7.3 Planning FORCAST Spectroscopic Observations

During Cycle 4, grism spectroscopy with FORCAST will only be available in single channel mode. By default, observations will be set up using NMC aligned along the slit in long-slit mode and perpendicular to the slit in XD mode. Due to the size of the PSF, neither chopping or nodding along the slit nor dithering are possible for high-resolution XD observations. For long-slit observations, it is possible to use NPC, either chopping or nodding along the slit, but GIs are discouraged from doing so unless they provide adequate justification for doing so. Those wishing to use NPC should discuss the possibility with their SMO Support Scientist. GIs should also bear in mind that for sources smaller than the slit length, NMC yields slightly improved efficiency over NPC. For larger sources and for targets embedded in crowded fields, GIs are advised to use C2NC2 mode.

The observing efficiency for FORCAST spectroscopic observations is currently unknown, but is expected to be of order 25 -- 50%, depending on the chosen observing mode. These efficiency estimates are built-in to the SPT and do not need to be specified.

It is important to note that due to the fixed position of the grisms/slits in the filter/aperture wheels, the orientation of the slit on the sky will be dependent on the flight plan and will not be able to be predetermined. Further, the slit orientation rotates on the sky with each telescope Line-of-Sight (LOS) rewind. These limitations may be especially important to consider when proposing observations of extended objects.

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