The IPIE Adaptive Optical System Application For LEO Observations

Transcription

1 The IPIE Adaptive Optical System Application For LEO Observations Eu. Grishin (1), V. Shargorodsky (1), P. Inshin (2), V. Vygon (1) and M. Sadovnikov (1) (1) Open Joint Stock Company Research-and-Production Corporation Precision Systems and Instruments, Moscow, Russia (2) Applied Science Center Femto, Zelenograd, Russia ABSTRACT Experimental results of the application of multichannel adaptive optical systems for observing LEO with the help of ground-based 1-meter telescopes under the natural conditions of sunlight are described. Given the typical turbulent state of the atmosphere confining the angular resolution to the limits of 1 2 arc seconds, images of different objects with angular resolution close to the diffraction limit were obtained, owing to the real time compensation of the wavefront atmospheric distortion with the help of adaptive optics. INTRODUCTION The dedicated optical facilities for surveillance of near Earth space are of great interest, for potentially they can give very important information on the images of space objects necessary for their classification, determination of their attitudes, and analysis of supernumerary situations. Besides, the current interest in optical observations is connected with assimilation of near Earth space and the intensive growth of population of space debris. The theoretical limit of the angular resolution of space object imaging is conditioned by the light diffraction at the confined input aperture of the telescope. For instance, for a 1-meter telescope the angular resolution in the visible spectral band is about 0.13 angular second, which corresponds to the linear size, ~0.5 m, at the slant range ~800 m. Such a value of angular resolution gives a chance to register a sufficient amount of resolved elements of the image for a large class of LEO objects, that is to be rich in the content of the space object image. In practice, the fluctuation of atmosphere turbulence refraction prevents reaching the diffraction limit of the angular resolution for ground-based optical telescopes. The real angular resolution is determined by the ratio λ/r 0, where λ is the wave length, and r 0 is the so called Fried parameter [1] which characterizes the spatial coherent properties of the light wave spreading in turbulent atmosphere. In typical conditions λ/r 0 1 2, that means that this resolution is 7 15 times worse than the diffraction resolution of a 1-meter telescope. One of the ways for attaining the diffraction limit of angular resolution is compensation for the phase distortion in the light wave with the help of the adaptive optical systems (AOS) [2, 3]. The key elements of AOS are the wavefront sensor which allows measuring the spatial distortion of the incoming wave in real time, and the multichannel deformable mirror making the proper correction of the wavefront distortions. Historically, the adaptive optics were developed for solving astronomy tasks. The specific character of using AOS for getting the LEO space object images, as compared with the astronomy measurements, is stipulated by the next essential factors. First, superfast-acting AOS are needed. Physically, this requirement is stipulated by the following. If a space object moves very fast, the rapid change of the inhomogeneous turbulent medium the light wave comes through, occurs. In other words, we have a virtual wind across the sight line, the linear velocity of which increases with the growth of the slant range. For example, for the space object angular velocity of 0.6 deg/s, the speed of virtual wind changes from 0 nearby the telescope through 100 m/s at height 10 km. In practice, it brings the necessity of increasing the rapidity of AOS as much as several times, in comparison with astronomy requirements. 1

2 Second, unlike the astronomy tasks, when observing the LEO space objects, there is no reference point source like a star. So, the estimate of the wavefront distortion should be obtained along the extensive object, the form of which is unknown a priori. There are papers in which instead of stars it was proposed to use the so-called laser stars tracking the space objects in the isoplanatism zone. But for that, the system becomes more complex and more expensive. Third, the fast angular motion of a space object forces one to use a high-speed rotary support, for which the visible angular position of the space object in FoV of the telescope is not time-stable, due to the residual errors of the mechanical targeting system, which results in smearing the image if no special measures are assumed. A concept of the application of AOS for observing LEO space objects, with the help of 1-meter telescopes, was developed by the Institute for Precision Instrument Engineering. The results of Applied Science Center Femto have formed the basis of this concept. In AOS a sensing element of signals for controlling the deformable mirror is used, on the basis of revealing the components of the sharpness function gradient, with the help of a parallel optoelectronic processor. The specific character of the AOS architecture has resulted in solving all the three problems above. The accepted ideology of constructing AOS was successfully tested on different telescopes. In this presentation the review of its basic characteristics and the results of its application to the 1-meter telescopes are given. STRUCTURAL LAYOUT AND TECHNICAL CHARACTERISTICS OF AOS Structural layout of AOS is shown in Fig.1. 2

3 Fig.1. Structural scheme of AOS 1 input lens, 2 multichannel deformable mirror with three tilt drives, 3 unit for forming the initial signals for the deformable mirror guidance, 4 beam splitter, 5 CCD-camera of the corrected image, 6 unit of electronic control, 7 optical unit, 8 workstation for AOS control, 9 workstation for controlling CCD-camera of the corrected image and recording its data, 10 operator s workplace. The sunlight reflected from the sun-illuminated space object and received by an optical telescope comes to the input lens 1. The input lens forms the necessary convergence of the input beam. In Fig. 1 one can see the case of parallel input beam. The lens forms a convergent beam behind the focus of which the multichannel deformable mirror 2 is installed. The deformable mirror is equipped with three drives for controlling the tilt of wavefront in a rather great range of angles. The deformable mirror has the initial concave form of its reflecting surface. The convergent light beam reflected from the deformable mirror comes to the unit 3 for forming the initial signals for controlling the deformable mirror. These signals, through the unit of electronic control 6, come to the deformable mirror and its three drives. In such a way an adaptive compensation of the coming wave front distortions is accomplished in the closed loop of feedback. The algorithm for controlling the deformable mirror is based on determining the components of the sharpness function gradient with the help of a parallel electro-optical processor which is described in detail in [4,5]. 3

4 Some portion of the light, with the help of beam splitter 4, is directed to the CCD-camera for the corrected image. The control functions for the AOS and the CCD-camera with the corrected image are accomplished by two PC, 8 and 9, arranged at the operator s working place, 10. All the optical-mechanical and photo-receiving devices are disposed in the optical unit 7. At present, the AOS, with different numbers of channels for controlling the deformable mirror, are developed and used (depending on telescope s diameter). One can see below the technical characteristics of AOS designed for joint operation with meter telescopes having the centrally shaded pupil: - number of channels for controlling the deformable mirror 30; - geometry of zones of the deformable mirror control within the two rings (Fig. 2); - number of zones within the inner ring 12; - number of zones within the outer ring 18; - type of piezo-drives. PZT; - range of control voltage. ±200 V; - range of the deformable mirror incline angles reduced to an input pupil of 0.6-meter telescope 15 angular seconds; - time constant of the deformable mirror control in the closed loop of feedback adjustable ms; - time constant of controlling the drives of the deformable mirror incline in the closed loop of feedback adjustable 2-10 ms; - interface with PC controlling AOS RS485; - interface with PC controlling CCD-camera of corrected image GigE; - overall dimensions of the unit being mounted on the telescope taking into account CCDcamera of corrected image and posts of fastening to the flange of telescope: x250x148 mm; - overall mass of the unit being mounted on the telescope 8 kg; - power supply 24 V, 0.8 A. Fig. 2. Geometry of zones of 30-channel deformable mirror The example of fastening AOS on the flange of alt-azimuth mounting of 0.6-meter telescope is shown in Fig. 3. For operation under low temperatures AOS can be mounted on the telescope in a thermocasing (Fig. 4). 4

5 Fig. 3. Mounting of AOS (optical unit, electronic unit, and CCD-camera with the corrected image) at the focus of the 600mm telescope. Fig. 4. Placing AOS on the telescope covered with a thermo-casing 5

6 The examples of images of different LEO space objects obtained with the help of AOS are given in Figs In Fig. 5 to the left typical images registered by the telescope, AOS being switched off, when the angular resolution is determined by the atmosphere turbulence. In this case the angular atmospheric resolution is about 2 angular seconds. Fig. 5 Seasat 1 image Fig. 6 Terra image 6