We are carrying out a 1.8-m aperture off-axis telescope project PLANETS. The PLANETS telescope is characterized by using an off-axis mirror to enable us high-contrast imaging and spectroscopy with low-scattering in the optical system. Our major scientific goal is to observe faint emission around a bright body, such as gases surrounding planets and satellites in the solar system (e.g., Jovian satellite Europa’s water plume) as well as exoplanets. Using a new technology of mirror polishing with a robot arm and 3-point drag probe, we performed the polishing of the off-axis primary mirror. We also developed the whiffletree support with warping harness for the primary mirror support. We will assemble the whole PLANETS telescope system and conduct the first light of stars as technical demonstration in Japan within a year. We have begun coordinating with several high-altitude observatory sites in which the PLANETS telescope will be installed after the technical demonstration in Japan.
KEYWORDS: Tunable filters, Feedback control, Control systems, Digital signal processing, Electronic filtering, Repetition frequency, Signal processing, Mirrors
In ground-based mid-infrared observations the background radiation must be removed. Chopping is a background removal method requiring fast switching of the observation field. For MIMIZUKU, the mid-infrared instrument for the TAO telescope, we have developed a cold chopper which switches the observing field by tilting a movable mirror inside MIMIZUKU, instead of tilting the large secondary mirror.
We require a short transition time, sufficient amplitude, high frequency and steadyness for observation in the chopper movement.
With Repetitive Control we significantly increase performance by iteratively improving a feedforward trajectory and continously adapting to changes in the nonlinear dynamics.
This allows for much shorter transition time (<30 ms) and more freedom in the design of a feedback controller. Furthermore, repetitive disturbances originating from the cryo-cooler can be countered thus improving stability on sky.
Controller design, stabilisation, choice of reference trajectory, real-time computability and performance trade-offs are subjects in this research.
MIMIZUKU, the mid-infrared instrument for the 6.5-m telescope at the University of Tokyo Atacama Observatory (TAO), employs a cold chopper to perform chopping, which tilts a mirror placed on the internal cold optics at about 30 K. The mirror rotates around two orthogonal axes, and its tilt angle is controlled by the balance between the restoring force of the flexural pivots and the magnetic force driven by the coils in the system. In this study, we developed a final-product model of the chopper and tested its onboard performance in MIMIZUKU. This experiment showed that the mirror could be operated with a stability of 3.83×10−4 and 3.29×10−4 degrees, and a transition time of 31.2 and 32.2 milliseconds for each rotation, when both rotations were driven at 5 Hz with an amplitude of 0.59 degrees, satisfying the performance requirements.
SMI (SPICA Mid-infrared Instrument) is one of the three focal-plane science instruments for SPICA. SMI is the Japanese-led instrument proposed and managed by a university consortium. SMI covers the wavelength range from 10 to 36 μm with four separate channels: the low-resolution (R = 60 – 160) spectroscopy function for 17 – 36 μm, the broad-band (R = 5) imaging function at 34 μm, the mid-resolution (R = 1400 – 2600) spectroscopy function for 18 – 36 μm, and the high-resolution (R = 29000) spectroscopy function for 10 – 18 μm. In this presentation, we will show the latest design and specifications of SMI as a result of feasibility studies.
PLANETS will be a 1.8-m off-axis telescope combined with contrast enhancement techniques, enabling us to observe faint emissions in the vicinity of bright objects. This “high dynamic-range” capability is largely dependent upon precision of telescope optics as well as atmospheric distortion. We present feasibility study of monitoring water plumes on Europa, neutral torus close to Enceladus, and ionosphere on Mars using PLANETS telescope. To test feasibility of high dynamic-range observation under actual conditions of wavefront error, we modeled propagation of light though the system based on Fraunhofer calculation taking into account for wavefront error made by atmospheric distortion and by primary mirror figure error. Then point spread function is calculated for several cases of figure errors under use of adaptive optics. The modeling result predicts that the moderate or high-precision primary mirror is mandatory to accomplish the high dynamic-range observation. We also present the latest design of PLANETS, especially focus on the support structures of primary mirror. We employ 36-point whiffletrees with 33 warping harnesses for axial support, and 24-point Schwesinger support for lateral support. The active support system is expected to reduce pre-polished RMS error from 1.51 μm to 0.66 μm corresponding to 70% reduction in total volume of final polish. The laboratory experiment using one third part of prototype whiffletrees shows supporting force RMS repeatability < 0.005 kgf, and drive hysteresis < 0.7% of load range, which are precise enough to control or to keep the primary mirror figure.
We have developed a new coronagraph using digital micromirror device (DMD) to observe faint emissions close to a bright objects in our solar system such as water plumes on Europa and Enceladus, plasma emissions in giant planet’s magnetospheres, escaping plasma and neutrals from Venus and Mars, and so on. The focal plane DMD mask enables us to occult planet’s disks and their moons even when their angular dimeter and geometry vary with time. The coronagraph composed of a DMD as an occulting mask on a telescope focal plane, a pupil stop, a narrow-band filter, and others. The DMD located at the focal plane produces color dispersion of entrance aperture on a pupil plane for an extended lightsource. Thus, we designed a proper shape of pupil mask to reduce remaining from the occulted light-source considering wavelength and bandwidth of observations. The coronagraph was installed on a Cassegrain focus of the Tohoku 60-cm telescope at Haleakala observatory in Hawaii. We have been using the coronagraph for observing sulfur ion emissions [SII] 671.6 and 673.1 nm from Io plasma torus since 2018. The DMD occultation reduces light from Jupiter disk and Galilean moons by 2.6×10-3. The system throughput is 56 % of a previous conventional coronagraph. In observation of Io plasma torus, north-south position of [SII] brightness peak shifted by 0.07 jovian radii toward the magnetic equator during three days. Increase of flesh pickup ion possibly makes higher anisotropy or higher ion perpendicular temperature causing the observed magnetic-equatorward shift of the plasma torus.
We report the current status of small-telescope activities and the 1.8-m aperture telescope PLANETS project at Haleakala dedicated to planetary and exoplanetary observations. Continuous monitoring is essential to understand the planetary atmospheric phenomena, and therefore, own facilities with even small- and medium sized telescopes and instruments are important. On the summit of Mt. Haleakala, Hawaii, we are operating a 40 cm (T40) and 60 cm (T60) telescopes for measuring faint atmospheric features such as Io torus, Mercury, and so on. It has uniquely provided long-term Io torus activities for more than ten years. T60 is now observing planetary atmospheres in visible and infrared ranges. The polarization imager DIPOL-2 is also installed to measure the weak polarization of exoplanetary light. In addition, we are carrying out a 1.8-m off-axis telescope project PLANETS at Haleakala. This project is managed by the PLANETS Foundation (www.planets.life) is an international collaboration of several institutes from Japan, USA, Germany, Brazil, and France. This off-axis optical system enables very low-stray light contamination and high-contrast in data, i.e., "high dynamic range". It will achieve unrivaled scientific capabilities on coronagraphy and polarimetry, aimed at detecting exoplanet reflected light and tenuous planetary exo-atmospheres in the Solar system. The main mirror is Clearceram ZHS with a diameter of 1850 mm, which is now on the final polishing process. We completed the telescope design and wind analysis of the mechanical support and tracking. The "split-ring" mount is so stiff that it has a first vibration mode above 50 Hz.
We report the development of infrared Echelle spectrograph covering 1 - 4 micron and mid-infrared heterodyne
spectrometer around 10 micron installed on the 60-cm telescope at the summit of Haleakala, Hawaii (alt.=3000m). It is
essential to carry out continuous measurement of planetary atmosphere, such as the Jovian infrared aurora and the
volcanoes on Jovian satellite Io, to understand its time and spatial variations. A compact and easy-to-use high resolution
infrared spectrometer provide the good opportunity to investigate these objects continuously. We are developing an
Echelle spectrograph called ESPRIT: Echelle Spectrograph for Planetary Research In Tohoku university. The main
target of ESPRIT is to measure the Jovian H3+ fundamental line at 3.9 micron, and H2 nu=1 at 2.1 micron. The 256x256
pixel CRC463 InSb array is used. An appropriate Echelle grating is selected to optimize at 3.9 micron and 2.1 micron for
the Jovian infrared auroral observations. The pixel scale corresponds to the atmospheric seeing (0.3 arcsec/pixel). This
spectrograph is characterized by a long slit field-of-view of ~ 50 arcsec with a spectral resolution is over 20,000. In
addition, we recently developed a heterodyne spectrometer called MILAHI on the 60 cm telescope. MILAHI is
characterized by super high-resolving power (more than 1,500,000) covering from 7 - 13 microns. Its sensitivity is 2400
K at 9.6 micron with a MCT photo diode detector of which bandwidth of 3000 MHz. ESPRIT and MILAHI is planned to
be installed on 60 cm telescope is planned in 2014.
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