Science Operations


Installed Masks

Each MODS reserves the first 12 mask slots for the standard facility slit masks. These are imaging stops, segmented long slits, and calibration masks that are always available in each MODS instrument. MODS1 and MODS2 are nearly identical, although MODS1 contains a DarkMask in slot1, while this slot remains empty in MODS2.


Position MaskID Description Files
1 DarkMask Dark Mask (uncut mask shell) [in MODS1 only] n/a
2 SieveMask 0.3-arcsec Pinhole Sieve Mask [Engineering] PDF | Gerber
3 Imaging 6×6-arcminute Direct-Imaging Field Stop n/a
4 LS5x60x0.3 0.3-arcsec Segmented long-slit PDF | Gerber
5 LS5x60x0.6 0.6-arcsec Segmented long-slit PDF | Gerber
6 LS5x60x0.8 0.8-arcsec Segmented long-slit PDF | Gerber
7 LS5x60x1.0 1.0-arcsec Segmented long-slit PDF | Gerber
8 LS5x60x1.2 1.2-arcsec Segmented long-slit PDF | Gerber
9 LS60x5 60×5-arcsec spectrophotometric “fat slit” PDF | Gerber
10 PinHole 0.4-arcsec Pin Hole [Engineering] n/a
11 Sudoku 9×9 Sudoku [Engineering] n/a
12 LS5x60x2.4 2.4-arcsec Segmented long-slit PDF | Gerber

Installed slitmasks are listed:

Masks scheduled to be installed in the near future are listed:

Mask Properties and Slit Tolerances

Masks are 230 ± 0.1 mm square to cover a 6 arcmin field of view. The mask curvature contains a 1040mm spherical radius which results in a 12.8mm sag from the center of the mask to the corner. This curvature is so they follow the curved telescope focal plane.

The best image quality is in the 4.5 arcmin diameter circle centered on the field. Outside this central region, the resolution and width of the spectrum will be somewhat degraded. The substrates that form the masks are electroformed on a spherical mandrel with NiColoy, which is a proprietary Nickel-Cobalt alloy used by NiCoForm with a nominal thickness of ~150 microns.

The properties and tolerances of the spectroscopic slits or apertures are summarized below:


Width 0.15-1.2mm
Length <10mm (typ)
Orientation ±30°
Shape Arbitrary polygon or circle
Width Tolerance ±15um
Position Tolerance ±15um
Edge Roughness ±1.8um RMS over 180um

Masks are often designed 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 reference stars) to about 1.8 millimeters (about 3”). If slits are too close, there is a risk of losing the bridge between them, which creates a very oddly shaped slits. If a user is unsure, please contact us (or have the PI contact us) prior to the deadline.

Mask Manufacturing and Laser Cutting

LBTO has a dedicated universal laser cutting workstation (Mask Manufacturing Unit aka MMU), which has been since installed at the University Research Instrumentation Center (URIC) located at the University of Arizona.

The GERBER file is converted to machine code (.pgm file) for the Mask Manufacturing Unit where the blank mask substrate is cut.

Mask Installation

Masks are installed in the instruments internal cassette upon request.

Mask exchanges for MODS are part of daily operations and can be performed for masks on hand on short notice, unlike LUCI which are scheduled well in advance.

A database listing all cut previously cut masks kept on site is available here, for users who desire to use previously cut masks and wish to check the availability.