The MIRA-I.2 is a 30m baseline optical interferometer located at the Mitaka campus of the National Astronomical
Observatory of Japan. After the detection of the first fringes with Vega in 2002, we have continued
improvement of system performance and have demonstrated stellar diameter measurement in wide band
(600nm-1000nm). Recently, we begin on development of two scientific detectors: spectrometer with separate
fringe tracking system and interfeometric polarimetry. Recent progress and performance of our two system is
MIRA-I.2 is a 30m-baseline two-aperture stellar interferometer working in the visible band (from 600 to 1000 nm).
In this article are presented the up-to-date progress and performance of MIRA-I.2 as well as some ongoing and future
plans. The fast and coarse delay lines are now both evacuated, and the maximum OPD (optical path delay)
compensations are about 16 m and 4 m long, respectively, for the fast and coarse delay lines. The current limiting
magnitude is about I=4.5mag, and stars within the declination range from +8 to +51 degree is possible to be observed
longer than one hour at the elevation angle of 60 degrees and higher. The OPD of the coarse delay line is modulated
by about 128 micrometers around the expected fringe center with the use of PZT, and 187 fringe packets are scanned
during one shot (= 60 seconds duration) to yield the mean visibility of about 10 % internal errors for each shot. The
thermal environment of the building that houses the delay lines and interference optics has been improved very much,
and readjustments of the optical alignment are not necessary for a whole night. The assembly and the setup of the optics
to be used for the fringe tracking experiment are nearly completed.
After the first fringe detection with MIRA-I.2 30m baseline for Vega in June 2002, fringes for Vega and Deneb has been confirmed and then construction continued. Fast delay line has been evacuated and extended from 4m to 8m long. Coarse delay line has been extended from 2m to 4m. Baseline vector has been determined with 0.1mm accuracy. Aluminized mirrors have been changed to gold-plated ones, and the total throughput has become four times larger than before at the 600nm-1000nm band. The photon rate is 150 per ms for a 2 mag (I-band) star and the present limiting magnitude is better than 3mag. A delay modulation PZT has been set to push a cat's eye retro-reflector. Observations have been made for 6 stars with successful fringe packet detections. Visibility stability has been being studied with artificial light sources and Vega, which preliminary results are better than a few percent. A three-color system between 600-1000nm is now on the half way of installation. Gregorian cat's eye retro-reflectors with fine delay line PZTs and fringe tracking control software is planned to be installed.
Developments of fiber linked optical interferometer are reported. This interferometer is a part of MIRA-I.2 interferometer (Mitaka InfraRed and optical Array). MIRA-I.2 is an optical interferometer with a 30 meters long baseline. It consists of two 30cm siderostats, tip-tilt mirrors, vacuum pipes delay lines and detectors. We plan to use two 60 meters long polarization-maintaining fibers for arms of the interferometer, instead of vacuum pipes. The developments include dispersion and polarization compensation of fiber and fiber injection module. In laboratory experiments, dispersion compensation succeeded. The fringe visibility was 0.93 for wide-band, where the central wavelength of light was 700nm, and bandwidth was 200nm, while 0.95 with a He-Ne laser. We used BK7 glass wedge for dispersion compensation. About fiber injection module, basic optical design has completed. The results of our fiber interferometer could contribute to OHANA (Optical Hawaiian Array for Nanoradian Astronomy) project. We present new science targets, white dwarves and T Tauri stars, and an 800 m delayline concept in CFHT for the project.
We propose a millisecond of arc optical/infrared array for stellar territory, MIRA-ST, with nine 4m-aperture off-axis telescopes, whose maximum baseline length is about 600 m. MIRA-ST will have the photon collecting area equivalent to that of a single-dish telescope of 12 m diameter, and the imaging capability better than 1 millisecond of arc resolution at 2.2 micrometers with a high dynamic range of reconstructed images.
Combining the light beams from each pupil telescope efficiently is one of the most difficult tasks. We compare the relative merits among a so-called pair-wise beam combining, an all-on-one beam combining, and a tree-structured beam combining. As for transferring the beams from individual telescopes to a beam combining facility with the loss of photons as small as possible, an optical fiber system is a most interesting substitute for the current mirror-and-vacuum-pipe combination. Specifically, the nature of spatial filtering of optical fibers has been under study in the light of deepening the limiting magnitude attainable without introducing an adaptive optics to each telescope.
With MIRA-ST we will be able to zoom in the stellar territory to unveil the detailed structures and lifecycles of stars of various kinds, and to examine the universality and/or diversity along the coarse of their evolutionary paths. The specific targets of most interesting for us are, among others, T Tauri stars, AGB and post-AGB stars, Cepheids, brown dwarfs, white dwarfs, stellar atmosphere/envelope of low temperature stars, accretion disks, and fundamental structures of main sequence stars.
The outline and the performance of the wavefront tilt correction system for the Mitaka optical and InfraRed Array, MIRA-I.2, is reported. The tilt control system consists of quadrant detectors and PZT-driven tilt mirrors. These components are controlled by three computers connected via local area network system. The control loop frequency of 2660Hz is achieved and this high loop frequency enables the stable operation of the control system with a bandwidth of approximately 80Hz in open loop at 0dB. Preliminary results of one night observation of α Lyr showed that this system suppress the relative wavefront tilt of incoming lights to 0.46 arcsec while it is 1.9 arcsec without control. With an aperture size of 90 mm and the wavelength of 750 nm, this means the tilt control system improves the visibility loss to 9% while it is 89% without control.
At Mitaka campus of National Astronomical Observatory of Japan, we are now constructing the MIRA-I.2 Interferometer, the second stage interferometer of MIRA (Mitaka optical and InfraRed Array Project). The MIRA-I.2 system is an interferometer consisting of two 300 mm siderostats which are placed on a 30 m baseline. Fringe detection is made at 800 nm with APD. Before setting up the regular system of 30 m baseline, we made the test system of 6-m baseline. This system was used for testing and solving the problems of the remote control of fast tip-tilt mirrors, the remote control of siderostats, and the remote adjustment of optics. After the fringe detection of stars with this system, in June 2001, we complete the regular system of MIRA-I.2 of 30 m baseline. In August, 2001, we started the setup of the regular system of 30 m baseline of the MIRA-I.2 interferometer by making the modifications and improvements of each element. From the end of October 2001, the test observations of some bright stars have been made. The fringes of α Lyr were detected with the regular system, in June 2002. Object stars, such as single star, binary star, Cepheid, Mira variable to be observed are investigated. Main subjects of this interferometer system are the observations of star radius and binary star.
Mitaka optical and InfraRed Array first stage (MIRA-I.1) was built following the first fringe detection with the previous first stage instrument called MIRA-I, rearranging its optics. It consisted of two 13 cm-diameter telescopes, collimator lens producing 8 mm propagation beams, long, fine, and fast delay lines, fringe and scintillation sensors, quadrant sensors and tip-tilt mirrors, and so on. MIRA-I.1 was operated from October 19998 to march 1999 and succeeded to get fringes of nine stars, to make stellar diameter observations, to make stellar fringe tracking experiments, and to obtain atmospheric characteristics of Mitaka site.
Fiber optics is useful for connecting the element telescopes of a long-baseline interferometer. Experiments with a fiber- connected interferometer are carried out as a part of MIRA- I.2 interferometer under construction. MIRA-I.2 is an interferometer consisting of two 300 mm siderostats which are placed on a 30 m south-north baseline. For this baseline, it is necessary to use fibers of about 100 m long. Fiber optics will be essential for the interferometric connection of large telescopes, such as Subaru, Gemini, and Keck on the summit of Mauna Kea. In this case, fiber length of about 1 km is needed. We achieved white light fringes using 10 m and 100 m fibers. With the 10 m fiber, visibility of 0.7 was achieved for both He-Ne laser and white light sources. Visibility of 0.7 for white light was obtained by compensating the dispersion effect of fiber with glass plates. Experiments will be extended to longer fibers such as 1000 m.
Long-baseline optical and IR interferometers are being considered as future astronomical instrument plans in Japan since 1994. They are called MIRA projects, indicating Mitaka or Mauna Kea IR array. The next Mitaka optical and IR array proposal is called MIRA-II. It consists of four fixed telescopes as an array for 1mas astrometry and three movable ones for 0.2mas imaging. They are placed in a sideways T- configuration with three 128m arms and extended lines getting the longest baseline of 680m. Each of the telescopes is a 30cm siderostat added with a 20cm beam compressing telescope. MIRA-III is a proposal of Mauna Kea optical/IR array including a 1.4km baseline with 1.5m telescopes. Its shape is a modified Y-configuration. It also aims at precise astrometry including many quasars as well as high resolution imaging of fainter stellar objects than MIRA-II. MIRA-SG, a future proposal of Mauna Kea optical/IR array connecting Subaru with GEMINI, is one of the largest interferometer with an 800m baseline by 8m telescopes. It became possible by using optical fibers fed from each Cassegrain focus with an adaptive optics system. Keck telescopes and other large telescopes on Mauna Kea are also candidates to connect with Subaru.
The Mitaka optical-IR array (MIRA) project is a series of optical and IR interferometers by the National Astronomical Observatory of Japan. We call the first step the MIRA-I project, and the following projects are the MIRA-1.2, MIRA- II, MIRA-SG, and the MIRA-III. The MIRA-I is a prototype for demonstration of stellar interferometry, and the other projects, which are described by Sato et al. and Nishikawa et al., are practical instruments for science. The main purpose of the MIRA-I is to get stellar fringes and skill of fringe tracking. The MIRA-I instrument is located at the National Astronomical Observatory in Mitaka, Tokyo.It consists of two element telescope with 25 cm aperture placed on four meters N-S baseline. The fringe detector operates at visible or around 800 nm with high-speed sampling. In laboratory, we obtained artificial white light fringes in December 1995, and we moved the instruments to the telescope dome at January 1996. At present, we have been trying to get stellar fringes. This paper describe the current status and the progress of the MIRA-I.
The MIRA-1.2 system consists of two siderostats, beam reducers, vacuum delay liens, baseline metrology system, tip-tilt mirrors, beam combine optics, and fringe detector. Two siderostats, of which aperture of the flat mirror is 300mm, are placed apart by 4 meters in the north-south direction. Beam reducer is a Cassegrain optics with the paraboloidal primary and secondary mirrors of the diameter D1 equals 200mm and D2 equals 30mm, respectively. The metrology system with laser interferometers is set up to stabilize the baseline length for astrometry. Piezo-driven tip-tilt mirrors are equipped for the correction of image motions due to the air turbulence and other errors. Delay lines are placed in a vacuum tube. Experiments of the fiber optics is carried out as a part of MIRA-1.2. Developing MIRA-1.2 system, it is aimed to establish the basic techniques of astrometry and future projects, especially of MIRA-II.