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Vol. No. Feature Vol. Vol. 39 No. 3 Journal of the Communications Research Laborato1 ア November 1992 Printed Printed in Tokyo, Japan pp. 465-476 Feature STUDY ON HIGH RESOLUTION INFRARED IMAGING TECHNIQUES By Tadashi ARUGA, Norihisa HIROMOTO, Shin YOSHIKADO, Hideki T AKAMI, Yoshinori ARIMOTO, Mitsuo ISHIZU, Tetsuo AOKI, Hirokazu KATAZA, Masao NAKA *, Hiroyuki FUZISADA へTadao AOKI*, Hiroshi SHIBAI*, Sh 吋i SATO 大Shunichi SATO*, Kazuhoro ASAI*, Motowo TUKAKOSH 戸， Arimichi MINOH 大Korehiro MAEDA 穴Toshinori MAIHARA 大 Denis GINGRAS へShu Wing LI*, and Jean SOUCHA Y* ABSTRACT Two techniques of high resolution infrared imaging have been studied concurrently. They are, are, a) diffraction limited imaging (which compensates for atmospheric effects using speckle interferometry), interferometry), and b) infrared baseline interferometer imaging (which attains higher resolution using using a two-telescope system). In the former, an infrared speckle imaging system with a 1.5 m diameter diameter telescope has been developed. The dif1 仕action limited imaging technique was verified by observations observations of a double starαGem and other objects. In the latter, some fundamental technologies such such as in 合・ared heterodyne detection, correlation and mterference have been studied on the basis of of an indoor experiment. 1. 1. Introduction Imaging ofobjects is divided into two categories, microscopic (e.g., using an electronmicroscope) and macroscopic. This study belongs to the latter and the aim is to achieve higher resolution (improved on order or more) than attained by usual techniques using a large telescope at infrared wavelengths. This This high resolution imaging is useful for many fields, including astronomy and optical remote sensmg. sensmg. This This study was accepted as one of the interdisciplinary studies called “Shosai-Kiso ・Kenkyu ” which is suppo 口ed by the Science Technology Agency of Japan, and was conducted during the 1988- 1990 1990 fiscal year. Scientists and engineers gathered at CRL from other institutes, universities in Japan and from foreign countries to participate in this study. The following two techniques were studied simultaneously: (A) (A) Diffraction Limited Imaging: recover a clear image from images blurred by atmospheric turbulence turbulence to obtain a resolution of the telescope ’ s diffraction limit using the method of speckle interferometry. interferometry. (B) (B) Inf 均red Baseline Inte ゆrometer Imaging: use a two-telescope system as an interferometer 噂 Researchers from outside of CRL (see Appendix). 465 466 T. Aruga et al. to to achieve higher resolution (which corresponds to the resolution of a large telescope with the diameter of the interferometer ’s baseline length). 2. 2. Principles of the Two Imaging Techniques (A) Diffraction Limited Imaging The resolution of a telescope with a diameter D at wavelength λis given by e’l、 Aθ ＝~ ・・、，，．． D where, where, LlO means angular resolution, which is confined by diffraction. In principle, one can get higher resolution resolution by using a larger diameter. However, images are blurred by atmospheric turbulence. The resolution resolution is limited by the seeing size of the atmosphere, even if a very large diameter telescope is used. used. Speckle Speckle interferometry is the best technique, at present, to compensate for atmospheric turbulence. “Speckle ”refers to the grainy struc 旬 re resulting 合om phase disturbance of an electro-magnetic wave. Speckle Speckle pattern is observed when one gets an image with a large diameter telescope at a short exposure time. time. (It should be noted that speckle does not appear for long exposures.) A speckle image includes the the information of diffraction limited image, so that atmospheric effects can be compensated for, i.e., a clear image can be recovered by means of statistical data processing of speckle images based on the speckle speckle interferometry. Since Since Labeyrie(I l first presented a method of speckle interferometry, other methods have been presented presented by Knox and Thompson<2>, Walker<3 l, Bates< 4 land others. The concept of diffraction Iimited imaging imaging is shown in Fig. 1. In terms of spatial frequency, the speckle interferometry is a procedure to H (U) : OPTICAL TRANSFER FUNCTION U : SPATIAL FREQUENCY uu，，u、‘．，， a、、 。、 、、 、 、 ．． 、、、 、、、 、、、 、、 、、 、、 、 、、 D u + 一AM ｛ F官正 MFU TT ON LT MYL T｝ LIMIT BY －品 ム ATMOSPHERIC EFFECT Fig. Fig. 1 Concept of diffraction limited imaging. Stu のp on High Resolution I司斤 ared Imaging Techniques 467 approach approach an improved optical transfer function (OTF) of a telescope to the theoretical OTF which is free 企om atmospheric turbulence. (B) (B) Infrared Baseline Interferometer Imaging As described above, the diffraction-limited resolution ofa telescope with diameter Dis λ ID. Can we achieve higher resolution? A technique to answer this question is interferometry using a multiple telescope telescope system. The two-telescope system shown in Fig. 2 is basic. If we use the system with a baseline baseline oflength L, then an angular resolution ofλ IL can be obtained. One can easily get L longer than D, so that a higher resolution imaging which corresponds to a very large telescope with diameter L, is is possible by the two-telescope interferometer. A basic theoretical study ofradio wave interferometry was done by Swenson and Mathur<5>. If we define define the radiation distribution of an objectj(x) and the correlator outputg(u), they are in the relation of the Fourier transform, where xis angle (or length) and u is spatial frequency. f(x) f(x) = F[g(u)], (2) where F[] means Fourier transform. By changing baseline length L, g(u) can be obtained as a function of of u (since u =LI λ） and finally j(x) can be retrieved using Eq. (2). This This technique has already been developed and realized using radio interferometers (in radio astronomy). astronomy). However, it has not been realized at the optical-wave region except for a fundamental study study by C. H. Towns' group<6l in the U.S. We a抗empted this technique at the infrared wavelength region region around 10 μm as his group. OBJECT f(X) TELESCOPE1 DIAMETER D DIAMETER D 9(U) Fig. Fig. 2 Two-telescope infrared baseline interferometer system. 3. 3. Outline of the Study 3.1 3.1 Organization of the Study Group Members who participated in this study are listed in the Appendix, and the distribution of the work is is shown in Fig. 3. 468 468 T. Aruga et al. TOTAL TOTAL MANAGEMENT (LEADER) Aruga Aruga DIFFRACTION DIFFRACTION LIMITED IMAGING INFRARED BASELINE INTERFEROMETER IMAGING IMAGING 1. 1. INFRARED CAMERA ・DETECTOR/OPTICS ・DETECTOR/OPTICS 1. INF RARED HETERODYNE Hiro 田口 to, Takami. ・LO C02 LASER Arimoto, Arimoto, Sato(Shu), Asa i. Tukakosh ,i Mi noh ij F ij isada, Sato (Shun) ・HETERODYNE DETECTION ・DATA ・DATA PROCESSING Yoshikado, Ishizu Aoki Aoki (Tetsu), Kataza 2. 2. INTERFERENCE TECHNIQUE 2. 2. SPECKLE INTERFEROMETRY ・SIMULATION ・OPTICS ・OPTICS Naka, Li Hiromoto, Hiromoto, Takami. Maihara ・EQUIPMENT ・ALGORISM/SIMULATION ・ALGORISM/SIMULATION Yoshikado. Shibai Gingras. Gingras. Souchay. Kataza Kataza RADIATION/PROPAGATION RADIATION/PROPAGATION ・Aoki ・Aoki (Tada) ・Maeda Fig. Fig. 3 Organization of the study group. 3.2 3.2 Study of the Two Imaging Techniques 3.2.1 3.2.1 Diffraction limited imaging (l) (l) Experimental study Several Several equipments have been designed and constructed for realizing the diffraction limited imaging imaging system using speckle interferometry. These are briefly described below. Inf トared Camera: An infrared camera with a two-dimensional a汀 ayofl28x 128HgCdTedetectorwas developedC7l. developedC7l. It was installed on the Nasmith focus of the l.5 m diameter telescope ofa new opticaI facility facility called the Space Optical Communication Research Center<8l (see Fig. 4). Speckle Speckle optics: An additional optical component, consisting of five lenses and having magnification 0, l0, was designed and fabricated. It was attached to the infrared camera for taking speckle images with 0.2 ”X 0.2 ”resolution. There are very few in 仕ared speckle camera systems in the world. CRL ’s system and and NOAO (U.S.)'s system<9l are two of them. S111 dr 011 Hi ghR eso lu1io11 !n fi- ar ed /111 agi 11 g Tec h111q11 es 469 Fig . 4 The 1.5 m diameter telescope system (A: Infrared camera, B: CCD camera). 2 3 ！す II' 4 5 6 Fig. Fig. 5 Sequential speckle images taken at 1/ 12 sec intervals at the wavelength of 2.2 μm . 470 470 T. Aruga et al. Data-acquisition Data-acquisition and analysis system: Short exposure speckle images are taken at 12 Hz and written into into computer memory at a data rate of about 400 kB/s. Data were analyzed using a UNIX-based workstation. workstation. An example of a speckle image of.a star using the imaging system is shown in Fig. 5. The experiment experiment was done at the wavelength of 2.2 μm. (2) (2) Theoretical study As mentioned in Section 2, there are many methods for speckle interferometry including Labeyrie, Labeyrie, Knox- Thompson, Walker methods and Shift-And-Add (SAA) method. We wrote programs for for these standard methods, and at the same time, presented a new method: The Cross 幽 Correlation SAA (CC SAA) method<10>, which is an improved method of the usual SAA method. A demonstrative result of this method is shown in Fig. 6, which shows a comparison between the CC SAA method and the SAA method. method. (3) (3) Results of diffraction limited imaging Combining the experimental study with theoretical study, we developed an imaging system which is is based on speckle interferometry. Actual observations have been done with several infrared stars in order order to verify the performance the imaging system developed. An example is shown in Fig. 7, which is an observation of a double starαGem. Figure 7(a) is an example of a speckle image taken with a short exposure time (1/12 sec), and Fig.
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