As the construction of the Subaru Telescope neared the end and the preparation of the first aluminum coating of the primary mirror on the ground floor of the telescope enclosure was in progress in 1997, dust particles blown into the enclosure became a serious issue. The source of the dust particles was mainly volcano cinder rocks in the immediate vicinity of the dome that were crushed through the construction activities, especially by heavy vehicle traffic around the dome. The mitigation measure proposed was to pave the immediate surrounding of the dome. The Subaru dome has a unique design with the special consideration to the airflow through the structure with a few ventilators for the best seeing condition possible. The heat retained by the pavement that may possibly cause thermals was an immediate concern. We examined several types of pavement materials to solve this problem and decided the most suitable materials and method. As a result, we paved the area using asphalt, and were able to improve seeing performance before midnight observation by painting the surface of pavement area white in 2003.
To get the strategy to confirm image qualities of Subaru Telescope, we have obtained the statistics of seeing measured with auto guider images obtained during scientific observations. In addition to this, we started a regular operation of a stationary DIMM at the Subaru Telescope site. From the data of natural seeing measured with the DIMM, we expect to reveal contributions of telescope vibration, inadequate enclosure ventilation, or optical aberrations including deformation of primary mirror by wind load. The stationary DIMM station consists of one 30 cm diameter DIMM, its enclosure, the local control unit and Linux based control PC. We put our DIMM station at the catwalk of the Subaru enclosure at the level of 12-m from the ground, because the high location from the ground can minimize the influence of ground layer. We describe details of our DIMM station and show seeing data obtained since June 2005 and comparison with the seeing obtained with Subaru auto guider images in order to check whether the enclosure of Subaru Telescope may affect the DIMM to measure the seeing.
The high plateaus in west China (Tibet) may provide good candidate sites possibly for ELT projects. According to satellite weather data, we found that a certain area in Tibet shows potentiality for good astronomical observations with less cloud coverage. We have explored through west Tibet to watch its topography in summer, 2004. We reanalyze meteorological data collected by GAME-Tibet project. We have started weather monitor in two candidate sites in west China; Oma in western area of Tibet and Karasu near the western boundary of China. Monitoring observations using modern astronomical site-testing techniques such as a DIMM and an IR cloud monitor camera will be started to catch up continuous monitoring of seeing and cloud coverage.
We have a plan to install a micro-crack alert system for the primary
mirror of Subaru Telescope based on the monitoring of the acoustic
emission from any incident events. We report the results of our preliminary experiment for characterizing the acoustic properties of actual Subaru primary mirror. The attenuation of acoustic wave was confirmed to be small enough to allow detection of such events at any locations of the mirror. The position of incident events that might lead to the generation of possible micro-cracks can be identified within less than 3 cm accuracy by placing seven acoustic sensors along the circumference of the primary mirror.
The Subaru Telescope has been stably operated with high image quality since common use began in December 2000. We have updated the following items in order to achieve further improvement of observation efficiency, image quality, and tracking. 1. High reflectivity of mirrors. The reflectivity of the primary mirror has been maintained, yielding 84% at 670 nm by regular CO2 cleaning (every two to three weeks). We successfully carried out the silver coating of the Infrared secondary mirror in April 2003 without over-coating. The reflectivity has been maintained at greater 98% at 1,300 nm. 2. Image Quality. Subaru telescope delivers exceptional image quality {a median image size of 0.6 arc-second FWHM in the R-band as taken by Auto-Guider Cameras at all four foci; Prime, Cassegrain, and two Nasmyth. We optimized parameters of the servo control system of the Elevation servo, reducing the amplitude of 3{8 Hz vibration mode of the telescope and improving image quality when using the Adaptive Optics (AO) system. 3. Acquisition Guiding. Dithering time was shortened by updating the control software. The slit viewer camera for HDS and the fiber bundle for FMOS are available for acquisition guiding in addition to Auto-Guider Cameras. 4. New instruments. We are developing a new prime focus unit for FMOS and will start functional tests in 2005. Moreover, we have started to prepare new interfaces and facilities for FMOS and the new 188 element AO natural/laser guide star system. The focus switching time
will be shortened by updating the hardware of the IR and Cassegrain Optical secondary mirrors from September 2004, reducing it to 10 minutes to switch the focus between Cassegrain and Nasmyth foci.
Institute of Astronomy, University of Tokyo is now planning to build a 6.5-m optical-infrared telescope in Atacama, Chile. This project is called "Univ. Tokyo Atacama Observatory (TAO)", and the site evaluation is now under way. As a part of this evaluation process, we started an investigation to compare the astronomical seeing at Atacama with that at Mauna Kea. Here, we report preliminary results of seeing measurements at several sites in Atacama, carried out on October 2003. In order to separate the temporal and site-to-site variation of the seeing, we used two sets of Differential Image Motion Monitors (DIMMs), each of which has two pairs of 7.4 cm sub-apertures with 20.5 cm separation. Three sites were investigated; the point near the TAO weather station (4,950m), the summit of Cello Chico (5,150m) and the point at 5,430m altitude on Cello Toco. Simultaneous measurements were carried out for three half nights out of four half nights measurements. Although the amount of our data is very limited, the results suggest following: 1) Seeing becomes better and more stable as time passing to midnight (eg. From 0."7 to 0."4 at V-band). 2) Higher altitude sites show better seeing than lower altitude sites.
We have been taking weather data at the location of the Subaru Telescope since 1999. We have also obtained the environmental data on many points in the telescope enclosure and on the telescope structure. Based on those, we will report the statistics of weather data and environmental condition around the Subaru Telescope as well as correlations among them. The statistics of nighttime clear sky ratios at Subaru Telescope site is presented. We have been gathering seeing data since the First Light of the Subaru Telescope in 1999, and we found a clear seasonal variation of the seeing size defined by FWHM method. A strong correlation between seeing sizes and wind velocities/directions is reported.
We report an infrared all sky cloud monitor operating at Subaru telescope at Mauna Kea, Hawaii. It consists of panoramic optics and a 10 μm infrared imager. Aspheric metal mirrors coated with gold (sapphire over-coated) are used in the panoramic optics, which is similar to the MAGNUM observatory's cloud monitor at Haleakala, Maui. The imager is a commercially available non-cooled bolometer array. The system is waterproof and (almost) maintenance-free. The video signals from the imager are captured, averaged over 50 frames, subtracted clear-sky frame and flat-fielded in two minutes interval. The processed cloud images are transferred to Subaru observational software system (SOSS) and displayed combined with telescope/targets information and also stored to Subaru Telescope data archive system (STARS). The processed images will be opened on Internet web site.
The SUBARU Telescope has four focal positions to allow different types of instruments. At present, there are four different Top Units; three types of secondary mirrors and one primary focus unit. These units have the weight of about 3 tons, and they need to be installed or changed high above in the air, with the telescope in its rest position, namely, pointed to the zenith. In order to carry out this exchange work safely and securely, in already a difficult working condition of high altitude place like Mauna Kea, we developed an automatic exchanger with remote control, called Top Unit Exchanger (TUE).
We developed two DIMMs (Differential Image Motion Monitor) for simultaneous seeing measurements at multiple sites. Simultaneous seeing measurements enable us to distinguish temporal variation and site to site variation of seeing, and clarify which site has better seeing. We set the aperture separation of the DIMM at 50 cm for accurate measurements under good (~0.3 arcsec) seeing conditions for Mauna Kea. Our system can also be tuned for moderate to bad seeing conditions by changing the effective focal length of the optics. The frame of our DIMM device is made of CFRP in order to avoid deformation with temperature change and to reduce its weight. We will present the details of our DIMM system and some results of simultaneous seeing measurements at Mauna Kea, Hawaii.
Subaru Telescope has currently achieved the following performances. 1. Image Quality. (1) Subaru Telescope delivers a median image size, evaluated by equipped Auto Guider (AG) cameras, of 0.6-0.7 arcsec FWHM in the R and I-band at all the four foci: Prime (P), Cassegrain (Cs), and tow Nasmyth (Ns). (2) The best image sizes obtained so far are 0.2 arcsecs FWHM without AO in near-infrared (IR), less than 0.1 arcsec FWHM with AO, and 0.3 arcsec FWHM in optical and mid-IR wavelengths. (3) Stable Shack-Hartmann measurement enables one to keep the errors of Zernike coefficients to less than 0.2μm which corresponds to ~0.1 arcsec image size. 2. Tracking and Pointing. (1) Blind pointing accuracy is better than 1 arcsec RMS over most of the sky. (2) Tracking accuracy is better than 0.2 arcsec RMS in 10 minutes. (3) Guiding accuracy is between 0.8 and 0.18 arcsec RMS with 12-18th magnitude guide stars. 3. IR secondary mirror (M2). (1) Chopping performances: typical figures are at 3 Hz, 80% duty cycle with 30-60 arcsec chopping throw. (2) Tip-Tilt performances: Position stability is about 0.030 arcsec RMS for the effective closed-loop bandwidth less than 5 Hz. 4. Others. (1) The reflectivity of the primary mirror has been maintained at higher than 85 and 95% at 670 and 1300 nm wavelengths by regular cleaning with CO2 ice every two to three weeks. (2) The reflectivity of the blue-side image rotator (ImR) at Nasmyth-optical focus was improved after re-coating of mirrors.
KEYWORDS: Mirrors, Telescopes, Control systems, Observatories, Weather forecasting, Space telescopes, Cooling systems, Fourier transforms, Temperature metrology, Computing systems
Based on the successful numerical weather forecasting performed by collaboration between MKWC and Subaru Telescope, we develop a temperature control system of the primary mirror of the Subaru Telescope. Temperature forecast is accurate 80% in 2 degrees. After to start the operation, the temperature of the primary mirror controlled below 1 degree centigrade compare by the ambient night air temperature in over 70% probability.
The effect of the temperature control for the improvement of the seeing of Subaru telescope seems to be moderately effective.The median of the seeing size of Subaru Telescope on May 2000 to July 2002 is 0.69 arcsec FWHM. We need further investigation whether the improvement is the result of our successful temperature control system of the primary mirror, or the effect of the annual variation of seeing itself. Thus, we need a long period data for verification the effect of the temperature control.
We describe a new system (WIZARD: Wide-field Imager of Zodiacal light with ARray Detector) for the zodiacal light observation developed by a Korean and Japanese zodiacal light observation group. Since the zodiacal light is faint and wide-spread all over the sky, it consists of a very sensitive CCD camera of a quantum efficiency of 90% at 460(nm) and a wide angle lens with the field-of-view of 49x98 (degree). WIZARD is designed to measure the absolute brightness of diffuse sky in visible wavelengths. The zodiacal component will be separated from the integrated starlight, the airglow continuum and the scattered light in the atmosphere in the data reduction procedure. We got a first image by WIZARD in 2001 at Mauna Kea (4200m, Hawaii) under the collaboration with SUBARU Telescope. We observed the zodiacal light and the gegenschein in 2002 again, and got the excellent images. In this paper, we describe the design of WIZARD and report the performance examined by the laboratory measurements and the observations at Mauna Kea in 2002.
In order to operate large telescope, it is crucial to have a good weather forecast especially of the temperature when the telescope begins preparation, i.e., open the dome to introduce new fresh air inside. For this purpose, the Mauna Kea Weather Center (MKWC) has been established in July 1998 by the initiative of Institute of Astronomy, University of Hawaii. The weather forecast is not a simple matter and is difficult in general especially as in the quite unique environment as in the summit of Mauna Kea. MKWC introduced a system of numerical forecasting based on the mesoscale model, version five, so called MM5, was running on the vector parallel super computer VPP700 of Subaru Telescope for past three years. By the introduction of new supercomputer system at Subaru Telescope, we have prepared new programs for the new supercomputer systems. The long term but coarse grid forecast is available through National Center for Environmental Predict (NCEP) every day, and the MKWC system get the result of simulations on coarse grid over the pacific ocean from NCEP, and readjustment of data to the fine grid down to 1km spatial separation at the summit of Mauna Kea, i.e. Telescope sites of Mauna Kea Observatories. Computation begins around 20:00 HST, to end 48 hours forecast around 0100am next morning. Conversion to WWW graphics will finish around 0500am, then, the specialist of MKWC would take into the result of the numerical forecast account, to launch a precious forecast for the all observatories at the summit of Mauna Kea, at 10:00am HST. This is the collaboration among observatories to find a better observation environment.
One of the major problems to retain the efficiency of a telescope is to achieve and maintain high reflectivity in the wide wavelengths of the coatings of the telescope optics. For coating the large mirrors of Subaru Telescope, we employed the conventional evaporation scheme, in the expectation of uniform coverage of the film. In this paper, we will report the installation and the performance verification of the coating facility. This facility consists of a washing tower for stripping off the old coating, an evaporation coating chamber, two trolleys and a scissors- like lifter for handling the primary mirror. To supply a large number of filaments loaded with uniform quality molten metal, the practical solution is to pre-wet the filaments with the agent metal and keep them in a controlled manner before the evaporation. The aluminum film deposit on the test samples in the 8.3 m coating chamber proved the film thickness uniformity matching with the specification. Reflectivity of the fresh surface was over 90% at visible wavelength. In September 1997, we re-aluminized 1.6 m and 1.3 m mirrors for the first time (at least for ourselves) application to the real astronomical telescopes. The resultant surface reflectivity confirmed the feasibility of using pre-wetted filaments.
A 2.5 m-thick cinder layer immediately below the pier of Subaru (Japan National Large Telescope, JNLT) now under construction on the summit of Mauna Kea in Island of Hawaii was improved by recompacting volcanic cinders obtained by excavation at the site mixed with cement. The purpose was to increase the Young's modulus at very small strains of the supporting ground to 500 MPa so as to make the lowest natural frequency of the pier-ground system in its rocking motion to be more than 4.3 Hz. This requirement is essential for the telescope control system to correct as quickly as possible for external and internal disturbance of the telescope tracking. The results of the field and laboratory geotechnical tests including the measurements of elastic wave velocities and cyclic triaxial tests, which were performed for the design of the ground improvement work and the prediction and evaluation of the Young's modulus of the constructed layer, are described.
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