Astronomical Spectroscopy - Physics - University of Cincinnati

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red (including at least 7770-8700Å) but also in the blue to obtain some of the classical luminosity
indicators used for Galactic yellow supergiants. To observe each fiber configuration
twice, on “blue nights” and “red nights” would require twice as many observing nights, given
the large overhead in each fiber configuration.
The fibers feed a bench spectrograph mounted in a dark room a floor below the telescope.
Changing the grating tilt might require refocusing the spectrograph (a manual operation,
impractical at night) but simply changing blocking filters could be done remotely. If the
filters were of similar thickness, and if the camera’s focus was fairly achromatic (which
would be expected of a Schmidt camera) then one could configure the fibers, observe in the
red, and simply by changing blocking filters, observe in the blue. No one was quite sure if
this would work, as no one could remember the spectrograph having been used this way, but
it would be easy enough to check on the first afternoon of the run. A 790 line/mm grating
blazed at 8500Å in first order was available, and would yield 2.6Å resolution in the red in
first order, and 1.3Å resolution in the blue in second order, providing wavelength coverage
of 7300-9050Å in the red and 3650-4525Å in the blue. Obviously the red observations would
require a blocking filter that removed light 7300Å. Among the available filters, an OG515
did an excellent job in the first case, and a BG39 did a good job in the second case while
still transmitting well over the region of interest (see Figure 3). (The same argument was
presented above in § 2.2.1.)
Prior to the observing run, thirty fiber configuration fields had been designed in order
to obtain as many of the target stars as possible. Since bad weather is always a possibility
(even at Cerro Tololo) the fields were designed in a particular order, with field centers chosen
to include the maximum number of stars that had not been previously assigned. Although
the fiber configuration program is flexible in providing various weighting schemes for targets,
it was found necessary to slightly rewrite the code to allow for stars that had been previously
assigned to be added “for free”, i.e., without displacing any not-yet assigned star. (It helped
that the first author had written the original version of the code some years back.) The
process took a week or more to refine the code, but the assignments themselves then were
straightforward.
The first afternoon at the telescope, the astronomers arrived to find that everything
appeared to be in good shape. Instrument support personnel had inserted the grating and
blocking filter, checked the grating tilt, and had focused the spectrograph, substantiating the
fact that the focus was unchanged between the red and the blue setups. A comparison arc
had been used to focus the spectrograph, and examination confirmed the expectation that
at the best focus the spectral resolution covered about seven pixels. The observers decided