Abstract:
The upcoming large astronomical telescopes are trending towards the Segmented Primary Mirror due to
technological advancements & manufacturing feasibility. We have designed a wide-field optical IR spectroscopic
survey telescope that can deliver spectra of several millions of astronomical sources. The baseline design of this
telescope is a 6.2 m segmented primary mirror with hexagonal mirror segments of 1.44m size, intersegment Edge
sensors, and soft positioning actuators. The telescope is designed to provide a 2.5deg FOV achieved through a
system of wide field corrector lenses with a design residual ~0.2". Also, it delivers an f/3.61 beam suitable for
directly feeding optical fibres. A mechanical concept of the telescope is designed with a truss-based mirror cell to
support the segmented primary mirror and keep the deformation to a minimum. As the primary mirror is segmented,
the deformation due to different disturbances like wind, vibration and thermal effects must be corrected to a
nanometer accuracy to make it act like a monolithic primary mirror. This is achieved through an active control
system using three actuators and six inter-segment edge sensors. A simulation tool, codeSMT, is built based on the
state-space model of a soft actuator with Multiple-Input Multiple-Output (MIMO) capability to incorporate dynamic
wind disturbance from the IAO Hanle site and vibration effects. A detailed error multiplier analysis is performed
numerically using this tool and is in good agreement with analytical calculations. A parameter sensitivity analysis is
performed to fine-tune the primary mirror control system variables. This paper presents the Optical, Mechanical and
Active Control system design approach of a 6.2m wide-field telescope currently under conceptual design.
