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NAOS is a VLT instrument (Rousset et al. 1998) doing real-time compensation of
the atmospheric turbulence using a deformable mirror and a tip-tilt mirror.
The RTC is a key component controlling the 185 actuators from a 144
Shack-Hartmann wavefront sensor subapertures at a maximum frequency of 500 Hz.
It also provides additional capabilities such as real time optimization of the
control loop which is the warranty for NAOS to achieve a very good Strehl
Ratio in a broad magnitude range ( = 8 up to 18), on-line turbulence and
performance estimations and finally capability to store and process the data
necessary to the off-line PSF reconstruction algorithm.
In this paper, we recall first the RTC specifications, then we present the hardware design based on SHAKTI boards and the associated maintenance concept. The software programming scheme is described and the different functionalities are associated to overall optimization of the adaptive optics system, performance estimation and off-line PSF reconstruction. Finally, we show how the RTC is designed to be easily upgraded for Laser Guide Star (LGS).
The RTC is fed by images of the Shack-Hartmann spots and is dedicated to the
control of the corrective optics. RTC functions are correction of detector
inhomogeneities, display of the corrected image and then computation of the
centroid coordinates in predefined zones. From these coordinates, the RTC
computes the commands to be applied to the actuators by means of a
matrix-vector multiplication. The RTC has to ensure a control-loop time-lag
smaller than 200 . In NAOS, two wavefront sensors are used, one
dedicated to visible range and the other to infrared range, each of them
having several read-out modes. For this reason, the RTC has to be fully
programmable. Moreover, the RTC performs pre-reductions on the various
measurements. They are:
For on-line optimization the different RTC tables used during data processing must be modified in real time, i.e. without opening the loop.
The SHAKTI boards are based on a modular architecture. Each board is composed of a motherboard which can incorporate up to 4 modules among the following:
The motherboard allows transfers between the CPU board of the VME bus and the 4 modules. It is also in charge of extracting the data from the front panel real-time bus. The format of this video bus is chosen dynamically when loading the programmable logic components. Such a programmable hardware offers a great flexibility and would enable the use of a curvature WFS, for instance.
The equalization/acquisition module is in charge of the flat-fielding correction and is able to grab an image before or after the equalization process. The graphic module displays the image on a monitor. The computation module is based on three Texas Instruments DSP TMS320C40 hereafter called C40. Finally the DAC module is based on 64 channels 12 bits voltage output converters and is accessible directly by the VME bus and also by C40 communication link.
Data coming from the IR or VIS WFS are transferred to the equalization module of the WaveFront Power Control Board (WF-PCB) through an interface board. The WF-PCB is a mother board equipped with one equalization/acquisition module, one graphic module and two DSP Computation modules. After equalization, the data are sent to the computation module of the same board. The data are shared between two processors which compute the centroids, then the last processor computes the slopes and re-emit the data to both Control Command boards (CC-PCB).
The CC-PCB is based on a mother board equipped with two computation modules and two Digital to Analog Conversion modules. This means 300 Mflops computing power and 128 DAC outputs for each board. The whole set of CC-PCB input data are transmitted to the two first C40 of the two modules. The first one computes a part of the command and the second one a part of the slope expansion on the Zernike basis. When the computation is completed, the two C40 send the results to the third processor. This C40 performs the temporal filtering, drives the actuators through a DAC module and then sends all the computation results to the Statistical board (SC-PCB).
The SC-PCB is identical to the two CC-PCB ones but without DAC module. The two computation modules perform voltage expansion on Zernike basis, Zernike autocorrelation, slope and voltage expansion on modal basis, covariance calculation, and also perform real-time acquisitions of the different type of data.
It's important to notice that none of the pieces of information that are necessary to the servo-loop use the VME bus. This one is only used to drive the different real time boards and to read data for a later off-line reduction.
The CPU board is a PowerPC board which read communicates with the WorkStation and performs the different functionalities described in Section 5. We include on RTC a monitoring system which allow to validate the different RTC real time functionalities in closed loop mode.
Software has been organized in independent layers in order to limit to the maximum the adaptations during an hardware or Software evolution. All these layers are written in C language and only the interfaces are dependent on ESO tools. The PowerPC board is controlled from workstation by a C++ software (Zins et al. 2000) which is entirely based on ESO basic libraries and Graphical User Interface developed in Tcl/Tk. All interactions with SHAKTI boards are controlled by one VxWorks task which has many different clients running on CPU board:
In order to do these, another additional SHAKTI motherboard with one C40 module allows to receive a digital signal. This signal can be either the straight values of tip-tilt, or measurements of a curvature (or Shack-Hartmann) wave front sensor. The tip-tilt signals are then sent to the CC-PCB. The numerical output SHAKTI bus is used in order to send the LGS tip-tilt at a high-frequency. The value of the defocus is derived from the data computed on the SC-PCB and sent through the VLT Software communication tools.
Rousset, G. et al. 1998, SPIE, 3353, 508
Zins, G. et al. 2000, this volume, 377