Astronomical Spectroscopy - Physics - University of Cincinnati

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Exposures of the dome flat quickly revealed that although there was plenty of light for
the red setting, obtaining a proper flat (one that would not degrade the observations; see
§ 3.2.6) in the blue would take on the order of days, not minutes.
Given this, an arguable decision was reached, namely that the observing would (provisionally)
rely upon the calibration screen flats obtained at each field during the night. There
were several arguments in favor of using the calibration screen flats. First, by having the
rest of the fibers stowed, and only the fibers in use deployed, the flat-field would be useful
in unambiguously identifying which fibers mapped to which slit positions on the detector.
Second, and more importantly, it provided a real-time mapping of the trace of each fiber on
the array. Third, it would cost little in overhead, as the radial velocities already required observing
the HeNeAr calibration lamps with the screen in place, and that most of the overhead
in the calibration itself would be moving the calibration screen in and out of the beam. The
definitive argument, however, was that the stars were very bright compared to the sky, and
so even if there was no sky subtraction, the science data would not be much compromised.
Had the objects been comparable to the sky values, the best alternative would have been
to do blank sky exposures, despite the use of extra telescope time. In any event, dome flats
in the red were run each afternoon as it provided a good chance to exercise the instrument
during the afternoon and ascertain that everything remained copacetic.
Prior to dinner, the observers configured the instrument to their first field. A problem
was immediately revealed: one of the assigned fibers did not deploy. Why Although
there are 138 fibers, several fibers have become broken over time or have very low throughput
and they are “locked” into the park position. A “concentricities” file is provided with
the software used to assign fibers and test the configurations, and after a little probing it
became clear that the concentricities file used in the assignments had been out of date.
Therefore, assignments for the entire thirty fields would have to be recomputed. Fortunately
most of the preparation work was simply in getting the software system set up, and before
dinner the observers had managed to get the first few fields recomputed, enough to get them
going, and the remainder were easily recomputed during the night. The new configuration
files were transferred from the observer’s laptop to the instrument computer, and the first
configuration was again configured, this time without incident. The observers began a series
of biases running and left for dinner.
Shortly before sunset the instrument assistant opened the dome to allow any heat in
the dome to escape. The first actual target would be a bright radial velocity standard star
(§ 3.2.7). Without disturbing the other fibers, the astronomers moved an unused fiber to the
center of the field, and deployed an unused alignment fiber to the location of another bright
star near the radial velocity standard. As discussed in § 3.2.3 there has to be some way to