Selecting HUT Guide Stars

With the general exception of bright stars (V<10), it will be desirable to select HUT guide stars for most targets. For each guide star (up to three can be selected), fiducial marks will appear on the HUT TV camera at the expected positions. This will assist the payload specialist in identifying the field and quickly centering the correct object in the aperture. In certain cases where an object is too faint to be seen, this is the only way to assure the target is within the aperture. In cases where the object is bright, once the object is placed in the spectrograph aperture, the guide stars can be used to hold it there. Also, error signals for each of the guide stars are generated for use in pointing the telescope and for post-flight analysis.

Most of the leg work in obtaining information on potential guide stars for each object is accomplished by HUT team members who obtain the information from the HST Guide Star Catalog (HSTGSC). While absolute positions from the HSTGSC are sometimes off by as much as several arcsec, the relative positions (say target to surrounding potential guide stars) are typically much better (sub-arcsec). Likewise, magnitude information from the HSTGSC may only be good to $\sim$0.5 mag, but for ``random" field stars surrounding each target, its the only game in town.

Because the HUT TV camera has limited dynamic range at each of its gain settings, guide stars should not span a range of more than 2.5 magnitudes. In addition, to protect the TV camera, it must be set to accommodate the brightest objects in the field. This requirement leads to the restriction that the target magnitude entered in the sequence database file can be no more than 5 magnitudes fainter than the brightest object in the TV field of view. We must be particularly careful in the region around the entrance slits. When observing bright targets, there is always a chance that they will pop out of the slit area, so the mean guide star magnitude in a sequence database file may not be more than 3 magnitudes fainter than the target.

The HUT TV field of view is roughly $9'\times 12'$. Based on analysis of the HSTGSC data for each object, information on potential guide stars within this region will be entered into each HUT sequence database file. This includes magnitude and relative positional information, and a ``flag", initially set to ``no" in all cases, that indicates whether or not the guide star is to be used. The person responsible for each target is tasked with inspecting the field of each object and selecting up to three of the guide stars for actual use during the observation.

Because the HUT TV camera contains small geometric distortions, it is best to select guide stars reasonably close to the target (but >30$^{\prime \prime}$), and surrounding the target (as opposed to all on one side of the field) whenever possible. Because field rotation angle may make guide stars outside 4.5$^{\prime}$ radius unavailable, one should choose such stars with caution. Unfortunately, Mother Nature does not always provide ideal situations, and one or two guide stars, or guide stars all on one side of the TV field, are better than no guide stars at all.