5.B Object Acquisition: Slitmask Alignment

5.11 Overview

Slitmask alignment is an extension of object acquisition, at a much higher level of precision. For direct imaging, positioning of the telescope to within a few arcseconds is usually sufficient. For single-slit observations, positioning must be precise to within a fraction of an arcsecond perpendicular to the slit width; usually the position angle is unimportant or perhaps accurate to, say, 1 degree. On the other hand, multislit observations require positional accuracy of order 0.1 arcsec in both axes over the entire length of the slitmask. Our problem is to position the telescope and instrument rotator to within the tolerances in a simple and efficient manner, with minimal operator involvement.

Slitmask alignment requires the use of both the guider and the DEIMOS CCD array. The guider is used for approximate positioning during the initial stages and for autoguiding during the final adjustments.

Goals

The following goals are included in mask alignment design:

The alignment process must be fast. Ideally, we would like an acquisition and alignment overhead of only a small fraction of a typical science integration. We set a target of 8 minutes for the process.
The process must be simple. All user inputs should be kept to a minimum, and as much as possible should be limited to oversight of automated routines.
The process must be accurate. Tolerances should be a fraction of the slit width -- of order 0.1 arcsec rms.

The procedure described here is based on that developed for LRIS slitmasks by the DEEP group at UCSC. It uses alignment boxes and alignment objects (stars) to position the mask against the sky, by centering the stars within the boxes. This approach has the following advantages:

Alignment stars are chosen to be fairly bright, so that relatively short exposure times (of order 30s) are sufficient to obtain accurate positions;
The mask remains installed throughout the procedure -- no overhead in moving the slitmask and no repeatibility worries;
Only a small area of the detector is involved, so we may employ subraster read-out of the CCDs to speed the process.

Steps in the Alignment Procedure

Suitable alignment objects are selected during mask design and corresponding alignment boxes cut during mask fabrication. To set up on a mask at the telescope:

The field is acquired and PA is set; the spectrograph is placed in direct imaging mode and the mask inserted. The guide star is placed in its expected position on the guide camera, and guiding is started. (This should be sufficiently close to correct alignment that the alignment star images will fall in the boxes on the mask.)
The observer takes a direct image through the mask. For DEIMOS, we will need to read out and analyse only those small regions near the alignment boxes.
The location of each star relative to its alignment box is measured, and a solution to delta-RA, delta-Dec and delta-PA is determined. These offsets are sent to the guider/offset control and the offsets applied.
Repeat the last 2 steps as needed until the alignment stars are centered in their boxes.
The grating is moved into place and the science integration begins.

With LRIS, CCD positions for the boxes are obtained from direct images through the masks taken with calibration lamps during the daytime, but in principle we should be able to adequately predict the CCD positions of alignment boxes.

The centering algorithm used by the DEEP group for LRIS is based on edge detections for both the stellar image and the alignment boxes. The only required inputs are star FWHM and box FWHM in pixels. For each star/box pair, a plot shows both x- and y-profiles through the box; the user sets a single sky level and the algorithm finds both the box and star centers. Generally only two keystrokes are needed for each alignment star/box. After all the alignment stars are examined, the solution is shown graphically and the user must simply use a single keystroke to exit. The entire alignment solution takes well under 1 minute to execute once the image is available for analysis.

Using this general procedure, we have performed successful alignments of LRIS in under 20 minutes, from the end of one science integration to the start of the next. LRIS requires close to 10 minutes for two grating-mirror changes and a mask-to-mask change. DEIMOS overhead for the equivalent operations will be about 4 minutes both because of simpler grating motions and because stages will run in parallel. Furthermore, it is likely that specialized read-out for DEIMOS alignment will be shorter (15 sec or less) than the entire LRIS read-out (40 sec?). Based on our experience, it seems likely that we may acquire and align on a field within our target of 8 minutes (not including telescope slews).

Note that the mask in the second barrel (when constructed) must be aligned relative to the first mask, ie., by internal motions rather than by repositioning the telescope and instrument rotator.

5.12 Figures

Fig. 5.15-1 -- Alignment Box/Star Centering Algorithm.

5.13 Nomenclature

5.14 Specific Functional Software Requirements

The needed functionality already exists for LRIS, with the exception of communicating the offsets directly to the telescope and instrument rotator. All of the steps will be executed by a single command.

Find precise box location (must be robust, but box size and approximate location are known a priori).
Find precise star location within box (must be robust; star FWHM is roughly known a priori).
Derive offsets and rotation from the values above. This function must be interactive in an easily visualized fashion, so that spurious points can be detected and removed easily.
Communicate offsets and rotation directly to telescope and instrument rotator. (We envision that the Observing Assistant or observer will be required to give a confirming "go ahead" command from the guider.)

The mask alignment for the second barrel differs only in that commands must be sent to motors in the spectrograph which reposition the mask.

5.15 Design Notes

With a priori knowledge of the size of the alignment boxes and approximate FWHM of the alignment stars, a very robust centering algorithm is available (similar to that used in the IRAF "identify" task). The profile is convolved with the profiles shown in Figure 5.15.1, and the zero-crossing indicates the center. This method weights toward the edges of the features (stars or box) provided the FHWM is appropriate. It is insensitive to errors in background level and the presence of most cosmic rays.

5.16 Existing Software

As noted above, comparable software for LRIS has already been developed at UCSC. Except for communication with the DCS/instrument rotator, no significant modifications would be required.

5.17 Additional Resources Required

(none)

5.18 Interfaces with Other Modules

The software must access the mask design and the current distortion maps in the database (Chapter 9) to get the approximate locations of the alignment boxes on the CCD array. It must also communicate with the "offsets" section of the guider or the DCS. The software will be interfaced with the image display in order to display residual vectors on top of the alignment object images.

5.19 Outstanding Issues and Concerns

(none)

Last modified: 13 Mar 96

phillips@ucolick.org