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.