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.