Magdalena Ridge passes test for world’s largest lens telescope

On July 10, a team from Colorado’s High Altitude Observatory completed the installation of testing equipment at Magdalena Ridge Observatory that moved the site into a second phase of consideration for COSMO, a long-planned telescope that will be the largest lens-type instrument in the world. Standing for COrona Solar Magnetism Observatory, COSMO is designed specifically for studying the Sun. While most modern research telescopes use mirrors, lens surfaces scatter less light than mirrors do. A lens-based telescope will thus perform better for solar astronomers interested in the glowing corona of light from the Sun’s extended atmosphere, in contrast to the nearly overwhelmingly bright apparent solar surface.

The planned telescope will have a 60-inch-diameter lens in its front, a whole 20 inches larger than what, for over a century, has been the world’s largest successful lens-based telescope (called a refractor) – the 40-inch housed at Yerkes Observatory, originally of University of Chicago and dedicated in 1897. In its specialized design, the COSMO instrument will be a coronagraph, a telescope made specifically for seeing the delicate corona of the Sun that normally is only possible to see during a total solar eclipse.

The site survey for COSMO has been running at Magdalena Ridge since 2022. Initially it involved a robotic radiometer that, in essence, measured how deeply blue the sky appears at the 10,300-foot elevation. In contrast to milky appearing skies normal at lower places, solar astronomers crave air that’s free from dust and clouds that scatter sunlight. Having passed through Phase 1 of the survey, Magdalena Ridge is now one of only three sites remaining under consideration, the other two being Cerro Tololo in Chile and Tenerife in Spain’s Canary Islands.

The new instrumentation, sheltered in a new dome also installed by the High Altitude Observatory, will measure how sharply the Sun can be seen at Magdalena Ridge. Air turbulence, along with scattered light, can corrupt the view. The Sun, in fact, twinkles like stars do in the night sky. But in the case of sunlight, the apparent effect is very small and difficult to measure. Yet it can be done by using sensitive devices called photodiodes. The twinkling proves to be related to how clearly a picture of the Sun can be recorded. The instrumentation for this and other related measurements is carried on a new robotic telescope, all of it sheltered in a new dome at the Ridge that will open robotically when conditions allow operation. The instruments are very similar to what were used some years back when astronomers at New Mexico’s National Solar Observatory in Sunspot conducted a survey that led to the world’s largest solar telescope being built at Haleakala on the island of Maui in Hawaii.

With the current grim prospect for science funding, no one knows how soon the COSMO project will actually be built. It would be a major development for local astronomy, however, if it comes to be here in Socorro County. One can at least consider that there are few elements of astronomy of greater human importance than space weather, given what scientists now understand about the Carrington Event of 1859 and other huge solar-powered geomagnetic storms like it. So perhaps there will be special consideration for projects like COSMO that are specifically concerned with the Sun and its direct impact on the Earth. And of Magdalena Ridge and its competition with the other sites, Professor Van Romero happened to host a tour of New Mexico Tech officials during the phase 2 installation by High Altitude Observatory. In no uncertain terms he remarked, “We want it here!”

High Altitude Observatory is a component of the National Center for Atmospheric Research in Boulder, Colorado. The Observatory evolved from a small installation started in 1940 in Climax, Colorado, at an 11,500-foot elevation on the Continental Divide. Observations from the initial solar telescope proved critically important to understanding radio communication during World War II.

Editor’s Note:

Reporting this story, astronomer John W. Briggs of Magdalena is Secretary of the Alliance of Historic Observatories, an Adjunct in the NMT Physics Department, and is the 2025 recipient of the G. Bruce Blair Award of the Western Amateur Astronomers. He provides technical support for the COSMO site survey at Magdalena Ridge.