2 Search for an astronomical site in India

The occurrence of monsoon and winter cyclones render very large tracts of India unfavourable for astronomical sites. Distribution of the number of clear (cloud free) skies in a year over India has been given earlier by Bappu et al. (1978) using ground based meteorological data. Recently, Sapru et al. (1998) have obtained the average annual spectroscopic nights for a number of places in India using the INSAT satellite cloud imagery database for the period 1989 to 1994. In India, maximum number of spectroscopic nights are at Gurushikhar, Mt. Abu in Rajasthan, though it does not have all the advantages from altitude point of view and is also affected by dust and light pollution. On the other hand sites in the Shivalik ranges of central Himalayas have these advantages as well as the number of spectroscopic nights are over 200 in a year.

 

 

Table 1: Preliminary survey parameters for the four sites investigated

Site	Altitude	Duration of	Annaul	% of nights having
	(m)	observations	variation	temperature	pressure	wind	prevailing
		(year)	in night	variation	variation	speed	wind
			temperature	$\le$ $2\hbox{$^\circ$ }$C	$\le$ 1 mb	$\le$ 3 m/s	direction
			( $\hbox{$^\circ$ }$C)
Chaukori	2130	1982-1989	27 to -3	44	87	-	NNW
Devasthal	2540	1986-1991	21.5 to -4.5	73	89	77	NW
Gananath	2090	1982-1990	24 to -3	71	92	82	NW
Mornaula	2250	1984-1990	24 to -3	62	88	62	NW

Manora Peak, just south of Nainital, headquarters of UPSO is located in Shivalik range at an altitude of 1950 m. It started functioning in late fifties. With time, light pollution at the present location has increased significantly. Seeing at the present site is also generally poor ($\ge$ 1 $.\!\!^{\prime\prime}$5). The site survey work to select a better site in Kumaon and Garhwal regions of Shivalik Himalayas (Uttar Pradesh) for setting up a moderate size optical telescope was therefore initiated in 1980. For this, contour maps (1:50000) of these regions provided by the Survey of India were studied and a total of six reconnaisance trips were made to 36 sites during 1981-82. Based on altitude of the site and its obstructions, if any, due to nearby hills, terrain of the surrounding regions and logistic reasons, like availability of reasonably flat land and/or presence of suitable water source at a manageable level, distance from the existing 6 m wide metalled road and possible disturbance likely to be caused by nearby city lights in foreseeable future, a total of four suitable sites namely Gananath, Mornaula, Devasthal and Chaukori (having altitude $\ge$ 2 km) were identified for preliminary investigations. Meteorological observations at these four sites were carried out during 1982-1991. The meteorological equipments installed at these stations were thermograph, hygrograph, barograph, sunshine recorder, rain gauge, snow gauge and wind speed and direction recorder. The cloudiness was recorded visually by the observers. Following are the findings of the site survey.

2.1 Meteorological parameters

Results of the meteorological observations for the four sites studied have been summarised in Table 1 and are discussed below.

2.1.1 Stability of night time temperature

The temperature measurements were made with a continuous recording type thermograph at each site. The variation in night time temperature is minimum for Devasthal site. The analysis of night time temperature data indicates that the temperature variation during night is within 2$^\circ$C for more than 60% of the time for all the sites except Chaukori. From this point of view, Devasthal is the most suitable site.

2.1.2 Relative humidity

Continuous recording hygrograph was used to measure relative humidity at a site. The yearly relative humidity measurements show that during photometric nights, humidity generally remains below 60% at Devasthal and Mornaula and below 70% at Gananath and Chaukori. However, during monsoon months, it is generally higher than 80% for most of the time at all the sites.

2.1.3 Atmospheric pressure

The atmospheric pressure was recorded with a continuous recording barograph. The data indicates that the pressure variation during night at all sites is generally within 1 mb and hence they are similar from this point of view.

2.1.4 Wind speed

The wind speed and direction were recorded with a continuous recording anemograph installed at a height of about 5 m from the ground level. The maximum wind speed during night time was found to be generally 6 m/s. It is found that more than 75% of night hours have wind speed below 3 m/s at Devasthal and Gananath. The prevailing wind direction at both places is NW.

2.2 Cloud coverage

The visual observations of cloud coverage were recorded at four hourly intervals. Based on these data, cloud coverage during the night time was estimated. The night time was defined as the duration between end of evening astronomical twilight to start of morning astronomical twilight. Following criteria had been adopted for cloud coverage

1.

Clear night: When the cloud cover is zero for the complete night;

2.

Partly clear night: When the cloud cover is zero for more than four consecutive hours;

3.

Night < 3 Octas: When the cloud cover is < 3 octas for more than four consecutive hours;

4.

Cloudy nights are other than (1), (2) and (3);

5.

Photometric nights include (1) and (2);

6.

Spectroscopic nights include (1), (2) and (3).

These observations indicate that spectroscopic nights at all the four sites are $\ge$ 200 in a year. A large fraction of them ($\ge$ 85%) are of photometric quality. Here it is important to mention that a recent study based on the INSAT satellite cloud imagery database for the period 1989 to 1994 (Sapru et al. 1998) suggest that the average annual percentage of spectroscopic nights is about 55% for Devasthal which is in agreement with the visual ground based observations. These numbers are comparable to those observed at high altitude ($\ge$ 2 km) sites in India (HIROT team 1996, Bhatt et al. 2000). A comparison of spectroscopic nights at all sites vis-à-vis some well known sites around the world such as Siding Spring in Australia (65%, Sadler et al. 1991), La Palma in Spain (80%, Murdin 1985), Cerro Tololo and La Silla in Chile (82%, Tapia 1992), Mauna Kea in U.S.A. (73%, Tapia 1992) and San Pedro Mártir in New Mexico (80%, Tapia 1992) indicates that all the sites can be called satisfactory.