The sensor assembly consists of a wavefront sensor and a tilt tracker integrated on a single baseplate. The system was designed to satisfy the following requirements;
Sensor Modes of Operation
* Track mode only (all source light available to tracker)
* Tracking and HO correction:
Track source = WFS source: star source split between sensors
Track source != WFS source: star track source, star WFS source
Track source != WFS source: star track source, sodium beacon WFS source
(WFS and tracker sources overlap)
Tracker Specification
* FOV (acquisition): 3 arc minutes
* plate scale: 20 arc sec per pixel
* track object anywhere in acquisition FOV
* focus adjustment
* remote operation of all functions
Wavefront Sensor Specification
* FOV: 30 arc seconds
* spatial resolution: variable
* tilt sensitivity: 1/20 wave
* dynamic range 3 waves
* source altitude: 90 km-infinity
* remote operation of all functions
The wavefront sensor and tracker optical trains split at the first optical component of the sensor system, the aperture sharing element (ASE), which divides the input by wavelength. In order to best meet the operational requirements, three different ASE elements are supplied. The ASE mount allows the element to be swapped without disturbing the optical alignment of the system.
All elements are 150 mm dia., 6 mm thick. A window may be used as the ASE when the wavefront sensor is not required for extended periods, but optimal performance of the tilt compensation system is desired (or the element may be removed entirely). The window allows all light from the star track source to enter the track sensor, which receives light in transmission through the ASE.
A dichroic element is provided for multi-purpose use:
* Tilt compensation with laser guide star (LGS): WFS receives all sodium
signal, tracker operates at approximately 50% maximum quantum efficiency.
* Tilt compensation with natural guide star (NGS): both sensors operate
at 50% efficiency.
The dichroic divides the input into two bands: the long wavelength band ( ~700 - 1100 nm) is transmitted to the track sensor, the visible band ( ~400 - 700 nm) is reflected into the wavefront sensor optical train. The dichroic reflects >99% at the sodium line and splits the reference wavelength band (600-700 nm) between the tracker and WFS. The dichroic should be used when both LGS and NGS sources are to be used and maximum flexibility is desired.
FIGURE 10.1 Transmission of Dichroic
A third ASE is a narrow-band sodium reflector, supplied by MPIA. This element
reflects the sodium signal with high efficiency to the wavefront sensor,
while allowing all but a narrow band at the sodium line to pass to the
tracker. This element should be used when the LGS is to be used exclusively,
as it allows the tracker to operate at maximum quantum efficiency.
The tracker supports two modes of operation: acquisition and closed-loop tracking. A two-lens system forms a telecentric lens which reduces the image at the f/24 focal plane by a factor of 3.65, giving a plate scale of approximately 20"/pixel at the tracker CCD. The telecentric lens consists of a dichroic, 80 mm dia., and a Nikon F-mount lens (50 mm fl.). A motorized filter wheel before the Nikon lens contains five ND filters: 0.04, 1.0,2.0, 3.0,4.0, plus an open position.
Figure 10.2 Calculated Spot Diagram at Tracker, 6 mm dichroic
FIGURE 10.3 Calculated Spot Diagram at Tracker, 19 mm dichroic
The Astromed track camera (supplied by MPIA) is supported by a three-axis motorized stage. The x and y axis movement allows any portion of the focal plane array to be positioned anywhere in the 3' FOV. The z-axis (focus) movement is provided for focus adjustment independent of the telescope. The Astromed camera has an internal electromechanical shutter.
During acquisition, the full focal plane array is read out and the console display continuously updated to aid the operator in locating the desired track source. Since the CCD is rectangular with a 2:1 aspect ratio, only half of the 3' field is displayed at one time. To obtain the full field, the camera must be translated one CCD width along the "x" (horizontal) axis in order to image the second half of the field.
Once the track source has been located, the CCD is positioned such that the track object lies at one corner of the array to facilitate high-speed subframing readout required for closed-loop operation.
The Shack-Hartmann wavefront sensor incorporates a Lincoln Laboratories 64x64 low-noise CCD array, capable of framing at 1200 KHz with approximately 10-15 electrons readout noise. Pixels are on 26um centers.
Five microlens arrays (MLMs) are provided on a single glass substrate supported by a motorized linear stage. The telescope pupil (defined by the 3.5m primary mirror) is relayed to the MLM and measures 5 mm diameter at the lenslet array. The MLM divides the pupil into subapertures with spatial resolutions of 3x3, 5x5, 8x8, 10x10, 12x12 over the telescope pupil. All arrays have lenses with approximately the same focal length: 45 mm. The linear stage positions one of the five arrays at a pupil conjugate, the choice of array depending on the seeing conditions during an observing run. The 3x3, 5x5 and 8x8 arrays arrange the lenses in a hexagonal pattern for maximum coverage of the circular telescope pupil, while the 10x10 and 12x12 arrays have square apertures.
A relay lens (Rodenstock Apo-Rodagon N, f/2.8 50 mm fl.) images the Hartmann focal spot pattern produced by the microlens array on to the CCD. The relay lens is mounted to a linear stage which in turn is supported by a stage for the Lincoln camera. In this way, the camera can image different planes along the optical axis with variable magnification. The system allows either the pupil plane or the Hartmann spot plane to be imaged. The variable magnification is required to remove "focus" from the Hartmann spot pattern. The nominal magnification, 0.346, is used for the LGS source. The wavefront at the MLM is collimated for a LGS range of approximately 110 km (a typical range). The dynamic range of the WFS accommodates focus induced by changing telescope altitude during a typical run without changing the relay magnification. With a NGS source (star at infinity) the wavefront is no longer collimated at the MLM. By changing the relay magnification (moving both the camera and relay lens), the focus in the Hartmann spot pattern is removed at the CCD and the full dynamic range of the sensor is restored. The camera/relay lens positions for LGS and NGS sources are saved as parameters called from the GUI.
A three-lens system and a tip-tilt mirror provides a means operating the wavefront sensor within a 30" FOV about the telescope boresight. Tilt at the MLM is removed by the tip/tilt mirror without changing the MLM-DM registration. The first lens relays the deformable mirror plane to the surface of the tip-tilt mirror, the second lens relays the pupil to infinity and forms an intermediate image of the NGS/LGS. The third lens collimates the source (at 100 Km) and relays the pupil(DM) to the MLM.
Two apertures and a fiber reference on a two-axis motorized stage (provided by MPIA) provide a stop for the low altitude LGS backscatter or a reference for calibration and testing purposes. Lenses are identical achromats (JLM P.N.: DBL14070/101) 120 mm efl, 50 mm dia. with high efficiency white light AR coatings averaging 0.25% reflectivity over the operational band of the wavefront sensor. The mirror, Newport P.N. 30Z40BD.1, is a 3" dia Zerodur optic with an "all-band" visible high reflector coating averaging 99% reflectivity over the WFS band. The mirror is supported by a stepper motor driven tip-tilt stage manufactured by Micro Controle (P.N.: SL8AVP), which has 2 degrees of actuated adjustment in each axis, sufficient for 30" FOV (initial course adjustments are made by releasing thumb screws on each axis).
A filter wheel located just after the MLM has the following ND filters: 0.04, 0.5, 1.0, 1.5 2.0 and 4.0. The 4.0 is required for the reference. The 0.04 may be removed to increase the WFS throughput. An electromechanical shutter (Uniblitz VSR25E1T) attaches to the Lincoln camera and is normally closed without power.
Major components of the sensor assembly were pre-aligned at AOA, however, some components must be remounted before the sensor is operational. Insert the ASE in the ASE mount and secure using three screws provided. A flexure ring holds the ASE securely against three pads on the ASE mount, assuring that the ASE is properly aligned to the wavefront sensor and allowing swapping of the ASE element without realignment. The dichroic should be used during alignment as it splits the reference source between the tracker and WFS.
The Astromed track camera mounts to the three-axis positioner using the camera-stage adapter plate. Camera connectors should be at the top of the camera (camera right side up). Verify that the Nikon lens focus ring is set to infinity (if not, the lens may interfere with the filter wheel).
The WFS 3" mirror is inserted in the Newport tip-tilt mount. Set screws secure the mirror from the side while three pads behind the mirror insure that the front surface of the mirror is in contact with contact points at the front surface located the mirror pivot axis.
The Lincoln camera and relay lens mount to their respective stages. The camera mounting bracket should be aligned flush with the side of the camera stage. The relay lens stage is mounted to the camera bracket, taking care to align the axis of the relay lens stage to the camera stage axis. The relay lens bracket is mounted to the relay lens stage last.
The sensor assembly is aligned to the rest of the ALFA optical system by positioning the sensor breadboard such that the edge of the ASE mount is approximately 5 mm (0-10 mm) from the science instrument dichroic mount. With the f/10 reference source at the infinity focal plane and centered on the optical axis, translate the sensor breadboard to place the beam through the center of the tracker optical train. The reference image plane should lie approximately 80 mm behind the second surface of the ASE.
FIGURE 10.4 Location of Optical Interface
Next, pivot the sensor head about the center of the ASE to center the reflected
reference on the first lens of the WFS optical train and position the pupil/DM
conjugate at the "center" of the 3x3 MLM (the MLM stage should
be positioned for the 3x3 array using the GUI). Secure the sensor assembly
using the eccentric clamp located near the center of the sensor baseplate.
With the actuators at mid-stroke, loosen the tip/tilt mirror mount locks for the two axis and pivot the mirror to direct the reference to the center of the Lincoln camera relay lens (removing the tilt in the Hartmann spot pattern), then secure the locking screws. Observing the RS-170 display from the Lincoln camera (with the camera/relay lens set for "Pupil"), verify that the pupil image is centered on the 3x3 lenslets and that the pupil is centered on the CCD. If the pupil is not centered on the CCD, loosen the four screws on the front of the Lincoln camera housing and translate the camera board to center the image on the CCD.