The site for ALPACA has not yet been selected, but it it likely to be on a mountaintop in Chile. One possibility that we are actively investigating is Cerro Tololo, home of Cerro Tololo Inter-American Observatory.

Site Testing

The quality of images obtained by optical telescopes is determined to a large extent by the local topography. Turbulence in the air flowing over the site blurs images, degrading the "seeing". In order to better understand the airflow over the summit of Cerro Tololo, we are conducting site testing measurements. Our primary instrument is a lunar scintillometer (SHABAR). This device employs an array of 12 photodiodes to record scintillation of moonlight 100 times every second. By comparing the intensity fluctuations seen by pairs of diodes with differing spacings, we can estimate both the seeing and and distribution of atmospheric turbulence.

The scintillometer is located in a small building on the summit of Cerro Tololo. Each clear night, the roof is opened by CTIO staff, and the telescope moves and acquires the moon. The moon is tracked continuously while it is 30 degrees or more above the horizon and the data are transmitted to the University of British Columbia for analysis. The scintillometer building can be seen on the CTIO webcam. More information, and data from the scintillometer, can be found on the links below.


Scintillometer data   Seeing plots   Summary statistics   Information   CTIO current weather

We have also employed a set of microthermal sensors that measure small temperature fluctuations in the air. These fluctuations are proportional to variations in the index of refraction, which is the fundamental property affecting astronomical seeing. These sensors consisted of a pair of coils of platinum wire mounted on a 30-m tower erected for the Thirty Meter Telescope project. At present we are not taking data with the microthermal sensors, since the tower has been relocated to another mountain.

Microthermal data   Cn2 plots   Summary statistics .



Last updated: 2006/11/27