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2 Acquisition & Slitmask Alignment

2.1 Overview

This subsystem is used to acquire a field and align a slitmask     with respect to the sky. In practice, field acquisition is performed by the Observing Assistant, using standard DCS commands to place user-supplied coordinates at a desired pointing origin. Therefore, the subsystem description here will focus primarily on the mask-alignment process, although one of the modules is useful for precise relative positioning for any purpose.

The modules described are XLATE and SMALIGN.

There is a dependency on the mask-design (database), specifically with the slitmask initialization procedure (see 2.5).

2.2 Subsystem Figures

See Figure 2.1, ``Mask Alignment Flow.''

 

  figure761


Figure 2.1: Mask Alignment Flow

2.3 Nomenclature

Alignment Star:
Preselected star or other object to be centered in an alignment box.

Alignment Box:
Small rectangular aperture on slitmask, in which alignment star is centered when the mask is properly aligned.

Guide Star:
Preselected star or other object to be placed at a particular location on the TV, either for object acquisition or guiding.

Database Extract:
A file on the local disk containing slitmask designs, extracted from the database by the slitmask initialization process.

2.4 Component Modules

   

2.4.1 XLATE

 

2.4.2 SMALIGN

2.5 Subsystem Procedures

 

2.5.1 Commissioning

Define the offsets between the telescope axis and the pointing origins for the slitmask, longslit, imaging field and TV centers.

Calibrate the actual slitmask-to-TV coordinate system mapping.

Calibrate the Position Angle zeropoint of the instrument to within about 0.1 degree (preferably much better than this).

2.5.2 Mask Initialization

Instrument Specialist loads the slitmasks in the instrument. Then, either the observer or instrument specialist selects the ``initialize'' option on the instrument GUI (Part I, Chapter gif). This causes the system to cycle through the loaded slitmasks, scanning barcodes and querying the database for the slitmask design associated with each mask. The slitmask info is placed on a local disk so that operation during the night does not depend on the availability of the database.

2.5.3 Mask Alignment Procedure

Expected Mode:

1.
OA acquires field, sets instrument PA
2.
Insert slitmask
3.
Translate to place guide star at its expected TV location
4.
Execute module SMALIGN

Fallback Mode (if placement of Guide star at expected TV coordinates fails to put the alignment stars within their boxes):

1.
OA acquires field, sets instrument PA
2.
Take direct image
3.
Examine image to get alignment star coordinates
4.
Execute module XLATE
5.
Insert Slitmask
6.
Execute module SMALIGN

2.6 Deliverable Documents

Instructions for the mask alignment procedure will be provided. These should be available in on-line form. Also, module SMALIGN will include built-in on-line help (eg., list of commands; sketch of normal operations; some troubleshooting).


next up previous contents index
Next: 3 Spectrograph Control Up: Part II: Subsystem Designs Previous: 1 Slitmask Design &

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