SODAR Study at Subaru Telescope
August 13, 2008
Have you visited the Subaru Telescope lately? If so, strange noises may have been heard like the song of an extraterrestrial bird. The sounds actually belong to an instrument that probes the sky immediately above Subaru. The instrument could be thought of “new age”, yet is simply a new wrinkle on an old idea.
Local Conditions at Subaru
The Subaru Telescope sits atop the summit of Mauna Kea, a solitary volcanic peak 14,000 feet above sea level. This mountaintop location is ideal for astronomical study because of superb geographic qualities and meteorological conditions. First and foremost is that the summit is above 40% percent of the earth’s atmosphere and 90% of the water vapor, making for clear and dry observing conditions. Other factors include stable winds, high clarity, low light pollution, and isolation in the middle of the Pacific Ocean. Although these positive factors make Mauna Kea one of the best locations for astronomical research, Subaru was designed to mitigate the minimal atmospheric influences that still remain.The shape of the cylindrical dome, the computer controlled ventilation system, and the windscreen baffling system contribute to astronomical observing under near-ideal conditions.
To better understand local site-specific conditions at Subaru that play a significant role in the quality of observations, in early 2008 a small team of Japanese researchers began studying atmospheric stability/turbulence and wind speed above the Subaru site. The first campaign into the atmospheric turbulence at Subaru was conducted as an essential part of the initial site survey for the observatory, starting in 1987. The Sound Detection and Ranging (SODAR) system remotely measures the vertical turbulence structure and the wind profile of the lower layers of the atmosphere near the telescope to obtain a vertical profile of optical turbulence. SODAR systems are like radar (radio detection and ranging) systems except that sound waves rather than radio waves are used for detection. Simply put, SODAR transmits a short pulse radio signal and measures the time it takes for the reflection to return. The aim of the SODAR system is to understand and monitor the lower atmosphere boundary layer (< 1 mile above) at the Subaru site and feed back that information for improved telescope and dome operations. In addition, because the motion of the atmosphere is the result of wind flow and turbulence, irregular fluctuations of horizontal and vertical wind currents, knowing its properties in the immediate vicinity of Subaru would assist in understanding data variability identified during research, which include distortion of images and measurement fluctuations.
How Does SODAR Work?
Essentially, SODAR measures reflection due to the scattering of sound by atmospheric turbulence. The systems normally operate by issuing an acoustic pulse as a single-frequency “ping” or a frequency-coded “song”, and then listen for the return signal. Subaru’s SODAR system uses a frequency-coded pulse, comprised of several different wavelengths, which are emitted serially, causing the system to make a singing noise when in operation (listen to audio file). Frequency coding of the transmit pulse is done to gain maximum altitude without losing altitude resolution. Generally, both the intensity and the frequency of the return signal are analyzed to determine the wind speed, wind direction and turbulent character of the atmosphere. A vertical profile of the atmosphere as a function of height can be obtained by analyzing the return signal at a series of times following the transmission of each pulse. The maximum vertical range of this SODAR system is approximately 5,250 feet (1,600 meters).
At Subaru, the SODAR system, installed in February 2008, is small, compact, and mounted on the roof of the control building immediately adjacent to the dome (see photos). The position is approximately 52 ft (16 m) below the telescope mirror, so the minimum altitude for SODAR measurement roughly corresponds to the position of the telescope mirror. During operation, the system emits a multi-tone acoustic pulse directly into the atmosphere at 10-second intervals and then registers the return signal. The frequency shift of the echo varies according to the wind speed, due to the Doppler effect, while the echo intensity varies according to the turbulence and structure. This system allows for real-time atmospheric data without any human intervention due to a pulse-coding technique and a fixed echo detection subroutine.
During the first three months of operation (Feb to May 2008), the stability of the instrument was considered to be adequate with valid data increasing in the initial few hours of the evening, decreasing toward midnight, and increasing in the morning again. Preliminary results show that a high refractive index layer with a high turbulence profile may exist around 2,000 ft (600 m) and 3,300 ft (1,000 m). However, the volume of valid data was insufficient to draw conclusions.One of the reasons for data loss could be background sound at Subaru, because SODAR is sensitive to acoustic noise and echoes. One of the possible causes is a nearby neighbor observatory with an exhaust fan facing Subaru that starts up around sunset time, and the noise from that fan is louder than the SODAR pulses. The results were first presented in June 2008 at the SPIE conference in Marseilles, France, and should soon be published.
After reviewing initial results, the astronomers operating SODAR feel that further characterizations are necessary to understand the observing conditions at Subaru. As of this writing, regular operations continue to monitor seasonal or otherwise long-term variations.