6 Discussion and conclusions

From the observations taken so far, the following parameters have been obtained for Devasthal site:

1.

In a year about 57% and 48% of nights are spectroscopic and photometric respectively;

2.

Air temperature varies from -4.5$^\circ$ to 21.5$^\circ$C in a year. However, variation during a night is $\le$ 2$^\circ$C;

3.

Relative humidity is below 60% during spectroscopic nights. However, during rainy season (end of June to middle of September) it goes to much higher values;

4.

Pressure variation during a night is $\sim$ 1 mbar;

5.

Wind speed during night time is generally below 10 m/s. For about 85% of the time it is below 5 m/s. The prevailing wind direction is from NW to SE;

6.

Average rainfall is 2000 mm in a year. Maximum rainfall could be $\sim$ 200 mm in 24 hour during rainy season;

7.

Snowfall is for few days in a year;

8.

Visual observations indicate that the sky at Devasthal is darker than that at UPSO where the values are 22.2 and 21.1 mag/arcsec² in B and Vphotometric passbands respectively (cf. Bhargavi et al. 1998; Mohan 1998).

On the basis of above, we conclude that Devasthal is a good astronomical site except during rainy season. Results of DIMM seeing measurements carried out for 88 nights during 13 months at Devasthal Site 1, 37 nights during 3 months at Devasthal Site 2 and 18 nights during 2 months at UPSO are presented. The corresponding median seeing values are 1 $.\!\!^{\prime\prime}$4, 1 $.\!\!^{\prime\prime}$1 and 1 $.\!\!^{\prime\prime}$6 respectively. The DIMM instruments were operated in open air and hence there may not be appreciable contribution of the so called dome seeing to the seeing values reported here. But at the same time the difference in the temperature of the primary mirror and ambient temperature (the cause of the mirror seeing) and heat generated by structures near the telescope may affect the values reported here. Moreover as no corrections have been made for these effects and instrumental noise and the measurements carried out near the ground, the derived values are indeed upper limits. At site 2, seeing is < 1 $^{\prime\prime}$ for $\sim$ 40% of the observing time in which a stretch of > 2 hour is for $\sim$ 55%. This study shows that Devasthal Site 2 is a good choice for locating the the proposed 3 m UPSO-TIFR optical telescope.

A comparison of our seeing results with those of other sites is given in Table 6. It is worth pointing out that our seeing measurements at Devasthal Site 2 are carried out at 2 m above the ground level while seeing measurements at other places such as La Palma, SPM etc. have generally been carried out at larger heights from the ground. The seeing degradation due to turbulence introduced by the surrounding trees, local topography and ground radiative transfer has been analysed by Avila et al. (1998). This and the estimation of surface layer contribution to seeing using microthermal fluctuations at Devasthal Site 1, by Pant et al. (1999) indicate that most of the contribution to seeing come from the 6 - 12 m height. Pant et al. (1999) also concluded that sub arcsec seeing of $\sim$ of 0 $.\!\!^{\prime\prime}$65 and 0 $.\!\!^{\prime\prime}$50 can be achieved if the telescope is located at a height of $\sim$ 13 m and 18 m above the ground respectively. This indicates that sub arcsec seeing can be achieved for most of the time at Site 2 by putting the telescope at a height of $\sim$ 15 m above the ground. To quantify the gain in seeing with height, we plan to carry out microthermal measurements at Site 2. Further efforts will be made to characterise Devasthal Site 2 more precisely.

Acknowledgements

The valuable comments given by the referees Dr. Rafael Costero and Dr. J. Vernin are gratefully acknowledged which helped in improving the paper. The authors thank the UPSO team of scientific and technical staffs for their efforts put in the site testing campaign. The financial help rendered by Indian Institute of Astrophyiscs, Bangalore in the initial stage of this campaign is gratefully acknowledged. The authors thank the district and forest authorities for their help, Prof. Whestpal for providing us the water vapour meter and Indian Institute of Astrophysics for providing us the AWS. We thank Dr. H.S. Mahra and Dr. T.D. Padalia for their help during the site survey work. The authors thank the Survey of India for providing their contour maps