Astronomical Spectroscopy - Physics - University of Cincinnati

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align (and guide!) fiber instruments. In the case of Hydra, these functions are accomplished
by means of any of 12 “field orientation probes” (FOPs). These are each bundles of 5 fibers
around a central sixth fiber. These are deployed like regular fibers, but the other ends of
these fiber bundles are connected to a TV rather than feeding the spectrograph. Thus an
image of six dots of light are seen for each FOP. When the telescope is in good focus, the
centering is good, and the seeing is excellent, all of the light may be concentrated in the
central fiber of the six. The telescope is guided by trying to maximize the amount of light in
each of the central FOP fibers. In principle, a single FOP should be sufficient for alignment
and guiding, since the only degrees of freedom are motions in right ascension and declination,
and not rotation. But in practice a minimum of 3 is recommended. The assigned SMC fields
had 3-5 each, but for the bright radial velocity standard a single FOP was judged sufficient
as the exposure would be a few seconds long at most and no guiding would be needed.
Once the two new fibers were in place, the focal plane plate was “warped”; i.e., bent
into the curved focal surface using a vacuum. (The fibers had to be deployed onto the plate
when it was flat.) The telescope was slewed to the position of the radial velocity standard,
and the “gripper”—the part of the instrument which moves the fibers, was inserted into
the field. The gripper has a TV camera mounted on it in such a way that it can view the
reflection of the sky. Thus by positioning the gripper over a deployed fiber (such as a FOP)
one can also see superimposed on the image any stars near that position. In this case, the
gripper was placed in the center of the field of view, and the bright radial velocity standard
carefully centered. The gripper was then moved to the single “extra” FOP and the presence
of a bright star near that position was also confirmed. As the gripper was removed from
the field of view, the light from the single FOP was visible. The telescope was next focused
trying to maximize the light in the central fiber of the bundle.
While this was going on, the observers carefully checked the spectrograph configuration
using the spectrograph GUI. Was the correct blocking filter in place for the red Were
the grating tilt and other parameters still set to what they were in the afternoon When
the operator announced that the telescope was focused, the observers then took a 5 second
exposure. At the end, the CCD read down, and light from a single fiber was obvious. A cut
across the spectrum showed that there were plenty of counts. The voltage on the TV was
then turned down to protect the sensitive photocathode, the calibration screen was moved
into place, and both a projector flat and HeNeAr comparison arc exposure were made. The
first observation was complete!
Rather than waste time removing the two extra fibers (which would have required going
back to the zenith), the telescope was slewed to the SMC field for which the fibers had been
configured. The gripper was moved into the field, and sent to one of the deployed FOPs.