The CFHT FTS uses two nitrogen cooled dewars mounted at each of the
interferometer outputs. Each dewar contains a hybrid type Cincinnati
Electronics InSb photovoltaic detector with a 
feedback
resistor. With a band gap width at 77 K of 0.23 eV these devices are
sensitive to radiation with 
5.5
. A typical response curve for such a device
is shown in Figure 2.4. During standard setups the nitrogen chamber is
pumped
and the detectors are operated at solid nitrogen temperatures (
K).
Three
preamp gain settings are available, and the appropriate
setting is selected based on the brightness of the source being
observed. The
lowest setting should be used for extremely bright objects,
the middle setting is for objects of intermediate brightness, while
the highest setting is intended for the faintest objects.
Four aperture cold stops are available with projected sizes of
2.5, 5.0, 8.0, and 12.0 arcsec. On special request a different size
aperture can be mounted on one of the two largest aperture stops.
Each dewar also contains a 10 position filter
wheel, of which 9 positions are available for filters. A current list of
infrared filters in stock
at CFHT for use with the FTS is given in Appendix A. If custom filters
are needed they should be
ordered in pairs (one for each dewar) and should have a minimum diameter
of
1 inch and a maximum thickness of 6 mm. Filters should also be coolable
to
liquid nitrogen temperatures, and have central wavelengths and
bandwidths specified at this temperature.
Each dewar contains a 
thermistor to monitor the temperature
of the detector. The temperature can be measured using an ohmmeter from
BNC
connectors located on the side of each preamp electronics box. The
electronic
boxes are blue in color, and are mounted immediately in front of the
dewars. When the nitrogen in the dewars is at atmospheric pressure, the
resistance should be in the range 
. Note that the two dewars
may give slightly different (i.e. 
) readings. When the nitrogen
is pumped to low pressures, the thermistors should register values in
the
range 
; values less than this indicate that the amount of
pumping on the nitrogen reservoir is too great and should be reduced
using the `throttle valves' on the Cassegrain Bonnette vacuum manifold
(see Section 4.2). As a reference, these valves should be opened about
two or three turns from their closed positions.