Radio Astronomy Observatory

[Picture of the 26 m] Color Image of telescope: 75dpi jpg file or 300 dpi jpg file

Radio Astronomy Observatory

The UMRAO Database

The above plot shows the evolution in both total and polarized flux of BL Lac over the last 15 years. The three symbols denote data at the three observing frequencies; frequency dependent behavior helps to determine the conditions in the emitting region. The bottom panel shows variations in the total flux, which are believed to result from the passage of shocks along a light-year scale jet associated with the nucleus of this galaxy. The middle panel shows variation in the polarized part of the emitted radiation, and by providing information on the degree of tangling of the magnetic field, supports the shock `picture', which predicts a compression of the magnetic field, effectively giving it more order. The top panel is the orientation of the plane of the polarized emission, which gives information on the direction of the magnetic field in the emitting region, and so provides information about the fluid flow pattern in the jet.

History

The primary instrument at the Observatory is a 26-meter (85-foot) diameter parabolic reflector with a 36-foot focal length. This instrument was constructed in 1958 at Stinchfield Woods as a research instrument, under contract from the Office of Naval Research. At the time of its construction, the Michigan dish was one of the largest radio telescopes in the world. Subsequent to 1968 the research and operation of the Observatory have been supported jointly by a series of grants from the National Science Foundation and by funds from the University of Michigan.

Technical Details

The surface of the dish is constructed from doubly curved panels of aluminum sheeting. Overall, it deviates by less than 0.125 inches from a true paraboloid. The telescope has a two-axis equatorial mounting (with a 40-foot diameter polar gear and a 38-foot diameter declination gear) constructed of galvanized structural steel; the counterweights are lead. The polar and declination axes are aligned to within an accuracy of 20 arcseconds. The total weight of the telescope including mounts and support structures is 400,000 lbs; the foundation is made of concrete blocks weighing 710 tons. Radiometers operating at the Observatory's primary observing frequencies of 4.8, 8.0, and 14.5 GHz are permanently installed near the focus. The cost of the telescope, including mount and concrete foundation but excluding radiometers, was about $270,000, a very low sum compared to present day construction costs.

Program

For the past two decades the 26-meter telescope has been dedicated to the study of total flux density and linear polarization from active extragalactic objects in the radio-wavelength region of the electromagnetic spectrum, and this telescope continues to be one of the few instruments in the world dedicated to that type of research. The discovery that extragalactic objects vary in brightness with time scales of weeks to a few years was made using the Michigan instrument in 1964-65, and the discovery of variability in polarization was made here in 1966. These discoveries opened up a new field of astronomical research, and understanding the origin of the variability continues to occupy researchers. Current research at the Observatory is focused on understanding the physical conditions in the relativistic flows in the light-year scale jets of these objects and on investigating the relationship between the radio-wavelength variations and the behavior of the radiation in other regions of the electromagnetic spectrum, observed with other ground-based and satellite-borne instruments. One of the most exciting discoveries in the past few years from data obtained with detectors aboard the Compton Gamma Ray Observatory, a satellite launched in 1991, is that several dozen radio-variable objects also emit high energy gamma-rays. A large group of confirmed and potential gamma-ray emitters are now observed at Michigan as part of a world-wide collaborative program dedicated to studying these enigmatic objects.

Because of the time scales (several years in some cases) of the phenomena studied, the Michigan observing program relies on essentially continuous observations over periods of many years. An important element in the success of the program has been the consistency and continuity of the data acquisition. In 1977 the telescope was placed under full control of a specialized computer system which allows it to acquire and record data completely unattended; this procedure has resulted in greater uniformity in the data, as well as an enormous increase in the number of observations, typically over 10,000 per year. Because of this automated mode of operation, the telescope is able to observe on a round-the-clock basis, without personnel at the observatory. The telescope has consistently operated at near the 90% time-level, with down-time only for radiometer changes, occasional weather-related problems, scheduled maintenance, or mechanical/instrumental upgrades.

Scheduling and Data Reduction

The lists of the positions of the objects to be observed during each run are changed frequently to reflect the current status of individual sources. These lists are generated using computer programs that simulate the functions performed by the telescope to obtain flux measurements (both on-source and background) and to move from source to source, and these lists are transferred via high speed modem to the control computers at the telescope site. The observations are processed in real time at the telescope site, and source parameters are inspected on a nearly daily basis to evaluate the operation of the equipment and to identify newly active objects. Final reductions of the data are carried out using a system of SUN computers in the Department of Astronomy on campus in Ann Arbor.

Teaching

In addition to the work described above, the telescope is used to provide hands-on training for both graduate and undergraduate students at the University of Michigan. Each semester, several laboratory sessions are conducted here as part of undergraduate courses, and typically two or three graduate students each term either conduct their own research projects with the telescope or are involved in the acquisition, reduction, and analysis of the data for the variability monitoring program.

Personnel

Hugh D. Aller (Director): haller 'at' umich.edu

Margo F. Aller : mfa 'at' umich.edu

Philip A. Hughes : phughes 'at' umich.edu

George E. Latimer: glatimer 'at' umich.edu