SITELLE Components

This page details the different components of SITELLE (optics, mechanics, detectors, ...).


SITELLE uses a single beam-splitter with flat return mirrors and that beam splitter is tilted away from 45 degrees, to achieve a very compact footprint for a 2-port interferometer. The alternative method of using retro-reflecting cubes for the return beams would have made the instrument significantly larger and costlier due to the very tight surface and angular requirements on the retro-reflector cubes. In the adopted design, the requirements on the beam splitter become very tight due to the risks of ghosts from the air gap in a conventionally fabricated beam-splitter. This was overcome by optically contacting the two halves of the beamsplitter in SITELLE; a very risky process on such a large optical part that must maintain lambda/30 flatness. The beamsplitter was fabricated by Zygo.
CaF2 was used in one collimator lens and three of the camera lenses. The collimator/camera lens combination maps a 91.8 mm x 92.5 mm input at the collimator focal plane onto the 30.72 x 30.96 active CCD image area. The camera focal ratio is f/2.68 and the beam there is approximately telecentric.


Installation at the Bonnette

SITELLE is mounted on Cassegrain bonnette C4 bolt ring as well as the C3 ring (6 bolts used) to address the large moment (close to the C4 limit) presented by SITELLE
The C4 bolts are accessible without removal of the instrument cover.
Access to the six C3 bolts located behind the truss tubes, requires partial removal of cover and the use of a wrench within the structure.


Composed of three aluminum benches interconnected by 12 pairs of carbon fiber tubes
The top bench bolts to the C3 and C4 bolt rings on the CFHT Cassegrain bonnette.
Tubes are combined in pairs (upper/lower stage) at a single aluminum termination cap.
The tube number and arrangement was carefully studied to meet the image motion requirement of 2 pixels at various pointing angles.

The aluminum benches are weight relieved on the inside surface then glued together.
The weight relief pockets are thus internal to each bench and not visible from the outside.
This allowed for the further mass reduction necessary to meet the telescope maximum torque load requirement.
The designed mass and moment of instrument is 340 kg and 2900 Nm respectively.

SITELLE will be wrapped in a rugged industrial grade neoprene blanket
The blanket seals at a dedicated ring at the top and a 2nd circular plate at the bottom using "dry suit"-like zipper.
Apertures in this blanket are fitted for: filter access, the interconnection panel & camera E-box, the second output port, and the C3/C2 bolt access
A polymer baffle sits atop the instrument. Its aperture mates with the calibration wheel mounted shutter


Components of the Optical Train

Laser diode: 1550 nm single frequency & high stability source (Thorlabs)
LD Fiber splitter (3x): Cascade of 2 fiber splitters allows to split the laser output in 4
Pigtail collimator: Parts offering ~2.5 mm beam
Waveplate: Zero-order 1550 nm 1/8 wave cut to custom size.
Fold mirrors: Front surface mirror cut to custom size
Polarization Splitter: 1550 nm optimized
Detectors: Board mounted FC/PC receptacle style photodiode

OPD Retrieval Method

Metrology fringe signals are digitized by an ADC, rectified and compared to yield interferometer tilt error and OPD at the center of the pupil
An ABB proprietary method based on the quadrature fringe signal provides continuous OPD information to better than 1/1000th of a fringe on each of the thee channels

Maintaining the commanded OPD position

The OPD servo mechanism consists of a large range positioner, the Physik Instrumente (PI) Nexline piezo-motor which is capable of 3 cm of total motion (in this application), and three fine motion positioners (conventional piezo positioners by PI) with much smaller range of motion (45 microns).
The three piezo positioners maintain the tip/tilt of the mirror and nominally operate at their mid-range of motion; a correction signal to the Nexline ensures that the mid-range position of these devices is maintained.
When a position is requested, the command is effectively sent to the fine motion piezos; these will "rail" in position forcing the correction signal to the Nexline to come into effect until the final position is reached.

Servo Control Loop

The heart of the servo control is a custom board developed at ABB called the MINT III board.
The board converts the metrology white light and laser signals into digital values; using this information, it computes the correction signal to be sent to the piezo amplifiers.
The information available from the MINT board through software is limited to a rate of 20 Hz; this is much lower than the metrology readout rate of 10 kHz, or the servo loop rate of ~300 Hz.

Alignment Initialization

The three metrology fringe signals must be initialized to a common reference in order to be used for alignment.
Since there is little modulation in the interferogram of a narrow band source, a broadband white light (WL) source is used during initialization to provide a common zero-path-difference (ZPD) center fringe to synchronize on the same metrology laser fringe.

CCD Cryostat
Key Readout Specs

Pixel size: 15 microns
Minimum time between start of exposures (including all overheads): 3.25 s
Dark signal (173 K): <3 e-/pixel/hour (not yet tested)
Well depth: 48,000 e- with nominal high sensitivity, low read noise tuned gain
Nominal CCD-limited readout noise: <4 e- with no binning, ~3 e- in all binned modes
Charge transfer loss: not yet tested, appears to be close to nominal E2V specification
Crosstalk: < 0.008 %
Operating temperature: -100 to -110 Celsius

Model Uniblitz NS65B and VDM1000 controller
65 mm aperture
Fits in a 5 inches housing
Total of seven moving parts (six are the blades themselves)
Bi-stable operation: no power required to hold shutter in open/closed state
Controllers are located within the main e-box

NS65B Electrical and Mechanical Specifications
Coil resistance1 each coil: 12 ohms
Pulse voltage : 36 V
Weight : 130.4 g
Temperature range : from 0C to 80C (< -10C is ok)
Open bounce : 15%
Close bounce : 5%
Number of blades : 6
Continuous frequency : 2 Hz
Burst frequency : 5 Hz

Wheels Mechanical Implementation
Two wheels located near the telescope focal plane
Filter wheel: 6 positions (1 open, 5 available for filters)
Target wheel: 2 positions (1 open, 1 integrating sphere target)
Actuator is sealed industrial grade COTS with planetary gear assembly compliant to operational conditions and wheel load.
Actuator can easily rotate a strongly unbalanced wheel, so that not all filters need be installed.
However, empty filters slots preferably use a dummy mass ring and lighter filters (thinner) are coupled to heavier mount to avoid 5-pixel back lash present in the planetary gear.

Filter Wheel
Filter access: Removal with 3 captive screws and hatch door accessed through the structure trusses.
Filters must be mounted in a dedicated holder.
Nominal dimensions : 9.525 mm thick, 163 mm diameter (17:1 diameter to thickness ratio), usable FOV is 131 mm diameter.

Calibration Wheel
The calibration wheel carries a custom polymer-based integrating sphere and telescope baffle (open or sky viewing position).
A cover mounted atop the integrating sphere partially seals the instrument enclosure when in position.
The telescope baffle also partially seals the instrument enclosure and only exposes the active filter surface to the dome environment.
As built, the integrating sphere has 2 optical fibers connected at all times to illuminate the instrument FOV.
One fiber provides a white light source, the other a 532 nm HeNe laser calibration source.
The filter wheel can be used simultaneously with the integrating sphere, for example to validate a filter bandpass using the white light source.

Last update: July 23rd, 2014 (nflagey)