4 Results and discussions

Microthermal data were successfully recorded on a total of 20 nights during March 1998 - June 1998. The values of $C_{\rm T}^{2}$ were measured every second and averaged over one minute. These data were used to evaluate the value of $C_{\rm N}^{2}$. It decrease sharply but nonlinearly with altitude. The seeing contribution due to a slab of turbulent layers is evaluated using Eq. (6) by assuming a power law variation for $C_{\rm N}^{2}$ within a slab. The height of the slabs range from 6 to 12 m and 12 to 18 m above the ground.

Temporal evolution of $C_{\rm N}^{2}$ on 03 May, 1998 during 13.6 - 17.0 UT at each of the three levels has been illustrated in Fig. 1. This shows that the microthermal turbulence decreased rapidly from 6 to 18 metre height. The average seeing for the lower (6 - 12 m) and upper (12 - 18 m) slabs are 1.08$^{\prime\prime}$ and 0.31$^{\prime\prime}$ respectively.

  

Figure 1: Temporal evolution of $C_{\rm N}^{2}$ during the night of 03 May, 1998. Dots represent the 6 m level, solid line represents the 12 m level and long dashes the 18 m level

  

Figure 2: Turbulence in the surface layer measured from microthermal sensors on the 18 m high mast on 05 June, 1998. The solid line gives the 6- 12 m contribution whereas the dots refer to the 12 - 18 m contribution

In Fig. 2, the derived values of $C_{\rm N}^{2}\Delta {\it h}$ for 05 June, 1998 are plotted against time in UT for the above mentioned two slabs. The upper curve indicates the turbulence contribution between 6 and 12 m whereas the lower curve indicates the contribution from 12 to 18 m slab above the ground. These plots also show that the significant contribution of microthermal fluctuations due to turbulence comes from the lower slab.

Figures 3a and 3b illustrate the statistics of seeing derived from both the microthermal and the DIMM measurements. The frequency and the cumulative distributions of the seeing due to the two slabs are shown in Fig. 3a. The mean and median values of the seeing due to 6 to 12 m slab are 1.28$^{\prime\prime}$ and 1.17$^{\prime\prime}$ respectively. The corresponding values due to the 12 to 18 m slab are almost identical 0.32$^{\prime\prime}$ and 0.30$^{\prime\prime}$ respectively. The attained values are remarkable. The seeing is better than 0.5$^{\prime\prime}$ for 88% of the time, while it is better than 0.3$^{\prime\prime}$ for 50% of the time, with minimum values around 0.1$^{\prime\prime}$. Figure 3b shows the frequency and cumulative distributions of the seeing obtained from the DIMM measurement and of the seeing contribution derived for the 6 to 18 m slab. The mean and median values for the 6 to 18 m slab are 1.36$^{\prime\prime}$ and 1.25$^{\prime\prime}$ respectively. The corresponding values for DIMM measurements are 1.51$^{\prime\prime}$ and 1.42$^{\prime\prime}$ respectively. The results of these measurements are tabulated in Table 1.

  

Table 1: The mean and median values of microthermal and DIMM seeing measurements at Devasthal site during March to June 1998

 

Figure 3a: Seeing data statistics: FWHM probability distribution and associated cumulative distribution function for two separate levels i.e. 6 - 12 m (upper plot) and 12 - 18 m (lower plot)

  

Figure 3b: Seeing data statistics: FWHM probability distribution and associated cumulative distribution function for combined level i.e. 6 - 18 m (upper plot) and DIMM measurements (lower plot)

In order to have a better insight into the atmospheric behaviour which gives rise to the optical turbulence, the data were analyzed for the temporal evolution of the seeing over two slabs 6 to 12 m and 12 to 18 m and compared with the DIMM measurements. Figure 4 illustrates the temporal variation of the seeing deduced from both microthermal and the DIMM measurements on 02 May, 1998. This and Table 1 show that significant contribution of the thermal deterioration of the optical seeing comes from the 6 to 12 m slab. The thermal disturbances affect this layer most as compared to the higher layers since it is located very near to the ground. For astronomers, it is desirable to have a characterization of the temporal evolution of the seeing i.e., to know the typical time interval of seeing variation and also the dependence of seeing quality with time (Muñoz-Tuñón et al. 1997). While examining individual nights we do not find any general trend in the seeing evolution as has been found by Muñoz - Tuñón et al. (1997) for the ORM site. This is in contrast to the general assumption that the seeing is worst in the beginning of the night and improves later.

  

Figure 4: (top) Seeing measurements obtained using DIMM at the focus of the 52 cm reflector telescope on 02 May, 1998. (bottom) Seeing contribution deduced from the microthermal measurements, solid line represents the 6 - 12 m and dots represent the 12 -18 m

A comparison of our results for the Devasthal site with those obtained at La Palma (Vernin & Muñoz-Tuñón 1994) and at the South Pole in Antarctica (Marks et al. 1996) is given in Table 2. The mean and median values quoted in the table for La Palma are calculated by us from the reported data for four nights by Vernin & Muñoz-Tuñón (1994). From the table it can be inferred that the surface layer contribution to seeing varies from site to site as it depends on the geography of the site under study.

  

Table 2: Comparison of surface layer contribution to seeing with other sites

Echevarria et al. (1998) have mentioned that the local value of the seeing is considered to be zero arcsec at a height of $\sim\!\!100$ m from the ground, and it increases as the starlight passes through the turbulent boundary layers of the atmosphere below 100 m. Above 100 m the star image is considered to be degraded only by the turbulent layers of the free atmosphere, which mostly reside at a height of 10 km or so. They have also concluded that the total value of the seeing $S_{\rm T}$ is given by the relation as

$$S_{\rm T} = \left( S_{\rm L}^{5/3} + FA^{5/3} \right)^{3/5}$$ (8)

where $S_{\rm L}$ is the local value of the seeing measured for a particular slab and FA is the seeing that one would observe at a height of 100 m. We have taken the value of FA = 0.52$^{\prime\prime}$ given by Cromwell et al. (1998). Adopting this value of FA in Eq. (8), we estimated the total seeing $S_{\rm T}$ for the Devasthal site as 1.52$^{\prime\prime}$ which is in good agreement with the value of 1.51$^{\prime\prime}$ obtained from DIMM measurements. It is relevant to mention here that both DIMM and microthermal measurements were carried out simultaneously at Devasthal. Considering the seeing contribution due to the 12 to 18 m slab as 0.32$^{\prime\prime}$ (see Table 1), we estimated the value of $S_{\rm T}$ as 0.65$^{\prime\prime}$. This indicates that if the telescope is located at a height of $\sim\! 13$ m above the ground, one can achieve sub arcsec angular resolution for a significant fraction of the observing time. This is comparable to the seeing evaluated by other investigators for various sites. If the height of the telescope is raised to 18 m, we can attain seeing of $\sim\! 0.5$$^{\prime\prime}$.