Science Operations

LMS Position Slits

Defining Slit Characteristics

The Slit Characteristics are all defined in the “Mask Config / Layout” menu.


From here you can select the Type of Slit (Straight, Circle, Curved), dimensions, some standard display and configurations options.


There are 3 slit type options: Straight (which includes parallelogram/tilted slits), Circular slits, and Curved Slits.  Laser limitations and maintaining mask integrity limit the spacing and minimum cut width of slits. 

Minimum Cut Width: 0.08mm (0.13″)

Minimum Spacing: 1.8mm (3″)

Select the type of slit and the corresponding slit width and/or length. Place slits by clicking on the position on the image, or using any of the methods described below: Auto-Slit, Copy Slit, or Center Slit. To edit a placed slit, click to the right of the slit on the slit width. A popup outlining changeable slit parameters will appear. To delete a placed slit, click to the left of a slit on the allocated slit number. 

About Straight/Tilted Slits:

Straight slits can be placed using Center Slit, Auto-Slit, or clicking in place, defining the slit width and slit length from the menus. The slit length and width can be edited from the slit parameters popup brought up for each individual slit. This popup also allows the user to adjust the inclination (tilt) of the slit. The spectral width and spacial length will be maintained as users adjust the tilt.

About Circular Slits:

The diameter of circular slits is defined by the width in the menu. Circular slits can be placed using Center Slit, Auto-Slit, or clicking in place, defining the diameter by setting the width though the menu. The diameter can be edited from the slit parameters popup brought up for each individual slit.

About curved slits:
The bezier points for curved slits must be defined as they are placed.  Once the next slit is placed , even if the following slit is deleted, only slit widths for previously placed curved slits can be altered.  
The 3 bezier points that define the curve are indicated by turquoise circles.  These circles can be dragged to adjust the curve.  You must move fairly slowly and be careful that your mouse tip remains on the fiducial as you are dragging.  
ProTip: To make it easier to make large bezier point motions, increase the slit width of your curved slit to 5″+ to make the bezier points larger and easier to grab. Once happy with the rough placement, readjust the size back to the desired slit width.
The slit length is a starting length for placement and can be adjusted by dragging the upper or lower bezier points.  Avoid making your curved slit >>20″ horizontally as the cut resolution may be impacted.  Contact if your science requires larger curved slits.


Auto slit requires the user to load a catalog.  Make sure you are using consistent and up-to-date astrometry!

In detailed discussion of Local Catalog Format can be found in the ESO Astronomical Catalog Library User Manual.

The format of Local Catalogs is a simple ASCII tab table. The line containing ID, RA, DEC, … specifies the header of each column. The keywords have to be tab separated. LMS requires an ID, RA, and DEC column, and for optimal performance the user should include a PRIORITY column as well.  Lines starting with ‘#’ are considered as comments and are ignored.

To load a Local Catalog, hit Ctrl-O to bring the File Dialog Box, or maneuver the “Data Servers” menu.  Auto-Slit item from the LUCIFER menu automatically places slits on the sources listed in the catalog. The slits are placed first in order of priority and then in order as they appear in the catalog.

They will be placed in a region 2.8″ wide and just shy of 4″ long (allowing for alignment holes at the top). The horizontal restriction in the FoV to 2.8’ is because the slit masks are held in a cylindrical shape to follow the curved telescope focal surface along the Y direction. As you move off the center of the field in X, the slitlets on a MOS mask become increasingly out of focus with respect to both the telescope (giving higher slit losses) and the instrument (decreasing the effective resolution). 


Keeping this in mind, spectral clipping is not flagged so the users must keep in mind the spectral regions of interest when setting up their masks.

Slit Constraints:

  • Auto slit is not functional with the curved slits
  • Slits must remain in the FOV
  • No overlapping slits
Auto-slit can run several times and with several input catalogs if desired, or several slit shape configurations. If using multiple catalogs, make sure your astrometry is consistent across your entire mask development.

Center Slit

This can be used to individually place slits on an image with an updated/calibrated WCS. The center slit option will significantly improve the slit positions for point sources and other relatively compact targets.

Toggle the “Center Slit” which opens the Pick Object window known from the Alignment target and Guide tar selection. Click somewhere with the left mouse button, hold the button while moving the mouse, wait until you see the target in the center of the Pick Object window and release the mouse button now. Centering may not be possible if the object extends beyond the Pick Object window. In this case, reduce the zoom factor of this window until the source is fully inside the window.



Copy Slit

This can be used to generated identical slit pairs offset by some vertical offset.  Selecting “Slit Copy” will open the Slit Copy Entry Form.  Enter the Slit Number of the slit to be copied (type, properties, length, width), and the vertical offset relative to the input slit.  Positive x will copy a slit x arc seconds above the original slit and negative will make a copy below.