Astronomical Spectroscopy - Physics - University of Cincinnati

– 23 –
Fig. 8.— Optical layout of Magellan’s IMACS. This is based upon an illustration in the
IMACS user manual.
2.4.2. Fiber-fed Bench-Mounted Spectrographs
As an alternative to multi-slit masks, a spectrograph can be fed by multiple optical
fibers. The fibers can be arranged in the focal plane so that light from the objects of interest
enter the fibers, while at the spectrograph end the fibers are arranged in a line, with the
ends acting like the slit in the model of the basic spectrograph (Figure 1). Fibers were first
commonly used for multi-object spectroscopy in the 1980s, prior even to the advent of CCDs;
for example, the Boller and Chivens spectrograph on the Las Campanas du Pont 100-inch
telescope was used with a plug-board fiber system when the detector was an intensified
Reticon system. Plug-boards are like multi-slit masks in that there are a number of holes
pre-drilled at specific locations in which the fibers are then “plugged”. For most modern
fiber systems, the fibers are positioned robotically in the focal plane, although the Sloan
Digital Sky Survey used a plug-board system. A major advantage of a fiber system is that
the spectrograph can be mounted on a laboratory air-supported optical bench in a clean
room, and thus not suffer flexure as the telescope is moved. This can result in high stability,
needed for precision radial velocities. The fibers themselves provide additional “scrambling”
of the light, also significantly improving the radial velocity precision, as otherwise the exact
placement of a star on the slit may bias the measured velocities.
There are three down sides to fiber systems. First, the fibers themselves tend to have
significant losses of light at the slit end; i.e., not all of the light falling on the entrance