Mississauga Astronomical Society

40th Meeting

     Speaker’s’ Night

Day:                Friday, May 13, 2005

Guest Speaker:   Dr. John Lester

Transit of Venus Exhibit at the National Museum of American History

Chris Malicki visited the Smithsonian Museum of American history in Washington D.C. on May 4 and chanced upon an exhibit on historical transits of Venus. Displayed were an Alvin Clarke refractor and a transit instrument taken to Kerguelen Island by the U.S. Naval Observatory in 1874. Among many original books were the Rudolphine Tables of Johannes Kepler (1627),  “Mercury Seen upon the Sun and Venus Unseen” of Pierre Gasendi (1727), Mercurius in Sole Visum of Johannes Hevelius, “Remarkable method for determining the solar parallax” of Edmund Halley (1677), Le Gentil’s account of his expedition and many others. Chris encouraged any members who were in Washington to visit this exhibit.

Optical Interferometry: A Revolution in Resolution

Dr. John Lester from the University of Toronto at Mississauga spoke of the revolution taking place in the era of the new giant telescopes. Light gathering capacity varies as the square of the diameter. The Canada-France-Hawaii Telescope (CFHT) is the workhorse of Canadian optical astronomy with a diameter of 3.6 meters. More expensive are the Gemini Telescopes with 8 meters diameter, and the Keck with 10 meters. Planned are the TMT with 30 meters and the OWL (overwhelmingly large telescope) with 100 meters.

In addition to diameter, sharper detail is desired. Detail size varies as the inverse of diameter. This the eye can resolve 1/30 of a degree, and Galileo’s telecsope 1/3000 enabling him to see lunar craters, moons of Jupiter and the crescent of Venus. However, the CFHT, Gemini, Keck, VLT with a resolution of 1/10,000 degrees are only 3 times better than Galileo, far below their potential. Why? The reason, Dr. Lester explained, is atmospheric scintillation.   

The atmosphere is made up of cells of air at various pressures and temperatures and moving across our line of sight. A large telescope mirror contains many individual rays of light and hence the final image composed of all these rays is blurred. To diminish this blurring, professional telescopes are located in places where the blurring is smallest as in Hawaii or Chile. We can do better by masking the mirror to create many small apertures. It is much more economical, however, to simply use many small mirrors at a much smaller cost and bring their light together. Detail varies as the inverse of separation of the mirrors. An interferometer is therefore composed of many small telescopes spread far apart and gathering light in a coherent manner. Whereas a large telescope focuses the light as a whole, the small telescopes are placed on the ground and there is a lag in phase of the light waves. These waves then have to be resynchronized to remove differences in arrival time using various adjustments.

Dr. Lester described several optical interferometer facilities.  The Navy Prototype Optical Interferometer (NPOI) at Lowell Observatory in Arizona has 4 permanent 50cm telescopes with separations from 19 to 37 meters and 6 movable 50cm telescopes with a resolution greater than Hubble or the OWL.  The Centre for High Angular Resolution Astronomy (CHARA) of the University of Georgia in Atlanta is located at Mt. Wilson California and has good seeing. Finally Dr. Lester described the Very Large Telescope Interferometer at Mt. Paranal in Chile, the Cambridge Optical Aperture Synthesis Telescope, and the Sydney University Stellar Interferometer.

What has been done with these interferometers? Double stars have been discovered and resolved with milliarcsec resolution and movement discerned in only a few months allowing calculation of stellar masses. The angular size of star discs can be measured and the temperature deduced from the radiation allowing comparison to theories of star structure. The angular diameters of stars at different orientations can be measured showing whether stars are round or showing bulges related to rotation. The size of the bulge depends on the star’s structure enabling comparison with theory. As examples, Altair rotates rapidly, is not round and has a bright region. Vega, also a rapid rotator lacks a bright spot. Stars are not uniformly bright due to limb darkening and temperature variations with depth can be compared to theoretical predictions. Many stars such as Be stars and aging stars have discs of gas surrounding them and these can be imaged.

In conclusion, small telescopes are making huge advances and are now measuring much finer detail. Dr. Lester said that we are entering a new era of much more detailed examination of stars. Many questions ensued.

Submitted by Chris Malicki, Secretary  Chris Malicki, Secretary                               back to M.A.S. meeting reports page
Mississauga Astronomical Society