Chile - 3 - Optical Observatories, Daytime

Friday, 12 December 2025

We toured two large optical telescopes on Cerro Pachón - the Vera Rubin and Gemini South Observatories.

The observatories on Cerro Tololo and Cerro Pachón share an entrance gate that is 41 km (25.5 miles) from the Rubin Observatory. At the gate you get a distant view of Cerro Tololo.

Cerro Pachón as seen from Vista Sidney Wolff. From left to right: SOAR, Gemini South, Vera Rubin.

The road splits at Vista Sidney Wolff. The road that goes towards the upper left goes to Cerro Tololo. The road going towards the bottom left comes up from the entrance gate. The road heading towards the bottom middle and left is the road up to Cerro Pachón.

Cerro Tololo and the road from Vista Sydney Wolff as seen from the Rubin Telescope.

Vera Rubin Observatory

The telescope at the Vera Rubin Observatory (formerly called the "LSST" - Large Synoptic Survey Telescope) is designed as a survey instrument - a wide-field imager that will cover the entire sky once every three days. It has the world’s largest digital camera (3200 megapixels, 20 terabytes of data every night). When fully operational, it will excel at detecting fast-moving objects such as comets and asteroids. When we visited in 2019, it was still under construction.

Closer view of the Rubin Observatory as we climb Cerro Pachón.

Full view of the Rubin Observatory.

Rubin control room.

Cables beneath the telescope.

The huge camera (3,400 megapixels) is mounted in front of the 8.4 m (331 inch) mirror.

The primary and tertiary mirrors are barely visible behind the SUV-sized camera. At the upper left is the calibration disk - it is imaged at the end of every observing session to create a reference image.

Gemini South

There are two other observatories on Cerro Pachón: SOAR (Southern Astronomical Research) and Gemini South. We had a great visit of Gemini - many thanks to Brian Miller for a great tour.

SOAR (left) and Gemini (right) Observatories as seen from the Rubin Observatory. I am wearing the required safety equipment for visiting some of the telescope sites - hard hat, safety vest, long sleeves, long pants, and closed toed shoes.

A better look at SOAR and Gemini.

The Gemini control room.

A total view of the telescope. The secondary mirror is at the top of the photos; the detector array is at the bottom; the primary mirror is hidden in the blue supporting structure.

A carousel of various detectors behind the primary mirror.

The primary mirror is 8.2 m (323 inches) in diameter. The telescope is optimized for detailed deep-sky observing, especially in the near infrared.

As we were leaving, clouds were building behind Gemini - not a good omen for the night's observing.