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The McDonald Observatory is located in the Davis Mountains of west Texas a few miles from the small, rustic village of Fort Davis. It formally opened in May, 1939 when the 82" telescope became operational. The 107" telescope was commissioned in 1968 and was used in establishing extremely accurate distance measurements to the moon in conjunction with the Apollo program. The Hobby-Eberly Telescope (HET), currently the largest instrument on the site, saw first light in 1998. The HET has a segmented 11 by 10 meter mirror and is currently the largest mirror in the world, although its aperture is effectively only 9.2 meters, due to the mounting design. It is a "single purpose" instrument, designed specifically for high resolution spectroscopy.

The three major mirrors currently in service at the observatory are shown in the illustration at the right. Both the 82" and the 107" mirrors are fashioned conventionally from a single glass slab. The 11 meter mirror is a composite of 92 hexagonal mirrors, polished to have spherical surfaces. This composite structure incorporates the same technology used in the two 10 meter Keck telescopes in Hawaii.

Here's a picture of the 82" telescope dome I took in 1965! It was the only large dome on the mountain then - quite isolated.

This historic instrument was the second largest telescope in the world when it went into service in 1939. For the first ten years it was used almost exclusively for spectroscopic studies of stellar structure. This played a major role in the development of our current understanding of stellar evolution. Famous astronomers associated with the observatory include Otto Struve, Gerald Kuiper, and S. Chandrasekhar.

The Harlan Smith telescope has a 107" primary mirror of fused silica and is based on the Ritchey-Chretien design. This design uses a primary mirror with a hyperbolic surface and compensates for image distortion with a complementary hyperbolic secondary. The advantage is good image quality over a wider field than is possible with a traditional parabolic mirror surface. When it went into service in 1969 it was the third largest telescope in the world. It was (and still is) used by NASA for laser ranging to obtain precise distances to the moon and planets.

The primary mirror of the Hobby-Eberly telescope is a mosaic of 92 one meter hexagonal mirrors, each with a spherical surface. To insure that each of these mirrors work together as one, a laser beam is sent from the secondary tower (upper right in the first picture) to each segment. The reflected beams are then combined in an interferometer, and the position of each mirror is adjusted as required. Since the telescope does not move in altitude (it is fixed at 45 degrees to the horizon), the alignment does not have to be performed during an observing run. Each individual hexagonal segment is flexible and can be adjusted to compensate for variable seeing conditions due to temperature fluxuations during the evening. (center image is a poster in the visitor's center; image on the right is from a postcard)

The new visitor's center at McDonald opened in March of 2002. It has a number of Mede 14" telescopes available for public observing every clear day (solar viewing) and night. Its exhibit area is unusual in that, unlike exhibition halls in most public observatories/planetariums, it concentrates on one subject: spectroscopy. This is appropriate, since spectroscopy has been the mission of McDonald observatory from the beginning. The displays are all very good and are appropriate for both children and adults.


HET from Visitor's Center	Tile replica of the HET mirror	Sundial at Visitor's center


Solar spectrum with absorption lines	Public solar viewing