First Light Adaptive Optics System for Large Binocular Telescope
The paper describes the design of the single conjugate Adaptive Optics system to be installed on the LBT telescope. This system will be located in the Acquisition, Guiding and Wavefront sensor unit (AGW) mounted at the front bent Gregorian focus of LBT. Two innovative key features of this system are the Adaptive Secondary Mirror and the Pyramid Wavefront Sensor. The secondary provides 672 actuators wavefront correction available at the various foci of LBT. Due to the adaptive secondary mirror there is no need to optically conjugate the pupil on the deformable mirror. This allows having a very short sensor optical path made up using small dimension refractive optics. The overall AO system has a transmission of 70 % and fits in a rectangle of about 400x320mm. The pyramid sensor allows having different pupil sampling using on-chip binning of the detector. Main pupil samplings for the LBT system are 30x30, 15x15 and 10x10. Reference star acquisition is obtained moving the wavefront sensor unit in a field of view of 3x2 arcmin. Computer simulations of the overall system performance show the good correction achievable in J, H, and K. In particular, in our configuration, the limiting magnitude of pyramid sensor results more than one magnitude fainter with respect to ShackHartmann sensor. This feature directly translates in an increased sky coverage that is, in K band, about doubled with respect to the same AO system using a Shack-Hartmann sensor.
Modeling the pyramidal sensor by a ZEMAXTM user defined surface.
After 20 years of wavefront sensors based on pyramid (PWFS), there are no straightforward ways to model such device in standard sequential ray-tracing software: modeling strategies tend to be oriented to the needs of the single user only and, in general, are unsatisfactory due to lack of flexibility. To overcome this problem, we have exploited the possibility of ZEMAXTM – one of the ray- tracing software mostly in use nowadays – to develop a user defined surface (UDS), whose properties are described in a dynamic link library (DLL) written in C language. The pyramid UDS approach greatly improves the versatility during the design and simplifies both quality and tolerance analysis. In order to prove the potentiality of our UDS-DLL surface, referred as PAM2R, we reproduced the optical layout of two PWFS systems already installed at LBT: the single-conjugate system FLAO, and the ground-layer system GWS of LINC-NIRVANA. In this proceeding we will highlight the main characteristics of the PAM2R surface, showing various results we obtained on the above case studies with the aim to establish a common design playground for the PWFS in the AO community.
Adaptive secondary mirror for the 6.5-m conversion of the Multiple Mirror Telescope: first laboratory testing results
We present the first results of test performed on a reduced size adaptive secondary prototype named P36. The full size unit, named MMT336, is ready to be assembled and it is planned to install it at the 6.5m conversion of the Multiple Mirror Telescope by the end of this year. The design of the final unit consists of: a convex thin deformable mirror whose figure is controlled by 336 electro-magnetic force actuators, a thick reference shell and a third aluminum shell used for actuator support and cooling. The force actuator response function is adjusted using both open and closed loop compensation to obtain an equivalent position actuator thanks to nearly co-located capacitive position sensors. The digital real-time control and the unit monitoring is done using custom-made electronics based on DSPs. The preliminary dynamical test aimed at identifying the P36 mirror response function to obtain a proper dynamics compensation were successful. In fact two main results have been obtained: 1) an accurate identification of the feedforward matrix used to control the mirror 2) settling time of approximately 0.5 ms, well within the specifications. We also complement these lab results with results obtained from simulations of the full size mirror dynamics.