Abstract.The Taiwan Oscillation Network (TON) is a ground-based network to measure solar intensity oscillations to study the internal structure of the Sun. K-line full-disk images of diameter 1000 pixels are taken at a rate of one image per minute. Such data would provide information on p -modes with l as high as 1000. The TON will consist of six identical telescope systems at proper longitudes around the world. Three telescope systems have been installed at Teide Observatory (Tenerife), Huairou Solar Observing Station (near Beijing), Big Bear Solar Observatory (California), and Tashken Observatory (Uzbekistan). The telescopes at the first three sites have been taking data simultaneously since October of 1994. The forth station started operation in July of 1996.
The Taiwan Oscillation Network (TON) is a ground-based network to measure solar K-line intensity oscillations to study the internal structure of the Sun. The TON project has been funded by the National Research Council of ROC since the summer of 1991. The TON will consist of six identical telescope systems deployed at proper longitudes around the world. The headquarters of the TON is located at the Physics Department of Tsing Hua University, Hsinchu, Taiwan, where the telescope systems are designed, built, and tested. The first telescope was installed at the Teide Observatory, Tenerife, Canary Islands, Spain in August of 1993. The second one was installed at the Huairou Solar Observing Station, near Beijing in January of 1994. The third telescope system was installed at the Big Bear Solar Observatory, California in June of 1994. The telescopes at three sites have been taking data simultaneously since October of 1994. The forth one installed at the Tashkent Obsevertory, started operation in July of 1996. The site selection and arrangement of the fifth telescope systems are underway.
The TON is designed to obtain informations on high-degree solar p-mode oscillations, along with intermediate-degree modes. The TON telescope system uses a 3.5-inch Maksutov-type telescope to observe K-line full-disk solar images with a 16-bit 1080 X 1080 water-cooled CCD. The solar images are taken and saved on magnetic tapes at a rate of one image per minute.The diameter of the Sun is set to 1000 pixels. The measured amplitude of intensity oscillations is about 2.5%. With the TON data one can study solar p-modes with spherical harmonic degree, l, as high as 1000.
The TON images are similar to the K-line images obtained at the south pole by NSO (Duvall et al, 1993), and at Alaska and Hawaii by University of Hawaii (Ronan and LaBonte, 1994). The TON is complementary to other existing and on-going ground-based network, such as BISON, IRIS, and GONG. The BISON (Elsworth et al., 1994) and the IRIS (Fossat et al., 1991) measure the integrated velocity. The GONG measures the imaging velocity and intensity with a solar diameter of about 220 pixels (Harvey and the GONG Instrument Development Team, 1988).
In Section 2, we will describe the hardware and controlling software of the
TON. In Section 3, we briefly discuss the scientific goal of the TON
project.
The TON telescope system is semi-automated. In the morning, observers have
to open the telescope cover, point the telescope to the Sun, and enter
control commands on computers. Then the telescope will guide itself, and
images will be taken and saved on magnetic tapes at a rate of one image per
minute. The whole system is controlled by a PC-486, running UNIX. Another
PC-486, running DOS, is used to monitor and display the brightness of the
Sun and the temperatures of circulating water, CCD head and CCD control box.
In the following, we will briefly describe the hardware and controlling
software of a TON telescope system.
A 3.5-inch ruggedized Questar telescope (Maksutov type) is used. The focus
of the telescope can be locked with six screws on three rods attached to the
main mirror. The size of the solar disk can vary by changing the separation
between the telescope and the CCD. For the present observations, the
diameter of the Sun is set to 984 pixels at aphelion so that the annual
average is 1000 pixels, corresponding to 22.5 mm on the CCD detector. It is
possible to observe a smaller area on the Sun since solar images
can be enlarged by moving the CCD away from the telescope. A neutral density
filter of transmittance 1/64 is used in the front of the telescope to reduce
the sunlight. A K-line filter, centered at 3934 A, of FWHM = 10 A and a
prefilter of FWHM = 100 A are placed right before the CCD detector. A
German-type equatorial mount is used. Two stepping motors of the equatorial
mount are controlled by a guiding system to be discussed later in this
section.
A 16-bit 1242 by 1152 CCD is used to take images, but only 1080 by 1080
pixels are read out to reduce read-out time such that one image would be
taken and saved on magnetic tapes every minute. The CCD is operated at a
level of about 240 thousand electrons per pixel cell to assure a linear
response and low photon noise. The analog-digital conversion is set such
that one unit corresponds to 6 electrons. The exposure time is set to 800
ms. The CCD is controlled by a PC-486 running UNIX, whose multi-tasking
capability is necessary for data acquisition and observing control. The
image data are recorded by two 8-mm Exabyte tape drives.
To reduce the thermal noise of the CCD chip, the temperature of the CCD
chip is kept at about 223 -- 233 K. Circulating cool water is required to
take away the heat from the three-stage thermal electric cooler in the CCD
module. Cooling of the body of the CCD module often creates condensation of
the moisture in the air on the CCD window. Thus dry air is constantly blown
on the window to prevent moisture condensation. For the TON operation, the
photon noise, which is about 0.2%, is greater than the the normal noise of
the CCD and its circuit. Since the CCD and the circulating water are located
outdoors, a heating device is required for sites whose temperature might go
below the freezing point to prevent the circulating water from freezing.
The telescope guiding system is a feed-back system consisting of a guider and an analog computing circuit. The guider consists of a lens which forms a 1-mm solar image on a quadrant photodiode. The analog computing circuit continuously determines the position of the solar image on the photodiode, and its signal is used to drive two stepping motors of the equatorial mount such that the solar image is maintained at the center of the photodiode. The fine adjustment of the alignment between the guider and telescope can be made by changing the offset voltages of the analog computing circuit.
The alignment between the guider and telescope may slowly change as the telescope moves which would result in a shift of CCD images up to 30 arcseconds over a day. This alignment change is compensated by the following way. The center of every CCD image is computed by software running on the PC (UNIX). The deviation of the center of the CCD images is fed into the analog computing circuit to modify its offset voltages.
A special characteristic of helioseismology is that very rich information on the solar interior can be derived from one set of good data (Deubner and Gough, 1984; Christensen-Dalsgaard et al., 1985; Toomre, 1986; Libbrecht, 1988; Hill et al., 1991; Gough and Toomre, 1991). The TON full-disk images have a spatial sampling window of 1.8 arcseconds per pixel. Thus it can provide information of modes up to 1000 with high frequency resolution. Because of the information on the wide range of l, the TON data can be used to study many helioseismology problems, including traditional helioseismology problems, such as the inversion of the sound speed, differential rotation, the global magnetic field in the convection zone, as well as problems required high-l mode information , such as sunspot helioseismology (Braun et al., 1987; 1988; 1990; 1992; 1994; 1995; Davila, 1990; Bogdan, 1992; 1994; Chou et al., 1995. local seismology (Braun et al., 1992; Lindsey et al., 1992),and ring-diagram analysis (Hill, 1988; 1989; Morrow, 1988). The continuous full-disk K-Line images can also be used to study some old problems in solar physics, such as the evolution of global magnetic fields and the rotation speed at high altitudes.
For more detail of this document, see also "Solar Physics" 160: 237-243, 1995.