3. Slitmasks: Mask Design and Fabrication Software

3.1 Overview

DEIMOS gains a large multiplex advantage through the use of multiobject slitmasks, allowing spectra for typically 80 objects (per barrel) to be gathered at once. However, the masks must be designed and fabricated on a field-by-field basis, and thus constitute a changeable component of the spectrograph hardware which must be constructed for each new set of targets and observing conditions. The design, manufacture, handling and storage of slitmasks is a major component of DEIMOS operations, some of which is controlled by the astronomer and some by the Keck support staff.

The software for slitmask design and fabrication is described below. The operational steps at Keck (fabrication through installation in the spectrograph) are described in Section 3.10.

3.1.1 Goals

The following goals are included in the software design:

The process must be simple and quick enough to produce masks on a timescale of a few minutes, but must also be sufficiently flexible to accomodate the specific needs of all observers. Specific needs include user control of slit-length (above and below target), slit position angle with respect to vertical, and violation of default minimum slit length if requested by the user.
The process must incorporate features necessary to rapid mask alignment. We adopt the procedure of square apertures ("alignment boxes") centered on suitably bright stars in the field. (See Chapter 5.B for the mask alignment procedure.)
The process must produce a single file per mask design which can be associated with the mask(s) and stored for database/archival purposes. This single file will contain all information about the mask necessary for mask alignment, for automated spectral reduction software, for producing graphical illustrations of the mask design, and for archival purposes.

The procedure for mask design and fabrication is based on that developed at UCO/Lick for LRIS observers. These elements are included:

Mask fabrication is divided into logical blocks. These blocks are the selection of targets and appropriate slits ("mask layout and target selection"); mapping to the slitmask coordinate system; and generating the milling instructions.
Mask design is highly interactive; all other steps are as transparent to the user as possible.

3.2 Figures

Fig. 3.1 - Slitmask Design and Fabrication Flow shows the mask design/fabrication process from a user's perspective. (See also figure 9.2.6 for a Keck-oriented perspective)

Fig. 3.2 - Normal Keck Slitmask Operations shows the mask fabrication/handling process at Keck.

Fig. 3.3 - Optical Distortions shows the expected distortions at the CCD.

3.3 Nomenclature

Target list refers to the list of targets or objects (including alignment objects) for which slits will appear on a single mask. For each object, the list includes ID, RA, Dec, magnitude, priority/code, optional slit PA, and optional slit lengths. (Priority/code is used to identify alignment objects or to assign a relative priority to program objects for use in automated selection algorithms.) A target list will also contain a line for the mask position (center RA and Dec) and position angle. NB: we explicitly assume all coordinates refer to a single equinox and to the user's intended epoch of observation.

Mask design file refers to a file which describes a slitmask. This file includes a physical mask layout (slit locations and PAs in slitmask coordinates; and object locations in three coordinate systems - slitmask (in mm), CCD (in pixels) and celestial. (Slitmask coordinates refer to a physical coordinate system tied to the slitmask.) A single mask design file is associated with a slitmask design and is used for mask fabrication, mask alignment at the telescope, automated spectral reductions, and archiving. The mask design file is a text file so that it may be edited by the user. Appended to this file are sections describing the target list (and possibly the entire input catalog - TBD), for inclusion in the archive. (Current plans call for the locating-pin holes to be described in this file as well.) NB: we treat the mask design file here as a single entity, but in practice the information contained in the mask design file will be stored in the database and recalled as necessary. The `mask design file' has sometimes been called a `map file.'

The term astrometry will be used to refer to the mapping of celestial coordinates onto the focal plane of the telescope (ie, at the slitmask) and onto the focal plane of the camera (ie, the detector). Astrometry involves the mapping of the celestial sphere onto a cylindrical slitmask surface, and is complicated by telescope distortions (up to the telescope focus) and collimator/camera distortions between the telescope focus and the detector.

3.4 Major Functional Requirements for Slitmask Design

These are the major functional requirements for mask design:

To enable the user to efficiently fill masks with highest priority objects, there must be
- automatic target selection;
- easy visual means of adjusting the mask center and PA; and
- means to easily handle all ranges of input samples (eg., large sparse samples and small dense samples);
Allow specific slit lengths and/or PA for individual objects (as well as default values);
Include reference objects, such as alignment and guide stars;
Fill extra space on the mask with lower-priority `filler' objects;
Avoid the mask frame;
Provide checks on atmospheric dispersion and telescope limit violations; and
Must account for all astrometric transformations (see 3.11)
- mapping celestial sphere to cylindrical mask surface;
- differential refraction (depends on HA, atmosph. environ.); and
- telescope distortions.

3.5 Software Elements

3.5.1 Mask Layout/Target Selection (Initial Mask Design)

The mask layout/target selection program allows the user to cull a specific target list from a larger list of potential targets, and to select coordinates for the field center and a position angle for the mask. It provides an approximate layout of the slits on the mask without concern for precise astrometric mapping (see "mapping to focal plane", next). This software is graphical and highly interative, allowing the user to adjust the position and orientation of the mask with respect to the targets, add/delete targets, etc. There will also be automated target selection options. The input and output target lists will have identical formats so that the program may be run iteratively, either to build up a final target list over several passes (perhaps with different catalogues), or to modify an existing target list. All inputs and outputs will be in celestial coordinates. A secondary list (eg., of lower priority `filler' objects) may be input if desired. In addition, there will be practical checks (eg, telescope limit violations) and a display of the atmospheric dispersion effects relative to slit width/orientation.

Inputs include:

potential target list (ID/name, RA, Dec, magnitude, priority/code; optional fixed PA, slit lengths);
optional secondary target list;
initial mask PA, RA and Dec;
spectral range of interest; and
HA, grating dispersion, slit width, exposure time (these are needed for atmospheric dispersion and limit checking, but are not used for mask layout).

Output is a target list in the same format as the input list. The program will have the mask outline (including the positions of the locating pins) and guider field-of-view built in.

The exact list of options for this program may grow with time. Target selection can be extended to include, for example, selecting specific slit lengths based on magnitude, etc.

3.5.2 Mapping to Focal Plane (Final Mask Design)

This process takes a target list and mask position/orientation, and produces a "mask design file" using precise astrometric mappings. In addition, slit lengths will be automatically extended to use non-assigned regions of the mask, or shortened to avoid overlap. All slit length adjustments will be reported. The output mask design file will generally be sent to Keck for mask fabrication.

Inputs are:

final target list, including information specific to individual slits (PA and minimum lengths), alignment and guide stars and locating-pin holes;
HA, central wavelength, air temperature and pressure (for atmospheric refraction calculations);
default minimum slit lengths, alignment box size;
constraints (minimum spacing between slits, minimum distance to edge of slitmask).

Output is a "mask design file" which contains a complete description of the logical slitmask. The program will have the mask outline built in, and access to current maps of telescope and collimator/camera distortion.

3.5.3 Generation of Milling Instructions

This program has minimal inputs from the user (the mask design file, tool diameter). It generates the specific instructions (in AutoCad DXF format) required by the CNC milling device. Generally this program will be run by Keck personnel, but it is provided as part of the general software for users who wish to have masks fabricated elsewhere.

3.5.4 Miscellaneous

Mill control: software will be required to actually drive the milling machine. Such programs are commercially available.

Coordinate utilities: software will be needed to update catalogue coordinates for precession and proper motion to the epoch of observation, outputting a format acceptable to the mask-design programs.

Slitmask layout: software will be provided to generate a graphical illustration of the slitmask using the mask design file. Such illustrations will include target locations (science targets, alignment and guide stars) as well as slits. Both single-page and truescale formats will be available.

Astrometry: software will be provided for using the distortion maps and known astrometric reference stars (if any) to provide celestial coordinates of objects on DEIMOS direct images. While errors in the absolute coordinates may be rather large, the relative coordinates will be sufficiently accurate to allow the construction of target lists suitable for designing slitmasks.

3.5.5 Calibrations and Internal Data Sets

The physical outline of the slitmask is required for both the mask design and mapping programs. In addition, the outline and relevant position of the guider field are required for the mask design programs, so that suitable guide stars may be assured to fall in good (and known) locations in the guider.

Maps of the telescope distortion (at the mask surface) and the collimator/camera distortion (mask-to-detector) will be required by the mapping program, and perhaps also by the mask layout program. Telescope distortion, ie., mapping the celestial sphere onto the mask, will be derived analytically, with empirical corrections obtained from astrometry when the instrument is placed on the telescope.

Spectrograph distortions can be obtained (at any time) by imaging a special "grid-of-holes" mask. These distortions are needed to predict positions on the CCD array for the purpose of slitmask alignment.

3.6 Existing Software and Tools

Software for mask design, mapping and mask generation have all been developed at UCSC for LRIS. It should be relatively easy to adapt this software for DEIMOS. Several enhancements to the mask layout/target selection program are intended, however.

3.7 Other Resources Required

Commercial software for controlling the milling machine will be required.

3.8 Dependencies on Other Components

The mapping process requires access to current distortion maps in the database. The milling and mask `check-in' process at Keck will also interface with the database.

3.9 Outstanding Issues

Nothing major. We need to settle on the exact nature of "slits", how to describe them and how they will be machined. For example, can we assume all slits are parallelograms, or should we specifically allow for arcs and circular apertures (which can present significant difficulties at the data-analysis stage)? Also, exactly how the slit-widths are entered into the mask description is TBD. There are no technical difficulties here, however.

3.10 Miscellaneous

3.10.1 Steps to Mill a Slitmask and Enter in the Database/Library

All blank slitmask stock will be labelled with a barcode (at UCO/Lick); the barcode and a description of the stock (eg, thickness, material, surface finish, etc.) are entered into an inventory table in the database.

Normal operation: observer sends mask design files to Keck.

Mask design file information, describing a mask design, is entered in database.
Mask design is selected by mill operator.
Stock is mounted and milled.
After milling, slitmask is inspected, checked against an illustration, barcode-scanned, and a quality (eg. good/reject) is assigned. If the milling was unsuccessful, the previous step is repeated. Acceptable quality means that a mask design in the database is now identified with the physical slitmask. We refer to this process as `mask check-in.'
Before each run, the Instrument Specialist calls up the requested mask designs from the database and retrieves the slitmasks for loading in the spectrograph. At this point, reconstituted copies of the mask design files are placed in the observer's directory.

Outside fabrication: Some users may want to manufacture the slitmasks elsewhere. In this case, barcoded stock is sent to the user, and the user arrives at Keck with mask design files and milled slitmasks. The steps are identical to normal operations except that no milling takes place:

Mask design file information is entered in database.
The mask design is selected from the database. The slitmask barcode is scanned, and the mask is verified against the illustration (quality "foreign" is assigned). This constitutes `mask check-in' for masks fabricated elsewhere. The mask design in the database is now identified with the physical slitmask.
For the run, the Instrument Specialist calls up the requested mask designs from the database and retrieves the slitmasks for loading in the spectrograph. At this point, reconstituted copies of the mask design files are placed in the observer's directory.

3.10.2 Slitmask Handling

A typical DEIMOS observing run may use 100 or more slitmasks. There must be a temporary storage facility for the masks, designed to protect the slitmask from damage, and in which the barcode labels are easily accessible for scanning. Some sort of carousel has been suggested. [Long-term storage is TBD.]

There must also be a table large enough to lay out 20 slitmask frames so that the slitmasks can be mounted.

There must be a cart for transporting the masks to and from the telescope, with sufficient storage space for 20 mounted masks. The cart should actually be designed with extra storage slots, so that old masks can be unloaded and new ones loaded while preserving some kind of physical ordering.

There must be 4 sets of 10 frames (plus extras) - these should be color-coded, to make it easy to distinguish barrel 1 vs. barrel 2, and old vs. new masks.

The standard mask handling procedure is this:

The Instrument Specialist mounts the slitmasks in their frames and loads them (in order) in the cart.
At the telescope, the Instrument Specialist cycles through the slitmask-changing mechanism. At each position, he/she removes the old mask from the spectrograph, loads it in the cart, and places the appropriate new mask into the instrument.
When all the masks are loaded, a special "initialize" script on the spectrograph control user interface will cycle through the slitmasks, reading each barcode and making the necessary association between mask "slot" and the mask design information in the database.

Chapter 3 sections 3.1-3.10 were written mostly by Drew Phillips.

(phillips@ucolick.org)

Last modified: 19 Mar 96