Astronomical Spectroscopy - Physics - University of Cincinnati

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tometric standard. The star would be relatively near the zenith, and so knowing exactly
when they could get started was not critical, as the allowed tolerance on the rotator angle is
very large for good spectrophotometry. They moved the platform out of the way, and slewed
the telescope to the star. When the sky was judged to be sufficiently dark, they carefully
centered it in the slit and begin a 5 minute exposure. Since all of the exposures would be
short, they decided not to bother with the considerable overhead of setting up the guider, but
would hand guide for all of the exposures, using the hand paddle to tweak the star’s position
on the slit if it seemed to be slightly off-center. They observed two more spectrophotometric
standards, each time first moving the telescope to zenith, unstowing the platform, rotating
to the parallactic angle, restowing the platform, and slewing to the next target. There was
no need to measure radial velocities (a difficult undertaking with broad-lined stars) and so
a single HeNeAr comparison would be used to reduce all of the data during the night.
After each exposure read down, the data were examined by running IRAF’s splot plotting
routine in a somewhat unconventional manner. The dispersion axis runs along rows, and
normally one would plan to first extract the spectrum before using splot. Instead, the
astronomers used this as a quick method for measuring the integrated counts across the
spatial profile subtracting off the bias and sky level, by specifying splot image[1600,*] to
make a cut across the middle of the spectrum. The “e” keystroke which is usually used to
determine an equivalent width is then run on the stellar profile to determine the number of
counts integrated under the profile, listed as the “flux”. This neatly removes any bias level
from the counts and integrates across the spatial profile. This could be checked at several
different columns (500, 1600, 2200) to make sure there are good counts everywhere. After
things settled down for the night, the the observers were in a routine, and used ccdproc to
trim the data and remove the overscan. Flat-fielding would be left until they had thought
more about the fringes. Nevertheless, doslit could be used to extract the spectrum with a
wavelength calibration.
The observers began observing their red supergiant sample. The splot trick proved
essential to make sure they were obtaining adequate counts on the blue side, given the
extreme cool temperatures of these stars. Every few hours they would take a break from the
red supergiants to observe spectrophotometric standards, two or three in a row. By the end
of the first night they had observed 28 of their program objects, and 11 spectrophometric
standards, not bad considering the gymnastics involved in going to the parallactic angle.
Some older telescopes have rotators that are accessible remotely (CTIO and KPNO 4-meters)
while all alt-az telescopes have rotators that can be controlled remotely by necessity.
The analogous observations at CTIO were obtained similarly. Since the detector there
was smaller, three gratings were needed to obtain full wavelength coverage with similar