FHiRE Mechanical Design
(left) SolidWorks model of the FHiRE optical bench and its individual
mechanical components. (right) A closeup view of the model.
For the Phase I implementation, the various sub-components of FHiRE will be
mounted upon carbon-fiber plates attached to a conventional vibration-isolated
optical bench via flexible fiberglass
struts. The struts isolate the low expansion carbon-fiber plates from the
higher expansion stainless steel mounting surface of the optical table. Small
clamps are used to ensure that the carbon-fiber plates are tied securely to
each other as the optical table expands and contracts with temperature
changes underneath. A slab of 2-inch thick insulation (not shown) will be
located between
the carbon-fiber and the optical table. A large, insulated cover (also not
shown) covers the entire optical bench and can be removed via a pulley system
when access to FHiRE is required. The optical bench will reside within an
insulated instrument room which is further insulated and accessible only via
an electronic combination. This ensures that the temperature of FHiRE's
components are kept as uniform as possible. This also ensures that the
wavelength
stability of the instrument is as constant as possble in order to reach FHiRE's
radial velocity performance goal of 10 m/sec, corresponding to approximately
10 nm on the detector.
Light enters the
instrument at the upper left via a 50-micron multi-mode optical fiber from the
GAM. A small manual focusing stage allows the fiber (not shown) to be focused.
The light leaving the fiber diverges and reaches the an off-axis
paraboloid that collimates the light. Near the fiber is a shutter used to
control the exposure time via a signal from the MONSOON controller and set by
jthe user interface.
The FHiRE collimator mirror is one of the primary optical components of the
instrument. The large size of this mirror requires considerable attention to
its mounting and alignment controls. The mirror is contained within a box-like
aluminum structure and held in place with a combination of
small springs that hold the mirror against three alignment screws located on
the
back of the mirror mount. The three alignment screws push against three
ceramic pucks glued to the back of the collimator mirror providing a kinematic
mount. Similar ceramic pads are glued to the top and bottom of the mirror
and against which the ball-spring plungers push in order to provide radial
support and registration of the mirror. The entire mirror mount can also be
positioned, though with low precision, upon its carbon-fiber support plate
by using small adjustment screws.
Upon reflection off the collimator the light is now a parallel beam with a
98-mm
diameter. This collimated beam travels to the FHiRE grating. The grating is
large, 204-mm by 408-mm and 75-mm thick. Being an echelle, it mounted so that
the light strikes it at a 74-degree angle of incident forming a large
ellipical footprint on the grating. The grooves of the grating run horizontal
so that the light reflected from the grating back to the collimator is
dispersed in wavelength in the vertical direction. The large weight of the
grating requires that it be carefully, yet securely mounted. The grating is
contained within an aluminum box and rest upon three teflon pads. Two
cermaic pads will be glued to two sides of the grating such that ball-spring
plungers push the grating against reference stops on the opposite sides of the
grating box. The grating box also features two shafts pentrating through
each of the long sides near the center of balance of the grating and box. An
outer wedge-like box, also of aluminum, features two bearings for
mounting the grating box via its two shafts. Two springs pull the inner
grating box against a micrometer mounted on the outer wedge enabling the
grating tilt to be precisely adjusted. The resulting assembly is mounted
upon a pivot within its carbon-fiber support plate. The grating mount can be
adjusted in azimuth via two small screws that push against the grating mount
to provide for optical alignment.
The dispersed light from the grating reflects off of the collimator a second
time but because of the azimuth tilt of the grating it is focused upon a
"white pupil mirror" adjacent to the shutter and the entrance fiber. The light
then reflects back to the collimator mirror where it is re-collimated as a
set of monochromatic beams spread according to wavelength. This third and last
reflection sends the light into the camera via a folding flat. A volume-phase
holographic (VPH) grating is used to disperse the light orthogonal to the
dispersion of the echelle grating in order to separate the overlapping
grating orders produced by the echelle. The VPH grating is mounted within a
box frame and rests on a thin sheet of teflon. The frame contains two small
shafts that are mounted within bearings within the frame. Two
small screws are used to align the grating such that the spectral
orders can be accurately positioned upon the detector.
The light from the VPH grating then enters the FHiRE camera which images,
i.e., focuses, the spectral orders upon the CCD detector. The camera is quite
large, with an aperture of about 100-mm. It consists of six elements made
from various types of optical glass in order to provide excellent image
quality over the detector. The lenses are mounted within individual lens
mounts with the mounts pinned together with spacing tubes to form the
complete camera assembly. The optical axis of each lens will be aligned to the
mechanical axis of its mount using nylon-tipped set screws as the mount
is rotated upon a precision air bearing. Once the screws are adjusted the
gap between the lens and its mount will be filled with space-grade silicone
adhesive. Once the adhesive has curred the screws are withdrawn and the optic
secure within its mount. By using CNC machining, each mount will be made to be
accurately fit together via alignment pins between each sub-component. The
resulting camera assembly will be held within support rings mounted to plate
resting upon its carbon-fiber support plate.
The light emerging from the camera lens enters the vacuum dewar containing the
CCD detector array. The dewar features a window to allow light to enter
while enabling the dewar interior to be cooled to 100 K via liquid
nitrogen (LN2). The instrment dewar rests upon a linear stage for focusing
with this stage in turn mounted upon a pivoting plate to allow the dewar and
its detector to be
tilted with two small screws in order to produce the best focus over the
detector. The VPH grating, the camera and the dewar are all mounted upon a
single carbon-fiber plate attached to the optical bench.