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1 Slitmask Design & Fabrication

1.1 Overview

This chapter deals with slitmask design and fabrication, both from the observer's and the observatory's perspective. Under normal procedures, the user will produce slitmask designs, and the observatory will fabricate the masks.

The user initially designs a mask on the sky (for an relatively arbitrary epoch) which includes a target list and optional ``custom'' information     for each object, e.g., a particular position angle and/or size of slitlet. This stage of mask design must be highly interactive, and provide not only the positions of potential targets on the sky, but also provide the user with information on atmospheric dispersion, telescope limits, wavelength coverage at the detector, location of gaps in the detector mosaic, suitable guide and alignment stars, etc. -- anything that influences the placement of targets on the mask. Once the target list, position angle and mask center are settled, the final design is committed to a physical slitmask description for a specific     observing epoch. This physical description, or ``blueprint,'' includes corrections for refraction at a specified hour angle, as well as aberration of starlight, proper motion updates, telescope distortions, etc.

The target list and mask blueprint, packaged as a single FITS file, are forwarded to Keck for ingestion in the database and for mask fabrication. Fabrication involves translating the blueprint into instructions for the NC mill, running the mill, and inspection of the result. Tracking of masks is provided by barcode labels on the physical masks, which also provide the association of mask design in the database with a physical mask. The mask fabrication procedures are designed for minimum impact on Keck staff, both in mill operation and in handling a large volume of slitmasks for many different observers.

This chapter describes (a) the mask design software, up to production of the mask blueprint; (b) the fabrication software; and (c) the overall handling of physical masks by Keck staff.

1.2 Subsystem Figures

See Figure 1.1, ``Slitmask Design - Overview.''

 

  figure530


Figure 1.1: Slitmask Design - Overview

1.3 Nomenclature

Fundamental Sky Coordinates:
Celestial coordinates tied to a fundamental coordinate system at a specified epoch and equinox.

Refracted Sky Coordinates:
Fundamental Sky coordinates adjusted for refraction, aberration of starlight and proper motion.

Slitmask Coordinates:
Physical coordinates on the flat slitmask (which becomes a tilted cylinder when loaded in DEIMOS). Defined at the nominal operating temperature for DEIMOS.

Camera Focus Coordinates:
Coordinates in pixel units at the nominal focal plane of the camera, easily transformed to pixel coordinates on a given CCD.

Mask Design:
The mask center, position angle, and list of slitlets and objects to appear on a slitmask, in fundamental sky coordinates. (See Part I, Section 3.6.3; and Part II, Chapter 2 of the Dictionary for details.)

Mask Blueprint:
The description of the physical slitmask design in the slitmask coordinate system. (See Part I, Section 3.6.3; and Part II, Chapter 2 of the Dictionary for details.)

1.4 Component Modules

   

1.4.1 SIMULATOR

 

1.4.2 MAPMASK

 

1.4.3 FABMASK

 

1.4.4 PLOTMASK

 

1.4.5 SKYCOORDS

1.4.6 MaskAcceptor

1.4.7 MillControl

1.4.8 MaskQuery

1.5 Subsystem Procedures

 

1.5.1 Pre-Commissioning

Verify/update mapping of slitmask-to-CCD coordinates (original mapping is theoretical; check in lab).

CCD defect list should be prepared.

1.5.2 Commissioning

Verify/update mapping of slitmask-to-CCD coordinates (original mapping is from lab; check following reassembly).

Verify/update mapping of refracted sky-to-slitmask coordinates (``slitmask astrometry''; see Part III).

1.5.3 Operational Procedure

Mask Design (performed by user for each slitmask)

1.
Execute SIMULATOR to design mask
1a.
Re-execute SIMULATOR, as desired to refine mask design
2.
Execute MAPMASK
3.
Send mask DesignFile FITS file to Keck (email, FTP, WWW)
(4)
Optional: user may follow own path to slitmask fabrication

1.5.3.1 Mask Fabrication

The purpose of this procedure is to ensure the correctness of the association between the barcode on the slitmask and the mask blueprint which was milled into the mask. When a dozen masks are milled during one session we want to avoid any problems of off-by-one or otherwise permuted identification between mask and blueprint.

We recognize that the mill operator may be involved in other tasks during the fabrication of DEIMOS slitmasks. We have minimized the activity required between the milling of one mask and the next. This procedure also permits slitmask Quality Control to be performed separately from slitmask milling.

Hardware Components

These are used for the mask milling procedure.

Software Components

The software required for this procedure is a collection of programs known as MillControl. MillControl can perform the following tasks.

The flow of information starting from the time of mask design through the time of mask production has been entered into the Agents database. This flow is depicted in Figure 1.3. See Chapter II.7 for more information about the Agents and meme flow.

  figure716
Figure 1.3:   Data Flow for Slit Mask Production

Slitmask Stock Check-in

The slitmasks loaded in DEIMOS are identified via barcode labels. Prior to milling, a barcode label must be affixed to each blank slitmask in a location suitable for reading by the barcode scanner within DEIMOS. This will be done whenever a new shipment of blank masks arrives. Then MillControl must be run to enter the entire shipment into the database. For each shipment we record the vendor, various physical characteristics of the stock, and the first and last barcodes in the sequence applied to that batch. (Note that we require the sequence of barcodes to be monotonically increasing.)

Milling Setup

These steps are required at the beginning of a milling session.

Milling Procedure

For each blueprint to be cut the following steps are performed.

Note that we do not permit a milling program to be restarted if it is interrupted in the middle. We expect that, for example, restarting a program after replacement of a broken tool would introduce unacceptably large systematic errors between the first and last milling sequences. Masks which are partly milled must be junked.

Quality Control Procedure

The quality control (QC) procedure could be done as an element of the milling procedure, or deferred until there is an entire batch of cut masks. If it is convenient, the mill operator may perform the Quality Control procedure on the previous mask while the next mask is being cut. If QC is done subsequent to the milling then all masks will already have been scanned as NOW_MILLED so that the association between blueprint and mask barcode is already in the database.

The following steps are performed for each slitmask.

Responses of MillControl to a MaskId barcode

Whenever the scanner scans a slitmask barcode, MillControl queries the database to see if a Mask record with that MaskId already exists. MillControl will then wait for a QC barcode, and if one does not arrive within a few seconds it will take action as if QUERY had been scanned. If MillControl receives two successive slitmask barcodes without an intervening QC barcode it will ignore the first slitmask.

Responses of MillControl to various QC barcodes

If the scanner receives a QC barcode but has not recently scanned a slitmask barcode:

MillControl displays an informational message indicating the QC barcode which was scanned and that no action was taken.
If the scanner reads QUERY then MillControl does the following:
If the Mask is already in the database then present a detailed list of its known attributes. If the Mask is not already in the database then report that fact.
If the scanner reads NOW_MILLED then MillControl does the following:
If MillControl is not awaiting an identity then this is an error. If the Mask is already in the database with a non-null MaskQual then MillControl must flag this as an error and require the mill operator to perform a detailed verification sequence. Otherwise, insert a new Mask record into the database containing MaskId, BluId, and all milling information except for MaskQual. MillControl retains the blueprint in the queue of things-to-do but marks it so that it cannot be selected for another DNC download.
If the scanner reads MASK_BAD then MillControl does the following:
If the Mask is already in the database with a non-null MaskQual then MillControl must flag this as a likely error requiring a verification sequence. If it is in the database with a null MaskQual then update the Mask record setting MaskQual BAD. Otherwise, insert a new Mask record into the database containing MaskID, BluId, milling info, and a MaskQual of BAD. MillControl retains the blueprint in the queue of things-to-do and marks it so that it may be selected again.
If the scanner reads MASK_GOOD then MillControl does the following:
If the Mask is already in the database with a non-null MaskQual then MillControl must flag this as a likely error requiring a verification sequence. If it is in the database with a null MaskQual then update the Mask record setting MaskQual GOOD. Otherwise, insert a new Mask record into the database containing MaskID, milling info, and a MaskQual of GOOD. MillControl removes the blueprint from the queue of things-to-do, and sends e-mail to the owner of the blueprint indicating that the mask has been manufactured.
In case of failure of the barcode scanner, MillControl will accept the mask ID (barcode) and quality via keyboard entry rather than via scanner.

The important element of the Milling Procedure is that MillControl will not permit a new DNC download to be started until a mask barcode is associated with the blueprint for the previous DNC download. This forces the operator to scan each slitmask as it is removed from the mill. The mask status of NOW_MILLED permits complete flexibility in the scheduling of the quality control phase. The QC can be done subsequent to the milling of all masks, or it can be done while the mill is cutting another mask.

Slitmask Stock Check-out

Some observers may wish to cut their own slitmasks. In that case the observatory will ship a number of blank slitmasks to the observer in advance of the run. MillControl will enter the identity of the observer. It will then permit masks to be scanned and tagged as SHIPPED. When the requisite number of masks has been checked out MillControl will print out the destination information for shipping.

Retirement of Slitmask Stock and Data

It will eventually become necessary to dispose of the accumulated mass of previously used slitmasks. In order that the database of available slitmasks remain intact MillControl must be used to track the disposal. As each mask is discarded the mill operator must scan its barcode followed by scanning DISCARDED. The mask blueprint records for discarded masks may be deleted as part of the same operation.

1.6 Deliverable Documents

There are three main documents of relevance:

1.
a User Manual for mask design (the ``cookbook'');
2.
an Operator's Manual for slitmask fabrication, describing data ingestion, execution of MillControl, and procedures for slitmask fabrication.
3.
The Calibration Document, which will describe the coordinate mappings used in SIMULATOR and MAPMASK.

next up previous contents index
Next: 2 Acquisition & Slitmask Up: Part II: Subsystem Designs Previous: Part II: Subsystem Designs

DEIMOS Software Team <deimos@ucolick.org>
1997-06-13T00:18:19