Slitmask Alignment Times

1. Introduction

The metrics system can be used to estimate the time it takes for a typical slitmask alignment on LRIS and DEIMOS. Note that the difference in techniques between the two instruments might be expected to show differences in the alignment overhead. As we will show, this is indeed seen; the typical DEIMOS slitmask alignment time was less than half that of LRIS during the last year studied, 2003.

2. Procedure

We define the slitmask alignment time to be from the end of a slew to a new field, to the beginning of the first spectroscopic exposure. These times are readily available from the metrics system. The actual tasks involved include initial field identification, rough centering (until the alignment stars are in their boxes), and fine alignment (adjusting telescope and rotator positions to optimize the alignment). Some observers may also require a final, confirming image through the mask. The instrument must then be set up for spectroscopy.

The metrics database from 2001–2003 for LRIS and 2002–2004 for DEIMOS was checked for slitmask spectroscopy and the numbers tabulated. Rather than use averages, which might be expected to be unduly influenced by mistakes in the metrics database, or by rare but highly inefficient observers, we use the median alignment times for each year.

3. Results

The annual results, in the form of median alignment times, are presented below for LRIS and DEIMOS.

First note that the LRIS alignment times show a consistent drop over the three years covered. This is likely due to improved alignment techniques, tools, and experience. However, even in 2003 the alignment times are significantly higher than DEIMOS alignments times. This is likely a result of a number of causes, one of which is the ability to predict a guide star position on the DEIMOS guide camera in advance, virtually eliminating the period between field identification and fine alignment using xbox. While the use of pickoff mirrors on LRIS can serve some of this purpose, the results are not consistent enough to make LRIS alignment with pickoff mirrors as efficient as DEIMOS alignment. Many observers resists the use of pickoff mirrors, however.

Field identification is arguably faster on DEIMOS as well, since it uses guider images rather than the science camera (with its slower readout). Finally, reconfiguring LRIS for spectroscopy in some cases is significantly more time consuming than reconfiguring DEIMOS. In principle, LRIS could be even faster, using the blue side as the alignment camera.

Over the course of a year, the difference of 181 seconds per slitmask alignment, multiplied over the 649 alignments done in 2003, produces an extra 32 hours of overhead for LRIS. This is roughly three nights of observing time.