astrometric solution as the targets.


Setup Alignment Boxes

To set up alignment boxes:

  1. Change the slit width and slit length to 4 arcsec. Users of MMS_v2.0+ can select Configure For Alignment Boxes which automatically configures the Mask Config for 4×4 arcsec boxes.
  2. Follow the same procedure you used to place slits on objects to mark the alignments stars. A good magnitude range is r = 15 to 18 mag. Such stars are very easy to see in a short acquisition exposure, yet are unlikely to saturate. Given that we are in the early stages of mask alignment with MODS, we presently recommend a minimum of 5 alignment stars distributed around the edge of the field.

Verify Alignment Boxes

 Verify Alignment Boxes is a routine (MMS_v2.0+ only) that can be run independently or when saving.  It performs the following checks:

  • Failure message if less than three 4×4 arcsec boxes or circles in the 5.6 arcmin diameter circle
  • Warning if there are three boxes in 5.6 arcmin diameter but less than 3 in the 5 arcmin diameter circle

This check runs automatically when saving. Note that Alignment Targets do not need to be written explicitly to the MMS file so coordinates or magnitudes are not verified in any way.  It is up to the user to verify that proper motions have been taken into account and that the alignment targets have appropriate magnitudes.


Guide Stars

The “Guide Stars” submenu and setup are set up nearly identically to the alignment targets.   The Guide Star patrol field will be outlined in red when adding targets.  Unlike alignment targets, only 1 is necessary to satisfy the verification.

Guide Stars will be highlighted with a yellow circle once selected.

It is not vital that guide star coordinates be taken from the same source as the science object as the alignment is done independently of the guide star.

MODS Guide Stars should be point sources in the mag range of 12.0-16.5 in R or r’ band. Brighter work better in poor conditions.

MMS does not pass on the guide star magnitudes so it is good practice to reassign your guide star during script generation in the Observing Tool.  But this step is important to ensure you have a suitable guide star for your science.  Some time it may be necessary to reorient 180 degrees to acquire a suitable guide candidate so it is recommended to assess options early on in the mask development process.
A second check of your guide stars may be done using the Observing Tool (OT).  Use one of the MOS templates and simply load the LMS file generated by the software. It will display the LUCI science field, the guider patrol field, as well as show if the guider may vignette science or reference boxes on your mask.  The OT allows you to verify that the guidestar you selected is acceptable for observing.