The "Locate_mode" parameter is used to specify how a given object is to be acquired. Whenever an object is bright enough to be seen on the HUT TV (basically V < 15), one should use "source locate." Some complex fields may require more detailed attention by the operator and the use of "manual locate" mode. For faint objects or extended nebulosity one has to position the slit blindly relative to your guide stars, and so "guide star locate" mode is used. Finally, "none locate" is used occasionally when another team's source is so faint for us that it doesn't make sense for the HUT TV to even acquire anything.
The SP_mode parameters are used to specify the mode by which the spectra will be read out. Normal usage modes include histogram (1) for count rates above 500 cts/s, and "high time" mode (4) where each photon is time-tagged (<500 cts/s). (Other modes are mostly used for calibration or instrument testing.)
A selection of five slit sizes is available for Astro-2 (see Appendix B). The 20" circular slit is the nominal slit to use for point sources, although 12" and 32" circular slits are also available. If IPS pointing is particularly good, we may switch to the 12" slit to cut down airglow (both intensity and line widths) for point sources, and if IPS pointing is very bad, the larger slit is available. Also, for diffuse sources two rectangular apertures are available (10" x 56", and 19" x 197"). Again, the narrow slit gives the best performance in airglow rejection. A 32" circular aperture with an Al filter is also available for rejecting the first order and observing in the 425 - 700 Å range (although the sensitivity in this mode is very low -- see the HUT Handbook).
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