"Updating Goethe Link Observatory"
By R. K. Honeycutt, J. E. Kephart, and A. A. Henden, Indiana University
From Sky and Telescope, December 1978, page 495.
Copyright (c) 1978 by Sky Publishing Corp.
Reproduced with permission of the publisher.
The designs for large modern research telescopes usually call for a closed control room overlooking the dome floor. This arrangement protects electronic equipment from the temperature and humidity variations in the dome and provides increased efficiency and comfort for the observer. Here at Goethe Link Observatory of Indiana University, we have recently added a new control room and other improvements for the 36-inch reflector.
The observatory was given to the university by noted Indianapolis surgeon and naturalist Goethe Link in 1948, and was described in SKY AND TELESCOPE for December of that year, page 34. Dr. Link, who celebrated his 99th birthday last May 20th, lives next door to the observatory grounds and still maintains an active interest in astronomy.
The original design of our dome provided a vacant space between the ground and observing floors. Here a 250-squarefoot room has been constructed with inclined glass windows that look into the dome. The room rises only four feet above the observing floor so that the telescope's view of the sky remains unobstructed. We took special care to make the room light tight and heavily insulated, with double-pane windows. A trapdoor in the ceiling permits hoisting equipment in and out, and ample electrical power is provided. The room is finished with natural pine paneling to match the observatory's existing rustic beam and paneled construction.
Originally the telescope was used as a Newtonian, but the height of the focus above the dome floor proved awkward for supporting research instrumentation. In 1966 an entirely new telescope tube with f/10 Cassegrain optics was installed. The original f/5 primary mirror was retained, but its ribbed construction prevented cutting the central hole needed for a normal Cassegrain focus. Instead, a tertiary flat was inserted to reflect the converging beam of light from the secondary and direct it toward any of three observing ports at the circumference of the tube.
The tertiary is mounted on a turret, allowing the beam to be redirected to a new port in a matter of seconds. Thus, several instruments can be mounted simultaneously on the tube, eliminating much tedious instrument changing and telescope balancing. This new configuration considerably eased the use of our two major research instruments, a Meinel stellar spectrograph and a rapid-scanning spectrophotometer. These telescope modifications were completed in 1967 under the direction of Martin S. Burkhead.
For some positions of the telescope the focus still rises to uncomfortable heights, and a remote television guider is desirable. We built one from a surplus studio TV camera, replacing the original vidicon with one made by coupling a 40-mm-diameter image intensifier to a 25-mm-diameter SEC vidicon. The vidicon is housed separately from the camera electronics, making it easy to transfer the TV guider from one observing accessory to another.
A custom-built integration unit controls the vidicon exposure times from 0.033 second to 100 minutes. For unfiltered operation of the guider, integration times of about 1/3 second reach sky background, and stars near 17th magnitude can be detected. A scan-converter memory is used to refresh the TV monitor during the integration periods, so the observer sees a nearly flicker-free image.
A new coordinate readout system has also been installed on the telescope. Shaft encoders on each axis produce a digital signal which is fed to a microprocessor. This in turn generates a TV display of the telescope's right ascension and declination. Also displayed are the date, sidereal and Universal times, and the air mass through which the telescope is looking. The microprocessor is part of an IMSAI microcomputer. Other telescope readout functions can be displayed by the computer as needed. The computer is also used to control the operation of accessories and collect data. An identical computer system on campus aids us in writing and debugging programs.
Our primary research accessory for the 36-inch telescope is a Cassegrain spectrograph with the multichannel-analyzer data acquisition system. To improve the spectrograph's performance, the highly sensitive vidicon detectors are cooled with dry ice. Additionally, some of the mechanical functions can be remotely controlled.
Our philosophy has been to provide remote operation of the telescope functions that must be attended to frequently. However, we have not aimed for total remote operation. The telescope is, after all, only a few steps away from the control room and adjustments which are required only occasionally (such as spectrograph slit width) are done by hand. With our present setup the observer can locate a star, set the spectrograph to the desired wavelength interval, guide an exposure, and monitor the quality of the data without leaving the control room.
Current research programs at the observatory include the study of the spectra of cool stars, high-speed spectroscopy of interacting binary stars, and spectrophotometry of unidentified absorption lines in the interstellar medium. Our new control room, which was funded by the National Science Foundation and Indiana University, should help these and other research programs by increasing the reliability of the equipment and the endurance of the wintertime observer.
©2005 Indiana Astronomical Society