The NCU Lu-Lin Observatory

 

Wean-Shun Tsay, Alfred Bing-Chih Chen[1], Kuang-Hsiang Chang and Huan-Hsin Li

Institute of Astronomy, National Central University,

Chung-Li, Taiwan 32054, ROC

 

ABSTRACT

 

    The NCU (National Central University) Lu-Lin Observatory is located at Mt. Front Lu-Lin, 120o 52' 25" E and 23o 28' 07" N, a 2862-m peak (Fig. 1) in the Yu-Shan National Park. The construction of Lu-Lin Observatory was finished on January 14, 1999. The initial study of Lu-Lin site was started since late 1989. Later on, a three-year project was founded by the National Science Council (NSC) to support the development of a modern seeing monitor for this site survey study from 1990 through 1993. The average seeing of Lu-Lin site is about 1.39 arc-second with average 200 clear nights annually. The sky background of this site is 20.72 mag/arcsec2 in V band and 21.22 mag/arcsec2 in B band.

 

    The Lu-Lin observatory is developed for both research and education activity. A homemade 76-cm Super Light Telescope (SLT) and three TAOS's 50-cm robotic telescopes will be the two major research facilities. We are proposing a pilot program to perform SLT observations of time-varying astrophysical objects exploiting the unique geophysical location (e.g. time and longitudinal coverage) of Taiwan at the Western Pacific Rim in such topics of emerging interest. The TAOS #1 telescope was installed at Lu-Lin in late March of 2000, while the other two will be installed in late 2001 for the survey work of Kuiper Belt Objects. A 90 KW/240 VAC power line and a water pipe system have been pulled to this remote site in early 2001 (Fig. 2). A wireless Network system through A-Li Shan has been operated at Lu-Lin Observatory while a faster wireless Network system with 11.5 Mbit/sec bandwidth is under designing and will be available in the near future.

Fig. 1: Lu-Lin site. Mt. Front Lu-Lin (2862 m) located in the Yu-Shan National Park, has a relative flat summit for developing astronomical facilities.

Fig. 2: Lu-Lin Observatory. The SLT is housed in the 6-m dome shown on the left hand side.  TAOS #1 telescope is on the right had side. The green channel contains power line, water pipe and local Network cable.

 

I. Introduction  

 

The NCU started the first site survey for a modern observatory in Taiwan since 1989. The original plan was trying to move the NCU 24" telescope from campus to a suitable site at the central high mountain range of Taiwan. Weather station data and infrared satellite cloud cover data of Taiwan were studied to pre-select potential sites (Table 1) for further investigation.

Table 1: Potential Sites for Taiwan Optical Telescope

        Mt. Front Lu-Lin       2862 m          Mt. Lu-Lin            2860 m

        Mt. Stone-Water        2770 m          Mt. Ali               2406 m

        Mt. Small Snow site #1  2550 m          Mt. Small Snow site #2  2990 m

        Mt. Pear              2600 m          Mt. Ho-Han           3400 m

 

    Among those potential sites, Mt. Front Lu-Lin [2862 m, (Fig. 1) hereafter referred as Lu-Lin, located within the vast region of the Yu-Shan (Mt. Jade) National Park and Forest Reservation, about five-hours driving from the home institution - Chung-Li] been considered with a relative flat summit and convenient transportation, was selected for a three-years seeing study.

 

 

II. Seeing study

 

"Seeing" is a special term of astronomy describing the phenomena of stellar image broadening observed from the ground.  When a stellar image is observed through a ground-based telescope with high power magnification, the observed image shape usually does not correspond with the theoretical diffraction pattern of that telescope, but looks like formed by many "speckles".  Instead, the diffraction ring may be resolved only partly or not at all, and an irregular motion of the image is often observed. The phenomena of image structure distortion and image motion may vary considerably with time and with the location of the observer. These random displacements of part or all of the light received by the telescope are called "seeing".

    Astronomers use the envelope diameter of the first order speckles to define the visible seeing α,        α = 2 * 105  (λ／ro)

where α is in unit of arc-second, λ is in unit of μm , ro is in unit of cm and called the Fried parameter (Fried 1965, 1966),  defined as the coherence radius of the wave-front distorted by the turbulent atmosphere. The Fried parameter ro is thus a function of the turbulence, the refractive index, the wavelength of the transmitted light, and the air-mass above the telescope. Typically at a really good site, the visible seeing, α, is about one arc-second or less.

    Seeing has been considered as the utmost importance factor for most observations (Woolf 1982). From direct imaging, spectroscopy, and photometry, to interferometric observations, seeing determines spatial resolution, limiting magnitude and instrument speed. There are many methods for studying seeing such as direct visual estimates, star trailing, high-speed photoelectric image scanning, monitoring of differential image motion and image profile width, speckle interferometry plus pupil imaging, and shearing interferometry (Ardeberg 1987).

    For the Lu-Lin site survey, we designed a portable optical seeing monitor which adapted a Celestron C-14 telescope mounted on Takashi NJP equatorial mount and a high-sensitivity CCD TV-camera (Philips Amprex NXA 1031/01) as detector (Fig. 3). This seeing monitor recorded the real-time images of objective stars. The profiles of these stellar images were then analyzed by an IBM-AT/compatible clone equipped with a video digitizer board at high speed (~ 1/30 sec). After three-years seeing study at Lu-Lin site, we concluded the average seeing to be about 1.39 ±0.34 arc-second, which is according to 757 CCD observations of 682 single star and 75 binary star measurements (Fig. 4 & Fig. 5 ). The annual clear nights at Lu-Lin are about 200 nights. The sky background of this site is 20.72 mag/arcsec2 in V band and 21.22 mag/arcsec2 in B band carried out from ST-6 CCD photometry observation.

Fig. 3: Seeing observation

 

Fig. 4: Binary star – Castor – measurements.

 

 Fig. 5: The average seeing of Lu-Lin site is about 1.39 ±0.34 arc-second.

III. Site Development

    Lu-Lin site, located within the vast region of the Yu-Shan National Park and Forest Reservation, was studied since late 1989. Further contact with the Yu-Shan National Park, Ministration of Interiors and the Bureau of Taiwan Forest took about one year to get the permission of constructing a site survey station within 100 square meters land on the Lu-Lin summit. Since there was no exiting driving access to Lu-Lin site, all construction material of site survey station and survey instruments must be carried up by manpower from the foot to the summit (Fig. 6). The observing station was first finished in 1991. With no electrical utility, power supply for all equipments were coming from a generator during the observation.

Fig. 6: Survey instruments must be carried up by manpower from the foot to the summit.

    After the site survey completed in 1993, the NCU proposed to develop the Lu-Lin site for a medium size 2-m telescope observatory. We continued to propose a feasibility study project of constructing a 2-m class telescope.  Following with the requirement of constructing medium size telescope observatory, the NCU continually rent 300 square meters on the summit of Lu-Lin for observatory (Fig. 7) and 500 square meters at the foot of Lu-Lin for remote control center and machine shop.

Fig. 7: The flat summit of Lu-Lin site.

 

    Until late 1997, a founding from NSC supported the development of Lu-Lin observatory. A 6-m dome was built for a homemade 76 cm reflector. The construction of Lu-Lin observatory was finished on January 14, 1999. After then, we started to contact the electrical power company. A 3.3 km underground electrical power line to the foot of Lu-Lin in the national park area was built by late 2000. Further electrical power line extended from the root to the summit of Lu-Lin was installed in early 2001 with funding from the MoE Excellency project. The TAOS #1 telescope was installed at Lu-Lin in late March of 2000, while the other two will be installed in late 2001 for the survey work of Kuiper Belt Objects. Another 50 cm telescope (similar to TAOS telescope ) from Yonsei University, South Korea is planning to join TAOS’s project as the TAOS #4 telescope.

 

Fortunately with the funding of MoE 4-year Excellency project, a 90 KW/240 VAC power line and a water pipe system can be pulled up to this remote site in early 2001. Also a wireless Network system through A-Li Shan has been operated at Lu-Lin Observatory, while a faster wireless Network system with 11.5 Mbit/sec bandwidth is under designing and will be build up for the near future remote observation.

IV. Research Activity

 

The Lu-Lin observatory is developed for both research and education activity. The homemade Super-Light Telescope (SLT, Fig. 8), adopted a 76-cm diameter HEXTEK Honey Comb lightweight gas-fusion mirror, is expected to be fully operated in the fall of 1999. The maximum field of view (FOV) of SLT, with fast primary focus (f/1.8) and Ritchey-Chretien focal ratio (f/9) and plate scale around 0.3”/10μm, approaches half-degree square. In the first phase, only one quarter of the FOV will be used with an Apogee AP-8 thin CCD ( 1K x 1K , 24μm/pixel). We are proposing a pilot program to perform the SLT observations of time-varying astrophysical objects exploiting the unique geophysical location (e.g. time and longitudinal coverage) of Taiwan at the Western Pacific Rim in such topics of emerging interest.

Fig. 8: The homemade 76 cm Super Light Telescope (SLT).

 

    The Taiwan-American Occultation Survey (TAOS, Chen et al. 1999) is a collaborative project to conduct a census of comet nuclei in the outer solar system. The TAOS experiment will initially consist of three 50 cm, wide field (f/1.9), robotic telescopes, each equipped with a 2K * 2K CCD camera. In the first phase, the three TAOS’s telescopes will be installed at NCU Lu-Lin site (2862 m) by the end of 1999 (Fig. 9). A year later, one of the three telescope will be moved to the West Peak of Jade Mountain (3450 m) with a 6 km base line in the east-west configuration. This experiment provides the only means to study the cometary population in the small sized end of the distribution. A great number of scientific products, notably variable stars, will also derive from the huge TAOS database.

Figure. 9: The TAOS # 1 robotic telescope.

V. Future Development

    With a 4-year funding supported by the MoE and NSC Excellency project, the NCU is going to build up the facilities at the Lu-Lin Observatory. This effort, which is coordinated by the Institute of Astronomy of the National Central University, will focus on the road construction, water and power supplies, communication links and other items necessary for the establishment and maintenance of the Lu-Lin Observatory as an inter-university astronomical facility for research and education. Members currently involved in this joint venture are Central University (which is operating the Lu-Lin Observatory), Tsinghua University and Taiwan University, but we have the vision that many more universities will be included in this consortium in the near future. We believe that the proposed national infrastructure will promote Taiwan's role in many first-class astronomical projects-from solar system astronomy to cosmology--in view of its capability to provide key longitudinal and time coverage.

 

 

Acknowledgements

 

This work would not have been possible without the effort of a lot of people working on developing the site. We wish to thank the long term supporting from the National Science Council of Taiwan, the Ministry of Education of Taiwan and the National Central University.

REFERENCES

 

Ardeberg  A. (1987).  "Identification, Optimization, and Protection of Optical Telescope Sites"

    Proc. of an International Conf. Flagstaff, AZ.

Chen W.P., Tsay W.S., Lemme C., Wang J. H., "Taiwan-American Occultation Survey",

    National Science Council Monthly, Vol. 27 No. 5, May 1999.

Fried D.L. (1965). J. Opt. Soc. Am., 55, 1427.

Fried D.L. (1966). J. Opt. Soc. Am., 56, 1372.

Woolf N.J.(1982). Ann. Rev. Astron. Astrophys., 20, 367.

Tsay W.S. et. al., (1999) “The NCU Lu-Lin Observatory and its Future Direction”, Proceedings of Fourth East-Asian Meeting on Astronomy, Supplement 1999, p24. 

 

 

DISCUSSION

 

Pei-Sheng Chen: How many nights around the year could be used for the Photometry? How about the water vapor in Lu-Lin?

 

W.S. Tsay: Around half of the clear nights could be available for the Photometry. Unfortunately we do not have the measurement of water vapor during the site survey.

 

John Gaustad: Is the absence of a road due to the cost of construction, or to regulations of the National Park which forbids it?

 

W.S. Tsay: Both. Construction of a road is estimated to cost US$5 millions. But also, current policy of the Yu-Shan National Park doesn’t allow the kind of road needed, which must spiral around the mountain. High level political discussion are needed.

 

Peter Mack: Burying the power line is a good idea environmentally. But do you expect problems of damage from earthquakes?

 

W.S. Tsay: The power line company will be responsible for maintenance of the underground power line.


[1] Current Address: Department of Physics, National Cheng-Kung University, Tainan, Taiwan 70101