The Function of the Tracker
The function of the tracker subsystem in the telescope is to carry and position the optical payload in the required position and orientation so that the light collected from the primary mirror can enter the Spherical Aberration Corrector and optical instrumentation in such a way that the image quality capability of these subsystems will not be degraded more than has been allowed for.
These position and attitude requirements encompass 6 independent degrees of freedom, which will be implemented by 10 dependent degrees of freedom. The tracker will be supported by the telescope structure at the nominal position of half the radius of curvature of the primary mirror. The telescope structure will support all the interfaces of the tracker to the rest of the telescope system.
The total mass to be supported by the tracker, inclusive of its own weight, will nominally be 4500kg. The mass of the optical payload alone will nominally be 750kg.
The required nominal linear and angular position accuracies are 5 microns and 1 arc seconds respectively.
Layout of the Tracker
The current layout of the tracker and payload is illustrated in the figure below.
The tracker beam will be positioned in a plane, 37 degrees with respect to the local vertical, by X any Y actuators each capable of a range of 3250mm. The angular and focus motions will be carried out by the hexapod system with an angular range of 17 degrees and a focus range of 200mm. A rotation stage, mounted on top of the hexapod, will counter the star rotation in the image plane due to the rotation of the earth. The autocollimators shown in the figure above will be the sensors to measure whether the optical axis of the payload is perpendicular to the primary mirror surface. The tracker's contribution to the error budget is specified to an image quality figure of 0.225 arc seconds, the closed loop guidance accuracy at 0.1 arc seconds and the open loop pointing at 5 arc seconds. These specifications translate to a control accuracy of 5 microns relative to the tracker local sensors. The sensors earmarked for the X and Y axes are incremental encoders with a resolution of better than 1 micron. To achieve the performance specifications will require sound mechanical design and control system implementation.
Reutech Radar Systems Pty(Ltd) in Stellenbosch South Africa has been contracted to design and build the tracker.
The Function of the Payload
The main functions of the payload system in the telescope are to:
a) Receive light from primary mirror and correct its optical aberrations by means of an optical system called a spherical aberration corrector (SAC).
b) Distribute the light to the various science instruments or ports:
- An acquisition system
- A guidance system
- A fibre instrument feed system
- The prime focus imaging spectrograph (PFIS)
- An auxiliary port
c) Provide the telescope with an acquisition capability
d) Provide closed loop guidance corrections
e) Correct atmospheric dispersion by means of an atmospheric dispersion corrector (ADC)
f) Baffle all stray light, especially light emanating from sources other than the primary mirror
g) Act as a supporting interface for all power, data, cooling and compressed air systems for the science instruments
h) Provide structural support
i) Provide communication with the telescope control system (TCS)
j) Provide the science instruments with a calibration facility (for flat fielding)
The payload will be supported and positioned by the tracker near the paraxial focus of the primary mirror. The tracker will support all the interfaces of the payload with the rest of the telescope system.
Layout of the Payload
The current layout of the payload is illustrated in the figures below. Note that the payload consists of a rotating and non-rotating part. The SAC, ADC and moving baffle are connected on the tracker hexapod side of the tracker rotating stage whilst the other components will be mounted on top of the rotation stage.
The PFIS will be designed and built by the University of Wisconsin-Madison, the imaging and guidance camera by the South African Astronomical Observatory and the SAC by the French company, SAGEM SA.
