For centuries, as astronomers have tried to cope with cloud and a turbulent atmosphere that made images swim like a penny on the bottom of a stream, they dreamed of having a telescope in space, above all the trouble. Now we have Hubble, and soon will have the James Webb Telescope. Space-based observations are a well-established part of modern astronomy. Why then do we still invest large amounts of money on building and operating telescopes on the ground?
There are actually lots of reasons. Firstly, developing a telescope for use in space takes a long time. The instrument has to survive the vibration of launch and then the radiation and large temperature changes in space, and finally it will have to work for long periods of time without someone being around to fix it or install new instruments. In general, space telescopes are smaller than ground-based ones. Instruments on the ground can be very large. Telescopes with 100m mirrors are under development, and those with eight to10m mirrors are more or less mainstream observing tools. Instruments can be deployed quickly on ground-based instruments, partly because it is possible to have people on-hand to support them. Ground-based telescopes can respond quickly to new astronomical needs. Making changes to telescopes in space can take decades. Finally, but by no means least, we can put students in direct contact with ground-based instruments, which is critical for raising the next generation of astronomical scientists and engineers. Ground-based telescopes are an important complement to space-based instrumentation.
Moreover, thanks to improved computers and new optical technologies we have come a long way towards overcoming at least some of the problems we get due to the atmosphere. If we know that a particular distorted object is supposed to be a star image, we can undistort the light passing through the telescope by using a thin, flexible mirror. This is called “adaptive optics” and is revolutionizing ground-based astronomy.
When there are no convenient reference stars available, we use lasers to excite sodium or other suitable atoms about 90 km up in the atmosphere, making “dummy stars” for the optical correction systems to use. However, one downside of this arrangement until recently is that only one dummy star does not provide all the information needed for cleaning the whole field of view of the telescope. This has led to the development of a new system which is currently under test at the Gemini South Telescope, in Chile. This is one of a pair of telescopes in which Canada has a share.
A method has been found to make a group of five artificial stars. This does two new things. Firstly there are now five correction points in the image, and knowing exactly how they are all positioned with respect to one another provides additional information. This “Multi-Conjugate Adaptive Optics System” was first fired up in January, and promises to go even further on getting space-based views of the sky using observatories on the ground.
Jupiter is now getting low in the southwestern sky in the evenings. Saturn rises around 11p.m., and Venus around 5 a.m. The Moon will reach first quarter on the 10th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.
Tel (250) 497-2300, Fax (250) 497-2355
E-mail: ken.tapping@nrc-cnrc.gc.ca.