The Telescopes

Holcomb Telescope

The Holcomb 38-inch Cassegrain telescope. The long scope riding piggyback to the right is Butler’s 6-inch Clark refractor. This 6-inch Clark dates back to 1883.

The 6-inch Clark Refractor

Butler University’s first telescope was a 6-inch refractor donated to the University in the late 1880s when the campus was located in Irvington on the east side of Indianapolis. The 6-inch refractor can be seen in the image to the left. It is the long telescope riding piggyback on the 38-inch telescope. The 6-inch refracting telescope was purchased from the estate of Robert McKim of Madison, Indiana by Irwin in 1889. The lens was made by Alvan Clark & Sons in 1883 and was originally part of McKim’s personal observatory located near his home. The telescope is thought to be the first in the state of Indiana. It has been refurbished several times, most recently 2014.  You can learn more about the history of astronomy at Butler and the telescopes here.

A refracting telescope uses a lens to gather light. This light is then focused at the back of the telescope where the observer looks through the eyepiece.

When Butler University moved to its current location in 1927 the 6-inch telescope was used on top of Jordan Hall. Once Holcomb Observatory was built in 1954, this telescope was put onto the 38-inch Cassegrain telescope as a spotting/wide field telescope. The telescope is still in very good operating condition and still provides excellent views of the cosmos.

The 38-inch Cassegrain Reflector

Ray trace image of the Holcomb Telescope Optics.

A diagram showing the new configuration of the 38-inch Cassegrain after refurbishment. In addition to a new secondary mirror, a corrector lens located at the back of the telescope was also installed.

The main telescope in use at Holcomb Observatory is the 38-inch Cassegrain. This telescope along with the observatory was built in 1954. A Frenchman, Nicolas Cassegrain (1625-1712), invented this two mirror telescope. Almost all modern telescopes follow this basic form to some degree. The diagram below illustrates the optical elements and the path that light takes through the telescope. The primary mirror has a parabolic shape and gathers light in the same way a refractor gathers light using its objective lens. The secondary mirror, an hyperbolic shape, reflects the light back through a hole in the primary mirror. The instruments and eyepiece are located near the focus behind the telescope.

Though the 38-inch Cassegrain is structurally and mechanically sound it was initially difficult for the observer to use. In order to go from object to object the observer had to physically move the large telescope by hand. In addition after many years of addition and subtraction of equipment the telescope was grossly out of balance and years of wear and tear made the telescope’s tracking suspect. With the advent of students using CCDs for data acquisition it became apparent that the telescope should be automated and incorporate precession tracking and positioning via a computer. This automation would allow students to more easily acquire targets and data making observing sessions a more pleasant learning/research experience.

In 1994 Butler University began the first phases of an upgrade the 38-inch telescope. Upgrading the large telescope proved a significant challenge for the university. The university hired AB Engineering of Ft. Wayne, Indiana for the upgrade. They proposed a retrofit of the mount with computer controlled polar and declination axis drives. This project required the replacement of the original polar axis worm with a new precision worm assembly and the declination tangent arm with a 26-inch diameter worm gear set. Each axis is driven by a single stepper motor providing both tracking and slewing operation. The encoders and stepper motors are controlled by a telescope control computer in the telescope dome. A dedicated, solid state controller orchestrates motion control and position monitoring.

The first phase of refurbished telescope was finished in April 1997 and commenced operation, with Mr. Tom Bopp, co-discoverer of comet Hale-Bopp, presiding. In 2001 the final automation was completed.

Since then age has taken its toll on this system and in 2013 the observatory launched another major tune-up for the telescope. Funding for this project was provided by former Butler physics student Dr. Frank Levinson through the Silicon Valley Community Foundation. One of primary items that have been addressed are the optics of the telescope. The primary mirror has had an astigmatism removed, a new 13-inch secondary mirror was fabricated and installed, and a corrector lens/focal reducer has been installed thus reducing the telescope’s focal ratio from an f/16 to an f/6.1. This reduction of focal ratio makes the telescope a much faster camera and increase the field of view of the telescope seven fold. The dome now has failsafe mechanisms allowing it and the telescope to be used remotely.