The Telescopes
The 6-inch 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
recfractor can be seen in the image to the left. It is the long
telescope riding piggyback on the 38-inch tellescope. 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 the Butler University moved to its current location in1927
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 good operating condition and now serves
as our guide scope using a CCD camera.
The 38-inch Cassegrain
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 hypebolic shape, reflects the light
back through a hole in the primary mirror. The intruments 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 to 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 telescope's position is displayed via
digital setting circles on a large character LED display, mounted
on the wall of the observatory and on Software Bisques' The
SkyTM via a virtual RS-232 connection or the local area
network. An integrated GPS receiver provides time and location
initialization. Operator control of the telescope is through a
hand-held paddle or through a PC via the Internet. A motion warning
system notifies personnel within the observatory when the telescope
is slewing and when slew limits are being approached.
The upgrade highlights included:
- Motorization of both the Polar and Declination axis to
accommodate variable tracking and slewing operation
- Optical encoders provide polar and declination position
feedback via digital setting circles
- Passive free weights to eliminate gear backlash.
- Computerized user interface for operation of the telescope
(remote and in dome)
- Dome automation and coupling to the telscope motion
- Primary telescope CCD and imaging and autoguiding
- Addition of an mirror cover
- Automated GOTO software with synchronized dome control
- Precision pointing using Tpoint
- Remote access via the Intranet
- Scripting for robotic control
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 automazation was
completed. The telescope is now fully automated so that it can be
run robotically and remotely and carry out an single nights's
observing program unattend.