Slitmask Alignment Procedure

Working Draft! See also the new Cookbook

Overview

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.

Using the procedure described here, 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.

Steps in the Alignment Procedure

NOTE: the following has been largely supplanted to use the new xbox task -- see the Cookbook.

Suitable alignment objects are selected during mask design and corresponding alignment boxes (typically 4 arcsec square) are cut during mask fabrication.

Afternoon Preparation

In the afternoon, take a direct image of the dome (or internal lamp) through each mask, and note the locations of all the alignment boxes. These need to be accurate to +/- 2 or 3 pixels. Put these coordinates into a text file for each mask (format x,y,ID; one line per box), eg:

# box centers for Mask A:
1121.  1718.   Align_star_1 
1308.  1672.   Align_star_2 
 996.   312.   Align_star_3

Use implot to measure the width of the boxes and epar qbox.

(After I have the text files and box widths, I like to run "qbox" on each image (letting it find whatever position for the star) to verify the box locations. Type "I" anytime to interrupt.)

Alignment

In the script below, I have assumed science integrations are being taken in single-amp mode; two-amp readout is faster and is preferred for alignment.

To set up on a mask at the telescope:

Slew to position and set correct PA. Place the instrument in direct imaging mode; change to two-amp read-out.
Take an image of the field. Identify an alignment star and use "mshift" to calculate the needed offset to the alignment box location. Insert the mask, and start guiding on the guide star.
Take a direct image through the mask -- stars should be visible in the boxes.
Use "qbox" to find all the star/box positions and produce the solution. Communicate the incremental offsets to the OA, or apply them yourself if you've printed out the DCS commands.
Repeat the last 2 steps as needed until the alignment stars are centered in their boxes.
Move the grating into place. Change back to one-amp read-out and begin science integration.

qbox

The script task qbox calls the two spp tasks mboxfind and maskalign. The calling sequence is simple:

qbox lris0054 maskA.box

... where lris0054 is the IRAF image taken through the slitmask,
and maskA.box is the text file with the box centers.

The centering algorithm is based on edge detections for both the stellar image and the alignment boxes. The only required inputs are star FWHM and box size 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. 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 below 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.

Pickoff Mirror Guide Star TV Coords

If you know the x,y coord (in arcsec) of the pickoff mirror star, you can predict the position in TV pixels on the slit-viewing camera. The formulae below assume 2x2 binning (ie 0.5" pixels) on the TV camera.

TVx =  85 - sim(y) / 0.5   (sim(y) is negative on mirror 2)
TVy = 204 - sim(x) / 0.5

NB -- these numbers have been derived from real data but not yet tested. Nicole Vogt at UCSC is developing a more accurate calibration of these numbers, and observers using her mapping have reported excellent results.

Andrew C. Phillips / Lick Observatory

Last modified: 06 jun 97

phillips@ucolick.org