Cerro Pachón & Gemini South Telescope

Gemini South is one of a two part observatory system, the Gemini Observatory. The northern counterpart is at Hilo, Hawaii, while the southern telescope is located on Cerro Pachón, a 2682m (8799 feet) tall mountain in Chile.  This is one large telescope, with a primary mirror diameter of 8.1 meters!  It collects a lot of light, and at this location, it can do just that. Cerro Pachón has some of the darkest and steadiest skies in the world. Combine that with the instrumentation on board this telescope, and the total integrated system produced results better than the Hubble Space Telescope!

Gemini South Dome

A view of the Gemini South dome from the far side of the mountain. That is one big dome!

The telescope is a somewhat unusual design in terms of mounting: it is on an altitude-azimuth configuration. The azimuth bearing is literally the steel rotating platform of the telescope resting on a steel azimuth ring. The ring is set atop a concrete telescope pier. To allow for smooth movement, oil is pressurized and pumped into the region between the two steel plates.  This setup has withstood all the seismic activity that Chile has thrown at it, but it was somewhat damaged during one of the strongest quakes in recent years which caused the mount to slide side-to-side with such force that the restraining plates were damaged.

Instrument package for Gemini South

This is the Cassegrain focus location for Gemini South. The instrument package hangs between the two stairways. The blue disk on the floor is the telescope’s azimuth bearing plate.

The altitude bearings are a little smaller but also float on a thin film of pressurized oil. The pumps for both axis can be heard constantly in the background of this rather industrial setting. Astronomers immediately know when something is wrong if those pumps are not making their usual noises.

Being an altitude-azimuth arrangement, this means that the field of view at the image plane rotates!  They compensate for this by de-rotating the entire instrument package at the Cassegrain focus of the telescope (hanging underneath the primary mirror). As the field rotates, this package rotates precisely in the opposite direction to counteract this.

One of the most amazing packages on board Gemini South is the GeMS, the Gemini Multi-Conjugate Adaptive Optics System. Now, you might already know about adaptive optics. A laser is projected into the sky to make a fake guide star, which the telescope system sees and senses. As the atmosphere makes this star wobble, small mirrors are used to move the image of that star back into place. This happens at a rate of up to 1000 times each second in both axis: up/down, and left/right. To make the system better, Gemini South uses five laser beams into the sky. This allows them to not only correct the errors caused by atmospheric seeing conditions, but also to correct for predicted motions in the stars’ positions!  This allows the telescope to image down to 0.1 arc-second seeing!  That is very sharp indeed!

GS primary mirror

Gemini South’s primary mirror. That’s 8.1 meters of diameter. The mirror is not coated with aluminum like most astronomical telescopes, but is coated with silver. This makes it more reflective to infra-red wavelengths. To the left of this image is a large metal “box”. This is where the laser is installed for the GeMS.

At this altitude, and given that it was winter there in June, snow was present on the mountain and those surrounding. The views here are lovely, as one would expect from an altitude of 8900 feet.

Winter here in June. Yes, that is snow!

Winter here in June. Yes, that is snow!

Controlling the telescope and all the associated equipment is done from the central control room. Eventually all this will be done from operations at AURA back in La Serena. The control system includes everything necessary for all observatory components, including: ventilation/cooling, instrument status and control, telescope pointing, oil system control, sky conditions and weather forecasts, and more. The system has redundancy with dual control desks.

Gemini South Control

Gemini South Control

Gemini South Control Room

Gemini South Control Room

All in all, the Gemini South observatory is an amazing place! All the instrumentation and systems that work together make for an astounding ability for data collection… and also a huge need for systems integration and maintenance. If you have the opportunity, I highly recommend a visit!

All of Gemini South

All of Gemini South: an indoor panorama.

About johnb

- Director of Grainger Observatory, Phillips Exeter Academy. - Variable-star-crazed astronomer, but have done research in other areas. - Drummer, archer, pilot, chef, friend, pet owner, husband, father, Train-nut.
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2 Responses to Cerro Pachón & Gemini South Telescope

  1. Your work is beautiful and instructive. You need to put up your own Facebook page. Please let me know if you do one, so I can follow you.

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