Naoko Ohishi

Long-baseline optical fiber interferometer instruments and science

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.

Polarimetric stellar interferometry by use of birefringent retarders

We propose a polarimetric technique for a stellar interferometer that uses two birefringent retarders inserted into each arm of the interferometer. The retarders shift the interference fringes of the two orthogonal polarizations to opposite directions along an optical path difference (OPD), which enables us to measure the polarized fringe visibilities separately. Laboratory demonstrations were carried out using quartz retarders with a thickness of 1.124 mm, and the separation of the interferometric fringes of +/-10 microm along the OPD was observed. We discuss a calibration method for the acquired fringes to obtain reliable estimations of polarized fringe visibilities. We also discuss feasibility of the proposed technique for the Mitaka optical InfraRed Array (MIRA-I.2).

Recent progress at the MIRA : development of fringe tracking system

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 reported.

MIRA Status Report: Recent Progress of MIRA-I.2 and Future Plans

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.

MIRA-I.2: recent progress

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 cats 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 cats eye retro-reflectors with fine delay line PZTs and fringe tracking control software is planned to be installed.

Wavefront tilt correction system for MIRA-I.2

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.

The instrumentation, object stars and the first observations of MIRA-I.2 (Mitaka optical and infrared array)

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.

Mira Project and the Observations of Mira Variables

We are now constructing the MIRA-I.2 interferometer, the second stage interferometer of MIRA (Mitaka optical and InfraRed Array Project) at Mitaka campus of National Astronomical Observatory of Japan. MIRA-I.2 system is an interferometer consisting of two 300mm siderostats which are placed on a 30m baseline. Fringe detection is made at 800nm with APD. Fringes of some bright stars were obtained with the test system of 6m baseline, on June, 2001. We are now constructing the regular system of 30m baseline. The fringes of Vega were detected with this system, on June, 2002. We discuss the possibility of observations of Mira variables with this interferometer

Large stellar interferometer MIRA-ST

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.