April 2007: Results of Seeing Measurements at the Summit of Co. Chajnantor

1. Summary of the measurements

We have successfully carried out seeing measurement observation in April 2007 on the 2m tower installed at the summit of Co. Chajnantor. The results present that the summit of Co. Chajnantor has a excellent seeing environment, comparable to those of other best sites on the Earth.

To be more precise,

• Median value of all the data is 0.69 arcsec, which is one of the best value obtained at observatories of South America.
• The best nights shows the median value of 0.37 arcsec, which is extremely good.

Considering its weather and the atmospheric transmittance, the summit of Co. Chajnantor is the best site for the infrared astronomy on the Earth.

# Member : Motohara, Aoki, Miyata, Minezaki
# Date : 2007/4/12-4/29

fig1: Two seeing monitor telescopes at the summit of Co. Chajnantor.

2. Measurement Setup

The measurement was carried out on the night of 17, 20, 22, and 23, April 2007.

The seeing monitor system has been upgraded since the previous run on 2006. The major upgrades are;

• newly installed a tower with a height of 2m on the summit, and installed a telescope on it, to avoid the influence of ground-layer turbulence as much as possible, and
• prepared one more set of the seeing monitor system, including a telescope, and installed it on the ground, to evaluate the effect of the ground-layer turbulence.

fig2: The newly installed 2m tower at the western edge of the summit.	fig3: The seeing monitor telescope installed on the 2m tower.
fig4: The seeing monitor telescope installed on the ground.	fig5: Two seeing monitor waiting for simultaneous observation.

3. Results

Due two the unstable weather during the run, sky was sometimes covered with patchy clouds and the gust of wind often shakes the telescope badly.Therefore, the telescopes easily lost their stars to measure, and the simultaneous measurement by two telescopes for a full night cannot be achieved.

Still, at lease one telescope successfully obtained data for a full night, which provides us valuable information.

What we found are: (1) On the good night, we can expect an excellent seeing as good as 0.4 arcsec in the optical wavelength at the height of 2m from the ground. For example, on April 20, we obtained a stable seeing around 0.4 arcsec with median value of 0.38 arcsec, although the measurement was interrupted due to cloudy weather.

fig6: Temporal variation of the seeing on the 2m tower on April 20, 2007.

(2) By going up to the 2m tower, the seeing improves by more than 0.1 arcsec. Figure 7 show the correlation of the seeing measured on the tower and on the ground. It can be seen that the seeing improves by 0.1-0.5 arcsec by going up to the tower.

fig7: The difference of measured seeing on the tower and on the ground.

4. Seeing at the Summit of Co. Chajnantor

By applying a correction of the 2m tower to all the data obtained so far and compiling them, the median seeing is found to be 0.69 arcsec. This is comparable with the value of 0.6 arcsec obtained at Mauna Kea, Hawaii, or 0.63 arcsec at Las Campanas, Chile, which are the best sites on the Earth, and better than 0.82 arcsec at Co. Paranal, where VLT is located.

fig8: Histogram of all the obtained data.

Furthermore, because our measurements are carried out only at the height of 2m, we can expect much better seeing with TAO 6.5m telescope, which is located at higher level from the ground.

Considering the higher atmospheric transmittance in the infrared than at the other observatories as well as the good clear fraction over 80%, the summit of Chajnantor should be the best site for the infrared astronomy.

We are now looking forward for the first-light image of of the 1m telescope which is scheduled to be installed at the summit in 2009.