The Doane Observatory at the Adler Planetarium & Astronomy Museum is home to the largest aperture telescope available to the public in the Chicagoland area. As such, it is the centerpiece of all telescope observing experiences at the Adler. With its 20-inch (0.5 m) diameter mirror, the Doane can gather over 5000 times more light than an unaided human eye, allowing visitors to see with their own eyes celestial objects like the Moon, planets, stars, and galaxies that are trillions of miles away. (In fact, the Moon can be so bright when viewed through the Doane that we use special filters to reduce the amount of light reaching your eye!)

The Doane is located outside of and behind the main Adler building, directly between the main building and the shores of Lake Michigan. The Doane is open to the Adler's visitors for viewing every Far Out Friday and for special observing events such as eclipses, planetary alignments, and comet watches.

The Doane Observatory was completed in 1977 through the generous support of the estate of Ralph E. Doane. When first completed, the observatory housed a 16-inch diameter Cassegrain telescope that was built and installed by the planetarium staff. That telescope served well until 1987, when it was replaced by a sophisticated new 20-inch (0.5 m) diameter Cassegrain reflector.

The Doane is a classical Cassegrain reflector. This means that it uses curved mirrors to collect and magnify light. Variations of the Cassegrain design are used in nearly every professional observatory today because they allow for long focal lengths (and hence, potentially high magnifications) with a relatively short telescope tube.

The Doane is mounted on a fork-style equatorial mounting that continually moves the 500-pound telescope and keeps it precisely tracking celestial objects. The tracking system relies on a clock drive that turns the telescope at the same rate as the earth rotates. This keeps celestial objects centered in the field of view. In fact, if special limit switches didn't prevent it, the telescope would rotate itself in a full circle once every 24 hours.

The Doane has a 0.5 m diameter aperture and 4 m focal length, which give a focal ratio of f/8 at prime focus. The mirrors are coated with a special aluminum compound to make them as reflective as possible at optical wavelengths. However, over time, oxygen and air pollution cause the coatings to tarnish and lose their reflectivity. So from time to time, the mirrors are entirely removed from the telescope, cleaned, and re-coated with fresh aluminum to keep them as shiny and efficient as possible.

While bright nighttime lights of a city like Chicago, with millions of residents, place some limitations on what can be observed with the Doane, we have very good views to the east over Lake Michigan. As a result, the Doane can be a powerful tool for astronomers. Here we present a small sampling of astronomical objects that have been observed with the Doane. These images have been minimally processed with a computer (using standard techniques to correct for camera "noise" and to improve sensitivity and contrast), but the colors and structures visible in these images are all real.

Caveat: because many of these objects are extremely far away from Earth (and thus very faint), they have been photographed using a series of time-exposure images. Not all of these objects will be visible using the naked eye at the Doane. Additionally, the human eye alone is not sensitive enough to detect the colors of these very faint objects when looking through even the largest of telescopes. We mention this not to be discouraging, but rather so that our visitors will have realistic expectations when they visit the Doane. The Doane is a wonderful tool, but even it is limited by the laws of physics and human biology!

The Doane's instrumentation has recently been upgraded, and includes a 1k x 1k Finger Lakes Instrumentation CCD camera, an SBIG STV high-speed CCD camera, and several video cameras with a standard complement of astronomical filters. The Doane also houses instruments for daytime solar viewing including a Coronado SolarMax 90 hydrogen-alpha telescope.