Study of turbulence at Cerro Pachon in January 2003

After a successful run of the MASS turbulence profiler at Cerro Tololo in 2002, it has been decided to make a short study of turbulence at the neighboring mountain, Cerro Pachon. This work was done for the SOAR telescope, in support of its future commissioning and possible adaptive optics system.

The picture shows E.Bustos with 3 instruments deployed in a wind-protected enclosure some 30m away from the SOAR dome. Those instruments are MASS and two DIMMs. The second DIMM was used only episodically to assure that both DIMMs indeed measure the same seeing. The main deficiency of this campaign is the location of DIMM only 1.5m above ground, thus being possibly affected by turbulence in the first 15m that is not of relevance to telescopes in higher domes. The seeing data are hence upper limits.

The observations were performed by J.Vasquez on 21 nights from January 8 to January 31, 2003 (with a small pause). All nights were clear. Typically, MASS measured over 400 profiles per night. There were nights with quite calm upper atmosphere during this period. The nightly average of free-atmosphere seeing (i.e. the seeing produced by turbulence above 0.5km over the mountain) was below 0.4 arcsec. on 10 nights out of 21, with the record of 0.205 arcsec. on January 15.

Two nights: calm (left) and perturbed (right)

The black lines show the total seeing measured by DIMM, the red dots and line - free-atmosphere seeing measured by MASS. On January 22/23, the total seeing was often dominated by turbulence in a layer at about 3 km altitude. The two lower panels show the corresponding turbulence profiles as "strip charts". On a calm and stable night of January 15/16, MASS senses some turbulence in the first few hundred meters above Pachon. Compensation of ground-layer turbulence with adaptive optics will be particularly efficient on such night.

Summary of the campaign

Comparison of free-atmosphere seeing (MASS, horizontal axis) and total seeing (DIMM, vertical axis) for the whole campaign (9475 points in common between MASS and DIMM).

Last modified: Jul 14, 2003 A. Tokovinin