CFHT, Instruments, Imaging, AOB, Short System Description

The CFHT Adaptive Optics Bonnette :
Short System Description

Note : This page has been adapted from the first section of the article "Performance of the Canada-France-Hawaii Telescope Adaptive Optics Bonnette", published in Publications of the Astronomical Society of the Pacific, 110, pp. 152-164, 1998. A compressed (389 kB) version of the article is available. This paper is a very extensive report on the performance of PUEO. It includes a short system description (as below) and an analysis of the wavefront and the image characteritics, on bright and faint guide stars. This can be easily used to find out what performance can be expected on a particular object (does not quantify the effect of the guide star spatial extension).

Introduction

The Adaptive Optics Bonnette (AOB), also called PUEO after the sharp vision Hawaiian owl, was developed for the Canada-France-Hawaii (CFH) Telescope, based on F.Roddier's curvature concept (Roddier et al, 1991; Arsenault et al, 1994). The ``bonnette'' (adaptor) is a facility instrument mounted at the f/8 Cassegrain focus of the CFH 3.6 m telescope on top of Mauna Kea (Hawaii). The instrument is the result of a collaborative effort between several institutes : The CFHT (managing the project and designing the general user interface); The Dominion Astrophysical Observatory (DAO, Canada) who designed and fabricated the opto-mechanical bench, the curvature wavefront sensor (Arsenault et al, 1994; Graves et al, 1991; Graves et al, 1994) and its electronics; the company Cilas (ex Laserdot, France) who provided the deformable curvature mirror and the Real Time Computer and software, including a high level maintenance interface; the Observatoire de Paris-Meudon (OPM, France) who manufactured the separate tip-tilt mirror and was in charge with the final integration, testing and calibration of the instrument. The UH adaptive optics team provided guidance throughout the project. The system was commissioned at CFHT during three runs in the first semester 1996.

Instrument description

The main characteristics are summarized in the table above. PUEO has only few optical parts, mainly reflecting ones. Making use of off-axis parabolic mirrors allows for a compact instrument with small optical components, favoring reduced flexures (see figure). The beam, which can pass straight through the bonnette, is normally diverted by a flat mirror - on a moving slide - to the AO system, allowing to switch rapidly the f/19.6 corrected beam to the direct f/8 beam, if required.

Optomechanics
Total number of mirrors in science train 5 + 1 beamsplitter (transmission)
Total number of mirrors in WFS train 9 + 1 beamsplitters (reflection)
Transmission Science train (Visible) 70% (V) excluding beamsplitter
Transmission Science train (IR) 75% (H), 70% (K) including dichroic
Input/Output F ratios 8 / 19.6
Overall Bonnette dimension Diameter 120 cm, Thickness 28 cm
Flexures Approximately 15 microns/hour
Optical quality lambda / 20 rms at 0.5 microns with DM flat
Instrument clear field of view 90 arcsec diameter
Wavefront sensor
Type Curvature
Number of subapertures 19
Detectors APDs (45% peak QE, approximately 20e-/s dark current)
Field of View 1-2 arcsec depending on optical gain
Deformable mirror
Type Curvature (+ dedicated Tip-Tilt)
DM Number of electrodes 19
DM Stroke approximately +/- 10 microns
TTM stroke +/- 4 arcsec
DM first mechanical resonance > 2kHz
Overall DM dimension 80 mm
Pupil size on DM 42 mm
Conjugation Telescope pupil
Control
Sampling/command frequency Selectable (1000Hz, 500Hz, 250Hz,...)
Max bandwidth 0dB rejection 105 Hz
Max bandwidth -3dB close-loop 275 Hz
Control scheme Modal, 18 mirror modes controlled
Close-loop mode gains optimization Update every 30 seconds
Instrumentation
IR imager HAWAII array. 0.035 arcsec/pixel
Visible imager 2K x 2K pixels. 0.03 or 0.06 arcsec/pixel
Visible integral field spectrograph
Table: Characteristics of the CFHT Adaptive Optics Bonnette

The optical design (Richardson, 1994) includes an F/8 off-axis parabola that collimates the beam and image the telescope pupil on the 19 electrode curvature mirror. A F/19.6 off-axis parabola, mounted on a fast tip-tilt platform, direct the beam to the science instrument. Prior to the science focus, a beamsplitter reflects part of the light to the visible wavefront curvature sensor. Optionally, an atmospheric dispertion compensator can be inserted in the collimated beam for observation at visible wavelength.

The optical bench during its integration at the DAO. Kind of crowded in there...

The geometry (19 electrodes/subapertures divided up into two rings plus a central electrode) is well suited to circular pupils; the inner ring and the central electrode allow to solve Poisson's equation over the pupil while the outer ring allows to measure the boundary conditions (Roddier, 1988; Rousset, 1994). Such a system, with few degrees of freedom but a high bandwidth, is particularly well suited to Mauna Kea seeing conditions where turbulence is weak yet fast.

Figure 1 : Optical path of the instrument. The central folding mirrors are on a movable slide, so that the direct and the corrected focus are co-incident. The wavefront sensor is remotely controlled along three axis and allows to select a reference star different from the science object.

Modal control and mode gain optimization (Gendron & Léna, 1994; Rigaut et al, 1994) maximize the instrument performance according to the state of turbulence and the guide star magnitude. We have modified the modal control as presented in Gendron&Léna (1994) to adapt it to close--loop operation. Using the mode coefficient power spectra over the last 30 seconds approximately and a model of the close--loop transfer functions (calibrated in laboratory), new mode gains are computed and updated on a time scale between 30 seconds and 2 minutes, allowing the system to track seeing variations.

The system has been tested in laboratory at 0 and 20 degree C for flexures, optical quality, and bandwidth (Lai, 1996). Figures are reported in the table above.

The quoted lambda / 20 rms at 500 nm refers to the optical quality of the science path from the input focus to the output focus, where the mirror shape has been adjusted using an interferometer as a wavefront sensor located at the science focus. When the curvature WFS is used to cancel the bench optical aberrations, the optical quality of the science path is lambda / 8. This degradation (lambda / 20 to lambda / 8) comes not only from the fact that the curvature WFS is unable to detect high spatial frequency aberrations (which the DM is not able to compensate anyway), but more that this high spatial frequency aberrations induce, through spatial aliasing an error in the estimation of the true low spatial frequency terms. To this have to be added non-common path aberations between the science and the WFS optical pathes.

During the engineering runs, we used focal enlargers (both in the visible and in the near IR to adapt the CCD/IR array sampling) which degraded the image quality down to approximately lambda / 4 rms at 500nm. This is still acceptable - although marginaly - compared to the residual lambda / 2 rms at 500 nm typical of compensated images.

System behavior and Tests on the sky

PUEO was extensively tested during three runs in the first semester 1996. The performance were evaluated in both the visible and the near infrared wavebands, using a 2K x 2K CCD and a 256 x 256 Nicmos array (loaned by the University of Montreal). The emphasis was put on the performance in the near IR, where the instrument was expected to give its full potential. The commissioning tests included purely engineering tests, performance evaluation and scientific programs. The latter were intended to test in "real life" what could be achieved by the instrument, and to set-up the data acquisition and reduction procedures. The engineering tests were to check that all the functionalities performed as expected and make the necessary calibrations (wavefront sensor motions, ADC calibrations, etc). In this section, we will report only on the result of the performance evaluation, in term of wavefront and image characteristics.

One of the main goal in designing this system was to make a user friendly, robust interface. The user is presented with a limited choice, simple interface : Basically, a one-button ``Start/Stop compensation''. This turned out to be achievable, and efficient both in term of system operation and performance. It covers all cases, from the brightest to the dimmest objets (R=17), thanks to the close-loop optimized modal control. In turn, the overhead of the system is very small : the set-up on an object, including the mode gains optimization automatic procedure, takes less than one minute. Because the instrument focus is taken care of by the adaptive compensation, the overhead is actually less than for a standard, bare CCD imager.

Created 4 March 95.
Page maintained by Jean-Luc Beuzit.
Please send comments to : beuzit@cfht.hawaii.edu