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.''

 

 

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

Function
Calculate and apply telescope offset to translate to .
Language
IRAF/spp
Type
tool
Author
A. C. Phillips
Revision
N/A
Essay
This basic module takes an input (x,y) in CCD pixels (``observed'' coordinates), and an instrumental position angle and target (x,y) in pixels, and calculates the necessary telescope offset in arcseconds to translate from the observed to target locations. The user is queried for permission to apply these offsets, and DCS commands are issued upon confirmation.
For mask alignment, the observed coordinates are those of an alignment star, and the target coordinates those of the corresponding alignment box. In practice, the positioning of the guide star at its predicted TV coordinates should be accurate enough that XLATE will not be required.
The source of the input parameters (input and target coords, PA) may vary. For example, the input (x,y) may be selected by mouse-click on the image display, the PA taken from the image header, and the target coordinates preselected from the database extract. Another source for input (x,y) might be a centroiding routine. Initially, we will provide only the simple interface - that is, the user enters all parameters by hand.
This module duplicates the functionality of modifying DCS keywords INSTXOFF and INSTYOFF, but is provided as an easier means for the observer to achieve the desired moves.
Inputs
observed/target CCD coord pairs; PA of observation
Outputs
printed telescope offsets; DCS commands
Deployment
Commissioning
Labor
20 hours

Keywords used by module
Modifies DCS keywords: RAOFF, DECOFF, REL2CURR
Waits on DCS keywords: AXESTAT

 

2.4.2 SMALIGN

Function
Calculate and apply offsets to telescope/rotator to align masks
Language
IRAF/spp
Type
tool
Author
A. C. Phillips
Revision
N/A
Essay
This interactive module analyzes an alignment image (that is, a direct image through the slitmask). It first calculates the relative position of each alignment star within its box, and then solves for rotational/translational offsets. The user is queried for permission to apply these offsets, and DCS commands are issued upon confirmation.
The user initially specifies an image to be analyzed. The image header is checked for slitmask name, and the alignment box locations for that slitmask are read from the database extract. [Alternatively, the box locations could be calculated from the subraster specifications, also in the image header.] Instrumental PA also comes from the image header. During the first interactive phase, the module presents x- and y-profiles across each alignment box. The user interacts by setting a sky level, and a centering algorithm finds the alignment star's position within its box. Options exist for ``forcing'' a position if the algorithm fails. Upon exiting this phase, the differential offsets (star-box) are compared to the box locations across the mask, and a rotation and translation are solved. Residuals to this solution are displayed in the second interactive phase. The user may modify the sample of star-box pairs to provide a more credible solution, if necessary. Upon exiting, the actual telescope RA, Dec and PA offsets are calculated, and DCS commands executed upon user confirmation.
Inputs
image name [FITS header provides needed PA, subraster coordinates]
Outputs
printed telescope/rotator offsets; DCS commands
Deployment
Commissioning
Labor
80 hrs

Module flow diagram
Figure 2.2, ``Module SMALIGN.''

 

 


Figure 2.2: Module SMALIGN


Keywords used by module
Reads FITS header keyword: ROTPOSN
Modifies DCS keywords: ROTDEST, ROTMODE, RAOFF, DECOFF, REL2CURR
Waits on DCS keywords: ROTSTAT, AXESTAT

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 ). 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).