The performance of the “weighted Fourier phase slope” centroiding algorithm at the subpupil image of a Shack–Hartmann wavefront sensor for point-like astronomical guiding sources is explored. This algorithm estimates the image’s displacement in the Fourier domain by directly computing the phase slope at several spatial frequencies, without the intermediate step of computing the phase; it then applies optimized weights to the phase slopes at each spatial frequency obtained by a Bayesian estimation method. The idea was inspired by cepstrum deconvolution techniques, and this relationship is illustrated. The algorithm’s tilt estimation performance is characterized and contrasted with other known centroiding algorithms, such as thresholded centre of gravity (TCoG) and cross correlation (CC), first through numerical simulations at the subpupil level, then at the pupil level, and finally at the laboratory test bench. Results show a similar sensitivity to that of the CC algorithm, which is superior to that of the TCoG algorithm when large fields of view are necessary, i.e., in an open-loop configured adaptive optics system, thereby increasing the guide star limiting magnitude by 0.6 to 0.7 mag. On the other side, its advantage over the CC algorithm is its lower computational cost by approximately an order of magnitude.
This paper reviews the EDiFiSE (Equalized and Diffraction-limited Field Spectrograph Experiment) full-FPGA (Field Programmable Gate Array) adaptive optics (AO) system and presents its first laboratory results. EDiFiSE is a prototype equalized integral field unit (EIFU) spectrograph for the observation of high-contrast systems in the Willian Herschel Telescope (WHT). Its AO system comprises two independent parallel full-FPGA control loops, one for tip-tilt and one for higher order aberrations. Xilinx's Virtex-4 and Virtex-5 FPGA's fixed point arithmetic and their interfacing with the rest of the AO components and the user have been adequately dealt with, and a very deterministic system with a negligible computational delay has been obtained. The AO system has been recently integrated in laboratory and verified using the IACAT (IAC Atmosphere and Telescope) optical ground support equipment. Closed loop correction bandwidths of 65 Hz for the tip-tilt and 25 Hz for higher order aberrations are obtained. The system has been tested in the visible range for the WHT with a 9 x 9 subpupil configuration, low star magnitude, wind speeds up to 10 m/s and Fried parameter down to 18 cm, and a resolution below the EIFU’s fiber section has been obtained.
EDIFISE is a technology demonstrator instrument developed at the Institute of Astrophysics of the Canary Islands
(IAC), intended to explore the feasibility of combining Adaptive Optics with attenuated optical fibers in order to obtain
high spatial resolution spectra at the surroundings of a star, as an alternative to coronagraphy. A simplified version with
only tip tilt correction has been tested at the OGS telescope in Observatorio del Teide (Canary islands, Spain) and a
complete version is intended to be tested at the OGS and at the WHT telescope in Observatorio del Roque de los
Muchachos, (Canary Islands, Spain).
This paper describes the FPGA-based real time control of the High Order unit, responsible of the computation of the
actuation values of a 97-actuactor deformable mirror (11x11) with the information provided by a configurable wavefront
sensor of up to 16x16 subpupils at 500 Hz (128x128 pixels). The reconfigurable logic hardware will allow both zonal
and modal control approaches, will full access to select which mode loops should be closed and with a number of
utilities for influence matrix and open loop response measurements. The system has been designed in a modular way to
allow for easy upgrade to faster frame rates (1500 Hz) and bigger wavefront sensors (240x240 pixels), accepting also
several interfaces from the WFS and towards the mirror driver.
The FPGA-based (Field Programmable Gate Array) real time controller provides bias and flat-fielding corrections,
subpupil slopes to modal matrix computation for up to 97 modes, independent servo loop controllers for each mode with
user control for independent loop opening or closing, mode to actuator matrix computation and non-common path
aberration correction capability. It also provides full housekeeping control via UPD/IP for matrix reloading and full
system data logging.
This communication reviews the participation of the Instituto de Astrofísica de Canarias (IAC) in the design of the
European Solar Telescope. Apart of being the coordinator institution of the whole project, and, as such, responsible for
the project managing, the IAC leads several tasks like overall instrument definition or characterization of the
atmospheric turbulence profile with height or the definition of adequate detectors. More in particular, the IAC will
design and build two long-base SHABAR (SHAdow BAnd Ranger), instruments to measure medium-altitude seeing.
The IAC is also responsible for the design, together with other institutions, of the design of grating spectropolarimeters
suitable for multiwavelength high spatial and spectral resolution.
Within the preliminary developments related to the Gran Telescopio Canarias, a test rig is being designed for the active control of the support system of the segmented primary mirror. The construction of this test rig will be divided in two phases: the first one will basically consist in the implementation and testing of a displacement sensor prototype as well as an actuator prototype. Phase 2 will consist in the development and characterization of the whole test rig, including two segment simulators and a number of displacement sensors and actuators. Also, a non contact optical system for the test rig behavior verification will be constructed during phase 2. This paper presents the conceptual design adopted for the active control system proposed for the telescope and a brief description of the development program, including the requirements of the displacement sensors and actuators. We intend the test rig not only for testing the active control system components, but also for checking different control strategies.