Optimal slit orientation for long multi-object spectroscopic exposures

Historically, long-slit spectroscopic observations were carried out using the parallactic angle for the slit orientation if slit loss was an important consideration (either to maximize the signal-to-noise or to do spectrophotometry). This requires periodic realignment of the slit position angle as the parallactic angle changes. This is not possible for multi-slit observations where one slit position angle must be chosen for the entire exposure. Common wisdom suggests using the parallactic angle at the meridian (HA = 0). In this paper, I examine what the best strategy is for long, multi-slit exposures. I find that in extreme cases (very long exposure time) the best choice is to orient the slit perpendicular to the parallactic angle at the meridian. There are two effects to consider: the increasing dispersion with increasing airmass and the changing angle between the parallactic angle and the slit. In the case of traditional slit orientation, the two effects amplify each other, thus rendering a significant fraction of the observation useless. Using the perpendicular orientation, the two processes work against each other, thus most of the observation remains useful. I will use, as an example, our 8 h Lockman Hole observations using the Keck telescope, but generic methods are given to evaluate a particular observation. I also make the tools available to the community.