Science Operations


Mask Overview

The LUCI instruments feature cryogenic MOS units housing a set of 10 “permanent” facility longslit and calibration masks, as well as another 23 exchangeable user-designed MOS slit masks. Mask exchanges to insert for each LUCI are performed based on demand.

See the table below for a list of “permanent” masks, and click here to view the current user-designed masks loaded in LUCI.

Mask Position LUCI1 UI Mask Name LUCI2 UI Mask Name
1 OptiSpec_Sieve OptiSpec_Sieve
2 N30LS_0.13arcsec N30LS_0.13arcsec
3 N30_FieldStop N30_FieldStop
4 Blind Blind
5 LS_2.0_1.5arcsec LS_2.0_1.5arcsec
6 LS_1.00arcsec LS_1.00arcsec
7 LS_0.75arcsec LS_0.75arcsec
8 LS_0.50arcsec LS_0.50arcsec
9 LS_0.25arcsec LS_0.25arcsec
10 Pixelflat_30 Pixelflat_30

The combined 2.0″ and 1.5″ slits are each 100″ long. There is an AFC spot at the center of the mask. The offsets from the mask center AFC spot to the slit center is 56.7″, with the 1.5″ slit below center and the 2.0″ slit above center.

Masks and dates for upcoming cabinet exchanges are listed here:

Mask Properties

Masks are 162±0.3 mm square to cover the 4 arcmin square field and allow for mounting within frames. The mask material is bent into a cylinder with a radius of 1030mm such that the full length of the long-slits remain in the telescope focus.

In 2018 we replaced all of the normal long slits (0.25, 0.50, 0.75, and 1.00 arcsecond widths) with new ones that include spots at the top and bottom that will allow active flexure compensation. These holes in the masks, plus implementing a new readout mode (sample-up-the-ramp) and other software changes, will allow tracking and correcting the internal flexure of the instrument during long spectroscopic exposures. As a result of the AFC spot inclusion, the usable length of the long slits has been reduced from 230 arcseconds to 205 arcseconds. There is quite a bit of work that still needs to be done before this is released as a regular operational mode, but it should be a significant improvement over the passive flexure algorithm currently in use.

Mask Positions

A MOS “robot” is used to move the masks between storage and the FPU. For the MOS status, note that there are three valid positions for a mask to be in: no mask in use when all masks are in storage (in the cabinets), mask in FPU when the named mask is locked in the FPU and ready for use in science observations, and mask in turnout when the mask is held in a staging position near the FPU during MOS or longslit acquisitions. This staging position is used to reduce the amount of time waiting for the mask to be moved into the FPU at the appropriate time. While the mask is in turn out, the mask does not obstruct the light path. However there is a holding current required to hold the mask in this position. This current can slowly warm components in the instrument. It is not recommended to leave a mask in turnout for longer than 10 minutes.

Custom masks are exchanged a few times per semester to accommodate different partner science programs. Each exchange involves installing the framed masks into a cabinet in an auxiliary cryostat. This cryostat is evacuated and cooled, bringing the masks to the same conditions as in the LUCI’s. All while maintaining cryogenic conditions, an empty auxiliary cryostat is mated with the LUCI to remove the old masks and the new cabinet is transferred to LUCI from the second.

The cabinet exchange process is scheduled well in advance. As each LUCI only houses 23 masks in the exchangeable cabinet, each partner has a mask allocation. Mask designs for use in a given time period must be delivered by strict deadlines to ensure that they make it in for the cabinet exchange.

Preparation and Submission

Custom multi-object masks can be designed using the LMS software, which may be downloaded here. Details and hints about the submission and preparation of Masks can be found on the Mask Preparation page.

LMS produces as output a Gerber file as well as an .lms file. The Gerber file is uploaded to our laser milling machine to cut the masks. The masks are cut out of 150 µm thick rolled stainless steel sheet that has been painted at black on the side facing into LUCI to reduce reflections. The .lms file is used by the LUCI observing software to know where the slits and alignment stars should be for mask alignment on sky.