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Rabaud, D., Chazallet, F., Rousset, G., Amra, C., Argast, B., Montri, J., Madec, P.-Y., Arsenault, R., Hubin, N., Charton, J., & Dumont, G. 2000, in ASP Conf. Ser., Vol. 216, Astronomical Data
Analysis Software and Systems IX, eds. N. Manset, C. Veillet, D. Crabtree (San Francisco: ASP), 373
NAOS Real-Time Computer for Optimized Closed Loop and On-Line
Performance Estimation.
D. Rabaud1, F. Chazallet2,
G. Rousset3, C. Amra4,
B. Argast5, J. Montri6,
P.-Y. Madec7, R. Arsenault8,
N. Hubin9, J. Charton10,
G. Dumont11
Abstract:
The Real Time Computer (RTC) is a key component of an adaptive optics system.
In the Nasmyth Adaptive Optics System (NAOS) for the ESO VLT, the RTC will
control the 185 actuators of the corrective optics from the 144 wavefront
sensor subapertures at a maximum frequency of 500 Hz. The RTC hardware
architecture is fully reconfigurable to switch between the two NAOS wavefront
sensors. The software includes an on-line control optimization allowing the
use in a broad magnitude range (up to
= 18). This RTC is designed to be
easily upgraded for Laser Guide Star.
1. Introduction
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).
2. RTC Specifications
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:
- Open-loop Zernike reconstruction on 2048 records and autocorrelation,
- Slopes and voltages expansion on modal basis for on-line optimization,
- Covariance calculation at 50 Hz for off-line PSF reconstruction,
- Real-time acquisitions of slopes, intensities, voltages and modes.
For on-line optimization the different RTC tables used during data processing
must be modified in real time, i.e. without opening the loop.
3. SHAKTI Boards Concept
The SHAKTI boards have been specifically designed for Adaptive Optics (AO) and
the RTC is based on a set of four. These boards are linked together by a
private video bus for the real-time transfer of the computed data. This bus
allows to realize parallel and/or serial architecture. Consequently, it is
possible to add computing power without any modification in the architecture.
It is also possible for several boards to emit data towards the same
multiplexed bus. This functionality can be implemented for a multi-output
CCD, where one wavefront calculation board per output can be used, or for a
multi wavefront sensor system such as in Laser Guide Star.
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:
- Acquisition/equalization module,
- Graphic display module,
- Computation module,
- 64 channels Digital to Analog conversion module.
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.
4. RTC Hardware Design
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.
5. Software Design Description
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:
- Tip/tilt off-loading: when the loop is closed, so as to off-load the
tip/tilt and focus correction, NAOS uses the telescope to compensate for the
very low frequency pointing and focus errors. These errors are determined from
the SC-PCB data and they correspond to average correction values.
- Sky background follow-up and threshold optimization: measurement of the
sky level on the WFS camera and threshold optimization is done on average
non-equalized images which are recorded without opening the loop.
- Turbulence and performance estimation: measurement of turbulence
conditions is performed on 2048 consecutive records at WFS acquisition
frequency every 30 seconds. The calculation is done on the 36 first Zernike by
recovering the open loop data. In this way, it is possible to determine the
noise level on the WFS, the turbulence correlation time at 50%, the Fried
diameter R0 and the outer-scale L0.
- Closed loop modal optimization: the PowerPC board acquire the result of
slopes and voltages projection on a modal basis done directly on the SHAKTI
boards. After application of an algorithm to reconstruct data in open-loop,
FFT is calculated and an optimal gain is determined for each mode. Then the
new command matrix is computed, loaded on the RTC boards and applied without
disturbing the loop. The duration of one full iteration is smaller than 3
minutes.
6. Laser Guide Star (LGS) Upgrade
Adding a LGS requires that the RTC can fulfill the following functionalities:
- Get the turbulent tip-tilt signal measured on a natural guide star,
- Send LGS tip-tilt extracted from VISWFS to the LGS/fast steering mirror,
- Compute the average defocus and send the result to the LGS system.
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.
7. Conclusion and Perspectives
Final design of NAOS RTC has been accepted by ESO in March 1999. Software
development will be completed by the end of 1999. Then, the computer will be
validated by using the ONERA adaptive optics bench before being integrated in
NAOS in March. It will thus be available for observations to be carried out by
the end of 2000. RTC hardware has been designed to answer many requirements in
real-time signal processing and could be easily upgraded for LGS. Software
represents the work of more than 10 man years. It contains experience gained
in the Adaptive Optics field during ten years by the laboratories involved in
the NAOS development. It has been developed in a very modular way so to be
reusable within other frameworks.
References
Rousset, G. et al. 1998, SPIE, 3353, 508
Zins, G. et al. 2000, this volume, 377
Footnotes
- ... Rabaud1
- ONERA, BP 72, 29 av. de la Division Leclerc, 92322 Chatillon Cedex, France
- ... Chazallet2
- SHAKTI, 27 bld Charles Moretti, 13014 Marseille, France
- ... Rousset3
- ONERA, BP 72, 29 av. de la Division Leclerc, 92322 Chatillon Cedex, France
- ... Amra4
- SHAKTI, 27 bld Charles Moretti, 13014 Marseille, France
- ... Argast5
- SHAKTI, 27 bld Charles Moretti, 13014 Marseille, France
- ... Montri6
- ONERA, BP 72, 29 av. de la Division Leclerc, 92322 Chatillon Cedex, France
- ... Madec7
- ONERA, BP 72, 29 av. de la Division Leclerc, 92322 Chatillon Cedex, France
- ... Arsenault8
- Observatoire de Paris/DESPA, 5 pl Jules Janssen, 92195 Meudon
Cedex, France
- ... Hubin9
- European Southern Observatory, K. Schwarzchild Str-2, D 85748
Garching, Germany
- ... Charton10
- LAOG, UJF - BP 53, 38041 Grenoble Cedex 9, France
- ... Dumont11
- ONERA, BP 72, 29 av. de la Division Leclerc, 92322 Chatillon Cedex, France
© Copyright 2000 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: NAOS Computer Aided Control: an Optimized and Astronomer-Oriented Way of Controlling Large Adaptive Optics Systems
Up: Adaptive and Active Optics
Previous: QuickLook Data Reduction Pipeline - Keck Adaptive Optics Real Time Data Reduction Software
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