November 2006: Results of Seeing Measurements at the Summit of Co. Chajnantor

1. Summary of the Measurement

In early November 2006, we have finally succeeded to carry out seeing measurement observation at the summit of Co. Chajnantor for 3 nights. The obtained results are:

• The altitude of 5640m is the highest measurement of the seeing ever reported.
• Out of 3 nights, we obtained median value of 0.6 arcsec for one night. Given that the location of the measurement is easily affected by the turbulence generated by the edge of the summit, as well as the the ground-layer turbulence, we can expect good seeing if we go to higher level from the ground.

# Members : Motohara, Aoki, Miyata, Sako, Mitani, Minezaki
# Date : 2006/10/31-11/18

fig1: The galactic center and the milky-way beautifully set down to the west during the seeing measurement, just after the sunset.

2. Measurement Setup

We have challenged the measurements on the nights from 117 to 13, and successfully obtained data on 11/8, 9, 12, and 13.

The seeing monitor system is almost same as that used during the run in 2003. However, as hard-disk drives are found not to function under the low pressure environment at the summit, we have modified the PC to a diskless system using a USB flash-memory key.

Also, the software is updated to incorporate a auto-guiding function, using which we can only introduce a star for measurement at the beginning of the observation. Then we can leave the system until the sunrise. Using this system, we succeeded to obtain data for 3 full nights. The new system is also good for the observer, as what we only have to do is to introduce a star just after the sunset, and go back to the hotel to sleep after that.

However, on the last night (11/13), patch clouds came in just after we left the summit and the observation was interrupted as the telescope lost the star.

fig2: The seeing monitor telescope waiting for observation.

fig3: Location of the measurement. The object covered with a green plastic sheet on the right is the telescope.

3. Results

The median values of the seeing each night show rather good value of 0.6 to 1.0 arcsec, which are comparable to that measured at Co. Toco (5400m) and Co. Chico (5200m) in 2003.

Especially on November 11, the median value was 0.6 arsec as shown in the figure below, and the best record was less than 0.5 arcsec.

Also, we have to consider the location of the measurement, 30m downwind from the edge of the summit and lower than the surroundings by 2m. Therefore, the measurement may be affected by additional ground-layer turbulence.

The measured seeing of 0.6 to 1.0 arcsec even under such not necessarily good condition may suggest that the summit of Co. Chajnator may have much better seeing.

fig4: The temporal variation of the seeing on the night of November 12, 2006. Median value is 0.68 arcsec, and the best record is 0.4 arcsec.

4. Future Prospect

The current measurement is more like a trial, and the setup is not an ideal one.

We are now planning to install a tower at the western edge of the summit, where the effect of the ground-layer turbulence is expected to be smaller, in 2007. Also, long campaign observations more than a week is scheduled to obtain more statistically reliable data.

Stay tuned for the update.