Vibrations are a key source of image degradation in ground-based instrumentation, especially for high-contrast imaging instruments. Vibrations reduce the quality of the correction provided by the adaptive optics system, blurring the science image, and reducing the sensitivity of most science modules. We studied vibrations using the Subaru coronagraphic extreme adaptive optics instrument at the Subaru Telescope as it is the most vibration-sensitive system installed on the telescope. We observed vibrations for all targets, usually at low frequency, below 10 Hz. Using accelerometers on the telescope, we confirmed that these vibrations were introduced by the telescope itself, and not the instrument. It was determined that they were related to the pitch of the encoders of the telescope drive system, both in altitude and azimuth, with frequencies evolving proportionally to the rotational speed of the telescope. Another strong vibration was found in the altitude axis of the telescope, around the time of transit of the target, when the altitude rotational speed is below 0.12 arc sec / s. These vibrations are amplified by the 10-Hz control loop of the telescope, especially in a region between 4 and 6 Hz. We demonstrate an accurate characterization of the frequencies of the telescope vibrations using only the coordinates—right ascension and declination—of the target and provide a means by which we can predict them for any telescope pointing. This will be a powerful tool that can be used by more advanced wavefront control algorithms, especially predictive control that uses information about the disturbance to calculate the best correction.
The reflectivity of telescope primary mirror is one of the fundamental parameters that shows the telescope performance. However, it has been difficult to obtain absolute value, especially the wide range spectroscopic performance measured in-situ on the primary mirror due to the lack of suitable measuring instrument. To overcome this challenge, we developed a portable spectrophotometer to measure the absolute spectroscopic reflectivity of telescope primary mirror. Its small dimension and light weight enable in-situ measurement on the primary mirror. This spectrophotometer covers the spectral range from 380 nm to 1000 nm with 2 nm resolution. The incident angle to the measuring surface is 12 degrees. The measurement beam size is about 12 mm in diameter. To obtain the absolute value, we adopted the principle of V-N method for the spectrophotometer. A sequential measurement also enables us to cancel the instability of the instrument.<p> </p> The Subaru Telescope primary mirror was recoated with Aluminum on October 20, 2017. It was the eighth coating work from its arrival at Maunakea, Hawaii in 1998 and was about four years from the previous recoating. Before the recoating work, the reflectivity measured with the spectrophotometer was 70~76 % (@400 nm), 75~80 % (@600 nm), and 73~78 % (@800 nm). The large dispersion of the reflectivity is from non-uniform contamination of the surface, especially from the accumulation of dust particles on the mirror.<p> </p> After the fresh coating of Aluminum, the values returned to 92.1 % (@400 nm), 90.5 % (@600 nm), and 85.8 % (@800 nm) with standard deviation less than 0.6 %. There were the data taken at the outside of the vacuum chamber right after the recoating.<p> </p> The great advantage of our spectrophotometer is its capability of getting absolute spectroscopic reflectivity of the primary mirror in-situ. We can continue to monitor the reflectivity of the primary mirror in-situ using this spectrophotometer, even after the primary mirror is mounted on the telescope. This helps us better understanding of the long-term reflectivity degradation.
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument, under development for the Subaru Telescope, has currently the fastest on-sky wavefront control loop, with a pyramid wavefront sensor running at 3.5 kHz. But even at that speed, we are still limited by low-frequency vibrations. The current main limitation was found to be vibrations attributed mainly to the rotation of the telescope. Using the fast wavefront sensors, cameras and accelerometers, we managed to identify the origin of most of the vibrations degrading our performance. Low-frequency vibrations are coming from the telescope drive in azimuth and elevation, as well as the elevation encoders when the target is at transit. Other vibrations were found at higher frequency coming from the image rotator inside Subaru's adaptive optics facility AO188. <p> </p>Different approaches are being implemented to take care of these issues. The PID control of the image rotator has been tuned to reduce their high-frequency contribution. We are working with the telescope team to tune the motor drives and reduce the impact of the elevation encoder. A Linear Quadratic Gaussian controller (LQG, or Kalman filter) is also being implemented inside SCExAO to control these vibrations. These solutions will not only improve significantly SCExAOs performance, but will also help all the other instruments on the Subaru Telescope, especially the ones behind AO188. Ultimately, this study will also help the development of the TMT, as these two telescopes share very similar drives.
We are now investigating and studying a small satellite mission HiZ-GUNDAM for future observation of gamma-ray bursts (GRBs). The mission concept is to probe “the end of dark ages and the dawn of formation of astronomical objects”, i.e. the physical condition of early universe beyond the redshift z > 7. We will consider two kinds of mission payloads, (1) wide field X-ray imaging detectors for GRB discovery, and (2) a near infrared telescope with 30 cm in diameter to select the high-z GRB candidates effectively. In this paper, we explain some requirements to promote the GRB cosmology based on the past observations, and also introduce the mission concept of HiZ-GUNDAM and basic development of X-ray imaging detectors.
Dome Fuji, on the Antarctic plateau, is expected to be one of the best sites for infra-red astronomy. In Antarctica, the coldest, driest air on Earth provides the deepest detection limit. Furthermore, the weak atmospheric turbulence above the boundary layer allows for high spatial resolution. We plan to perform site-testing at Dome Fuji during the austral summer of 2010-2011. This will be the first observation to use an optical/infra-red telescope at Dome Fuji. This paper introduces the Antarctic Infra-Red Telescope with a 40cm primary mirror (AIRT40) which will be used in this campaign; it is an infra-red Cassegrain telescope with a fork equatorial mount. AIRT40 will be used for not only site testing (measurement of seeing and sky background) and daytime astronomical observation during this summer campaign, but also for remote scientific observations during the 2012-2014 winter-over campaign. For this purpose, AIRT40 has to work well even at -80 degree Celsius. Therefore, we accounted for the thermal contraction of the materials while designing it, and made it with special parts which were tested in a freezer. For easy operation, many handles for transportation and a polar alignment stage were installed. Moreover, we confirmed that this telescope has enough pointing, tracking, and optical accuracy for the summer campaign through the test observations at Sendai, Japan. Because of these preparations AIRT40 is suited for observations at Dome Fuji. In the 2010-2011 campaign AIRT40 will be used to measure the seeing, infra-red sky background, and to observe Venus.
In Antarctica the cold and dry air is expected to provide the best observing conditions on the Earth for astronomical
observations from infra-red to sub-millimeter. To enjoy the advantages in Antarctica, we have a plan to make
astronomical observations at Dome Fuji, which is located at inland Antarctica. However, the harsh environment is very
problematic. For example, the temperature comes down to as low as-80 degree Celsius in winter, where instruments
designed for temperate environment would not work. In this context, we have developed a 40 cm infra-red telescope,
which is dedicated for the use even in winter at Dome Fuji. In designing the telescope, we took account of the difference
of the thermal expansion rate among materials, which were used for the telescope. Movable parts like motors were
lubricated with grease which would be effective at -80 degrees. Most parts of the telescope are made of aluminum to
make the telescope as light as possible, so that it makes the transportation from seacoast to inland and assembling at
Dome Fuji easier. We also report the experiment that we have done at Rikubetsu (the coldest city in Japan) in February