Astronomical Spectroscopy - Physics - University of Cincinnati

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wants can be a real challenge.
2.3.2. Coude Spectrographs
Older telescopes have equatorial mounts, as it was not practical to utilize an altitudeazimuth
(alt-az) design until modern computers were available. Although alt-az telescopes
allow for a more compact design (and hence a significant cost savings in construction of the
telescope enclosure), the equatorial systems provided the opportunity for a coude focus. By
adding three additional mirrors, one could direct the light down the stationary polar axis
of an equatorial system. From there the light could enter a large “coude room”, holding
a room-sized spectrograph that would be extremely stable. Coude spectrographs are still
in use at Kitt Peak National Observatory (fed by an auxiliary 0.9-m telescope), McDonald
Observatory (on the 2.7-m telescope), and at the Dominion Astrophysical Observatory (on
a 1.2-m telescope), among other places. Although such spectrographs occupy an entire
room, the basic idea was the same, and these instruments afford very high stability and high
dispersion. To some extent, these functions are now provided by high resolution instruments
mounted on the Nasmyth foci of large alt-az telescopes, although these platforms provide
relatively cramped quarters to achieve the same sort of stability and dispersions offered by
the classical coude spectrographs.
2.4. Multi-object Spectrometers
There are many instances where an astronomer would like to observe multiple objects
in the same field of view, such as studies of the stellar content of a nearby, resolved galaxy,
the members of a star cluster, or individual galaxies in a group. If the density of objects
is relatively high (tens of objects per square arcminute) and the field of view small (several
arcmins) then one often will use a slit mask containing not one but dozens or even hundreds
of slits. If instead the density of objects is relatively low (less than 10 per square arcminute)
but the field of view required is large (many arcmins) one can employ a multi-object fiber
positioner feeding a bench-mounted spectrograph. Each kind of device is discussed below.
2.4.1. Multi-slit Spectrographs
Several of the “long slit” spectrographs described in § 2.2 were really designed to be
used with multi-slit masks. These masks allow one to observe many objects at a time by