Should this not be the case, Service observations are to be preferred. Pre-imaging needs to be as deep as to allow identification of all spectroscopic targets. It is expected that this objective will be reached in typical 5 minute long exposures, but use of the ETC is required for a proper assessment of the pre-imaging integration time. Pre-imaging integration time, including overheads, is to be accounted for by the users in their Phase I Proposals. Pre-imaging is done only in Service Mode.
Associated overheads have to be accounted for by the users.
Basically, the slits have to be oriented on sky in the average direction of the atmospheric dispersion during the observations. This clearly correlates the mask design to the hour angle at which the observations will be performed. This constrains in turn the position angle on sky, with the slits aligned accordingly. This constraint will apply to pre-imaging, since no image rotation will be allowed by the mask preparation software. Particular situations with special requirements on orientation on sky need to be clearly identified, as they will require special observing procedures, if at all possible.