The FTS was designed and constructed by J.-P. Maillard for use at the infrared focus of the CFHT, and is bolted to the underside of the Cassegrain Bonnette. The focal plane is located 450 mm below the adaptor. The observer must specify on the Observing Time Request form whether the guide pellicle or mirror is to be mounted in the Bonnette. The choice depends on the nature of the observing program. The pellicle beam splitter permits guiding on the source directly while the guide mirror may be needed when off axis guiding with the Cassegrain Bonnette is needed, most often because of the faintness of the program target at optical wavelengths. In practice, the pellicle can be used for guiding on objects with V magnitudes brighter than 13 or 14 if the moon is not too close to the target area. Offset guiding is also possible with the pellicle but the area of the accessible guide field is somewhat smaller than for the guide mirror (for a more detailed discription see the discussion of the Cassegrain Focus in the CFHT Observers Manual).
The CFHT FTS is a stepped-scan interferometer. The movable mirror is held stationary at each sampling point and then moved rapidly to the next sampling position. The interferogram signal is produced by integrating the detector output signal during the time interval that the mirror is held stationary. The general optical layout of the FTS is shown in Figure 2.1. Two different light paths are indicated in this figure; the solid lines trace the paths followed by the sources on the sky, while the dashed line shows the path of the signal provided by the reference laser. A mirror (not shown) can be injected into one of the input beams just before the entrance aperture to direct radiation from a white light source into the FTS instead of a remote source. Not illustrated in the diagram is the path followed by the signal from a second laser used for alignment of the FTS.
A more schematic layout of the FTS is shown in Figure 2.2. The basic FTS design utilizes both inputs of the interferometer. The two entrance apertures have a projected separation of 52 arcsec and are oriented along the E/W axis when the Cassegrain Bonnette has a rotation angle of . An adjustable uncooled iris, with a maximum opening of 24 arcsec, is located at each entrance aperture. One aperture samples the object being observed, while the other samples the sky. As a result, the sky background is automatically subtracted from the source signal since the radiation from the two inputs arrives in antiphase at the detector because of the different number of reflections and transmissions along the two paths through the interferometer (just as the two output signals from a single source are in antiphase; see Section 1.2). Beam switching can also be performed for situations requiring extremely accurate background subtraction, such as for observations in the thermal infrared where the sky background becomes very important - at wavelengths beyond 2.5 the sky becomes increasingly bright, until longward of 3.5 it is brighter than most astronomical objects. Beam switching is also recommended in the H and K bands for sources with K magnitudes fainter than 8, particularly if a large aperature is used.
The mirrors directly beneath the entrance apertures deflect incoming light to collimators which, like all mirrors in the interferometer, are sapphire-overcoated silver. The collimated signals, which have a beam size of , are then directed to one of three beam splitters permanently mounted on a sliding carriage inside the spectrometer. The general characteristics of the three beam splitters are summarized in Table 2.1. The beam splitters are positioned using push buttons on the Auxiliary Control Panel mounted on the side of the FTS.
Each arm of the interferometer contains a cat's eye mirror, a typical example of which is shown in Figure 2.3. A cat's eye system consists of a focusing primary mirror, , and a secondary mirror, , with centers of curvature located at and respectively. If collimated light is incident on the primary mirror these retroreflective devices return the beam in the original direction, almost independently of the angle of incidence. As a result, the arrangement is insensitive to tilt adjustments and ensures alignment of the input and output beams. It also offers two other advantages: the output beam is displaced from the input beam so that both outputs of the FTS can be utilized, and a small path difference change in the interferometer can be realized without moving the entire cat's eye assembly. In the case of the CFHT FTS the secondary mirror of each cat's eye is mounted on a piezo-ceramic stack, allowing the path difference to be modulated at very high frequencies (Section 1.3). One cat's eye assembly (the `moving' mirror) is mounted on a screw-driven carriage with a travel, which therefore permits a maximum path difference of . This assembly is translated along the carriage by a DC motor. The other cat's eye assembly (the `fixed' mirror) is suspended from the optical plate and can move up to . The motion of this mirror is controlled with a loudspeaker motor.
The fixed mirror is mounted with Bendix flexural joints, a setup which is almost entirely free of friction; however, this is not the case for the moving mirror. Adjustments to the overall path difference are made by first moving the fixed mirror assembly. A position transducer monitors this movement and sends a signal to the DC motor which adjusts the position of the moving cat's eye by a similar amount.
The relative position of the moving cat's eye assembly is controlled by using a fringe pattern produced by a single mode He-Ne laser (with a reference wavelength ). The basic procedure for generating the reference fringes is briefly discussed in the next section.