Time-domain astronomy is important in the field of modern astronomy, and monitoring observations in the mid-infrared region with 1% photometric accuracy to study the variables and transients is becoming essential. The non-uniformity of the sensitivity caused by the optical characteristics of instruments and differences in the response curves of individual detector pixels degrade photometric accuracy. Therefore, to achieve 1% photometric accuracy, a flat-field correction for the non-uniformity with an accuracy of better than 1% is required. We developed a flat calibration unit (FCU) consisting of a silicon lens, a blackbody source, and two flat folding mirrors. We conducted proof-of-concept tests of the FCU by measuring the accuracy and stability of flat frames obtained using the FCU. The accuracies of the flat frames were 0.23% at 7.7 μm, 0.43% at 9.6 μm, 0.34% at 11.5 μm, and 0.84% at 20.9 μm, which are sufficient to achieve 1% photometric accuracy. The flat frames obtained using the FCU were stable over a period of 29 h within the accuracies of 0.13% at 7.7 μm, 0.12% at 9.6 μm, 0.22% at 11.5 μm, and 0.52% at 20.9 μm, indicating that it is sufficient to obtain flat frames once per night.
MIMIZUKU is a mid-infrared instrument for the TAO 6.5-m telescope under construction in the Atacama Desert, Chile, and will be the world’s first mid-infrared monitoring observation station. We aim to achieve a photometric accuracy of 1%. For this purpose, highly accurate flat fielding with an accuracy of 0.1% is needed. Although flat fielding has been conducted using sky images and dark images conventionally, the correction has uncertainties of several percent. The reason is that the non-linearity of the detector is not considered. To improve this, it is necessary to create flat frames from data in the same count level as during observation. Highly accurate flat frames were derived by taking differential counts against the time variation of the atmospheric radiation. However, this method cannot be used under stable conditions suitable for observations. Therefore, we developed a flat calibration unit which irradiates the detector uniformly and vary the irradiation intensity with time to enable the improved flat fielding under any conditions. We designed the unit that irradiates the detector uniformly by placing a silicon lens and a blackbody source in front of the camera. The blackbody source is put at the pupil position of the optical system. We made some tests to create flat images with the unit. By improving flat fielding, we have successfully corrected for patterns originating from the detector, which appeared in the conventional one. We also clarified that the accuracy of the improved flat fielding was 0.29%, while the accuracy of the conventional one was 1.3%.
Cold choppers are fast beam-switching tip-tilt mirrors installed in the cold optics of mid-infrared instruments. They enable chopping observations, required for ground-based mid-infrared observations to subtract the bright background radiation, without moving telescope mirrors. In the era of next-generation extremely large telescopes, the telescope mirrors cannot be moved due to the size. Therefore, cold choppers are a key technology for groundbased mid-infrared instruments for such large telescopes. In this study, we develop a prototype cold chopper for TAO/MIMIZUKU, the mid-infrared instrument for the TAO 6.5-m telescope, and evaluate the performance in a cryogenic environment at 20 K. It is confirmed that the prototype shows almost the same response as at room temperature and achieves 2-axis square-wave motion with an amplitude of 0.84 deg, a settling time of ∼40 ms, and a frequency of ≥2 Hz. The evaluated power dissipation is ∼5mW. Stability is found to be slightly worse than required (6 × 10−4 deg) due to mechanical vibration caused by the cryocooler used in the experiment. We plan to mount this chopper on MIMIZUKU to check the effects of such vibrations in the on-board environment.
The University of Tokyo Atacama Observatory (TAO) is a project to build and operate an infrared-optimized 6.5m telescope at the summit of Cerro Chajnantor (5640 m.a.s.l). This is promoted by Institute of Astronomy, Graduate School of Science, the University of Tokyo in collaboration with many universities and institutes. The project is now approaching the final phase of the construction. Production of major components are almost completed. The primary mirror fabricated by Steward Observatory Richard F. Caris Mirror Lab in the University of Arizona was temporarily assembled in its support system and confirmed its performance by the optical test in the laboratory. The telescope mount, the enclosure system, and the mirror coating system were fabricated in Japan and already shipped to Chile. They are now stored in an open yard located in the foot area of Cerro Chajanator. The expansion of the summit access road, the summit leveling, the foundation work was completed. Now the construction work of the summit facilities is on-going. TAO will equip three instruments in early science phase. A near-infrared instrument SWIMS is completed, and now used as a PI-type instrument of Subaru telescope. A near-infrared spectrograph NICE which was used on the 1.6m Pirka telescope in Japan is being refurbished for TAO. A mid-infrared instrument MIMIZUKU successfully saw the first light on Subaru telescope and is being prepared for TAO in Japan. We expect to start science operation in FY2023.
MIMIZUKU is the first-generation mid-infrared instrument for the TAO 6.5-m telescope. It has three internal optical channels to cover a wide wavelength range from 2 to 38 µm. Of the three channels, the NIR channel is responsible for observations in the shortest wavelength range, shorter than 5.3 µm. The performance of the NIR channel is evaluated in the laboratory. Through the tests, we confirm the followings: 1) the detector (HAWAII 1RG with 5.3-µm cutoff) likely achieves ∼80% quantum efficiency; 2) imaging performance is sufficient to achieve seeing-limit spatial resolution; 3) system efficiencies in imaging mode are 2.4–31%; and 4) the system efficiencies in spectroscopic modes is 5–18%. These results suggest that the optical performance of the NIR channel is achieved as expected from characteristics of the optical components. However, calculations of the background levels and on-sky sensitivity based on these results suggest that neutral density (ND) filters are needed to avoid saturation in L ′ - and M′ -band observations and that the ND filters and the entrance window, made of chemical-vapor-deposition (CVD) diamond, significantly degrade the sensitivity in these bands. This means that the use of different window materials and improvements of the detector readout speed are required to achieve both near-infrared and long-wavelength mid-infrared (>30 µm) observations.
Since the University of Tokyo Atacama Observatory (TAO) is located in extremely high altitude (5,640 m.a.s.l.), safety management is one of the most important issues for the project. We have developed safety management program for work at the site in order to prevent medical illness for humans. In this program all staff have to take medical examinations such as Electrocardiogram and Hipobaria before their work starts. The results are reviewed by medical doctor. Only authorized staff can be permitted to work at the site. During stay in site, all staff need to always use oxygen supply because there is only half of the oxygen at the site. It is also important to understand physical workload at the site. Our safety staff reviews it and determines necessary resting time for each worker, e.g. great load works need to work 50 minutes and rest 10 minutes. In addition to low air pressure, very low temperature, extremely high UV radiation, and extremely dry atmosphere should be concerned. Our program requests all staff to use winter clothes, sunblock and UV protection sunglasses and a lot of potable water consumption. Keep communication is also very important to secure the safety. Normal telephone communication is not available as well as internet connectivity. We have established satellite communication as well as UHF internal communication for general works and safety coordination with other observatories in Atacama area.
‘Field Stacker’ is a unique system mounted on MIMIZUKU, a mid-infrared instrument for the TAO 6.5-m telescope. This system obtains a pair of distant targets simultaneously and aims at performing relative photometry with an accuracy of a few percent. A key to achieve the accurate relative photometry is precise flat fielding. We have developed a new method for the flat fielding using time variation of the sky background. We analyzed the data obtained in an engineering observation at the Subaru in 2018. The error of the flat fielding and the total error propagated from the flat fielding are estimated to be 0.2–0.3% and 0.5%, respectively.
The Near-Infrared Cross-dispersed Echelle spectrograph (NICE) is a first light instrument for the TAO 6.5 m telescope. The instrument covers a wavelength range of 0.9 to 2.4 µm and has a resolving power of λ/∆λ ~2,600. NICE was first used on the 1.5 m infrared telescope at the National Astronomical Observatory of Japan from 2001 to 2005 and on the 1.6 m Pirka telescope in Japan from 2009 to 2018. We are now upgrading the cryogenics and computer system for the installation of NICE on the TAO telescope. Here we report on the current status and future schedules of instrumentation updates, control system, and modified specifications of NICE for its transfer to the TAO 6.5 m telescope.
The Mid-Infrared Multi-field Imager for gaZing at the UnKnown Universe (MIMIZUKU) is developed as the first-generation mid-infrared instrument for the University of Tokyo Atacama Observatory (TAO) 6.5-m telescope. MIMIZUKU performs medium-band imaging and low-resolution spectroscopy in 2-38 microns and enables highest-spatial-resolution observations in the long-wavelength mid-infrared beyond 25 microns. In addition, MIMIZUKU has a unique opto-mechanical device called ‘Field Stacker’, which enables us to observe a distant (<25 arcminutes) pair of target and reference objects simultaneously and improves accuracy of atmospheric calibration. This function is expected to improve photometric accuracy and quality of spectroscopic data even in the long-wavelength mid-infrared regions, where the atmospheric absorption is severe. In 2018, engineering observations of MIMIZUKU were carried out at the Subaru telescope, and its first-light was successfully achieved. In the engineering observations, the imaging and spectroscopic functions in the mid-infrared wavelengths (7.6-25 microns) were confirmed to be working almost as expected, although the sensitivity is still worse than the background-limited performance by a factor of a few due to high readout noise. The Field Stacker was also confirmed to be working as expected. It is confirmed that the photometric instability can be reduced to a few percent by using Field Stacker even when the atmospheric transmittance varies by 10% in time. It is also confirmed that spectroscopic observations can be performed not only in 10-micron band but also in 20-micron band, where the spectroscopic observations are difficult even at the Mauna Kea site. We report the results of these on-sky performance evaluations.
Institute of Astronomy, Graduate School of Science, the University of Tokyo is promoting the University of Tokyo Atacama Observatory Project, which is to construct an infrared-optimized 6.5m telescope at the summit of Co. Chajnantor (5640m altitude) in northern Chile. The high altitude and dry climate (PWV-0.5mm) realize transparent atmosphere in the infrared wavelength. The project is now approaching the final phase of the construction. Production of major components are almost completed: Production and preassembly test of a telescope mount and dome enclosure have been completed in Japan, and they are being transported to Chile. Three mirrors, the 6.5m primary, 0.9m secondary, and 1.1m-0.75m tertiary mirrors and their support systems have been all completed and tested in the USA. An aluminizing chamber have been fabricated in China, and its tests have been carried out in Japan. Development of two facility instruments, SWIMS and MIMIZUKU, are also completed. They were transported to the Subaru telescope, successfully saw the first light in 2018, and are confirmed to have the performance as designed. On-site construction work at the summit is now underway. Expansion of a summit access road from the ALMA concession was completed in 2019. Installation of foundation will follow, and then erection of the dome enclosure and a control building. The construction works are delayed by COVID-19, and we expect to complete the dome enclosure by Q3 of 2021. The telescope will be installed inside the dome and see the engineering first light by early 2022.
The telescope of the University of Tokyo Atacama Observatory has a primary mirror with a diameter in 6.5m. In order to fabricate the reflecting film initially on the mirror surface and to maintain its optical performance over a long period, a mirror{coating facility will be installed in operation building beside enclosure of the telescope at the summit of Co. Chajnantor (5,640m). The facility consists of mirror coating chamber, cleaning unit for stripping off the old film and clean-up the mirror, and a cart with a lifter for handling the primary mirror cell. Almost all equipment, including the main chamber, was completed by early 2020 and engaged as a mirror coating facility. In order to optimize the coating parameters, comprehensive performance tests were carried out (without primary mirror cell which plays a role of a part of chamber). The evaluation items are how long it takes to reach the target vacuum level and parameters of the current, voltage, and application time during ion bombardment and aluminization. Through iterating test, we were able to obtain each parameter that ultimately met the requirements of the TAO telescope mirror.
The University of Tokyo Atacama Observatory Project is to construct a 6.5 m infrared-optimized telescope at the summit of Co. Chajnantor (5640 m altitude) in northern Chile. The telescope optics uses a Ritchey-Chretien type layout, with an under-sized secondary mirror to reduce stray light caused by thermal emission from the telescope structure. The primary mirror is a F/1.25 lightweight borosilicate glass (Ohara E6) mirror with honeycomb structure, which is developed by Steward Observatory Richard F. Caris Mirror Lab. The telescope has two Nasmyth foci and two folded-Cassegrain foci, which can be switched by rotating a tertiary mirror. The final focal ratio is 12.2 with a field of view of 25 arcmin in diameter. The telescope mount is a tripod-disk alt-azimuth mount. Both the azimuth and elevation axes are supported by and run on hydrostatic bearings, and they are driven by friction drives with servo motors, which are controlled by the telescope control system. It also controls the hexapod mount of the secondary mirror and the pneumatic actuators of the primary mirror support to keep good image quality during the observation. An off-axis Shack-Hartmann sensor installed in each focus measures the wavefront aberration of the telescope optics, then the misalignment between the secondary and primary mirrors is corrected by adjusting the hexapod mount while other aberrations are corrected by the deformation of the primary mirror. The force distribution of the actuators for correction will be calculated by fitting the wave-front errors with a series of bending modes of the primary mirror.
The Simultaneous-color Wide-field Infrared Multi-object Spectrograph (SWIMS) is one of the 1st generation facility instruments for the University of Tokyo Atacama Observatory (TAO) 6.5 m telescope currently being constructed at the summit of Cerro Chajnantor (5,640 m altitude) in northern Chile. SWIMS has two optical arms, the blue arm covering 0.9–1.4 µm and the red 1.4–2.5 µm, by inserting a dichroic mirror into the collimated beam, and thus is capable of taking images in two filter-bands simultaneously in imaging mode, or whole nearinfrared (0.9–2.5 µm) low-to-medium resolution multi-object spectra in spectroscopy (MOS) mode, both with a single exposure. SWIMS was carried into Subaru Telescope in 2017 for performance evaluation prior to completion of the construction of the 6.5 m telescope, and successfully saw the imaging first light in May 2018 and MOS first light in Jan 2019. After three engineering runs including the first light observations, SWIMS has been accepted as a new PI instrument for Subaru Telescope from the semester S21A until S22B. In this paper, we report on details of on-sky performance of the instrument evaluated during the engineering observations for a total of 7.5 nights.
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