The problems with large telescopes


hen we refer to the “power” of a telescope, many think we mean magnification. We don’t. We are referring to the telescope’s ability to collect light. Most objects we observe in astronomy are faint, and as we dig further and further out in space, back in time towards the beginning of the universe, objects become incredibly faint. Because there is inevitably a sensitivity threshold for any detector or imager, whether it is the human eye or a state of the art cooled CCD (charge coupled device) camera, to observe faint objects, we need to collect enough light to exceed that threshold.

The light collectors and image formers on telescopes, which are normally called the “objectives”, are either lenses or mirrors. Because lenses have to be supported around the edges and obviously we cannot put anything in front or behind them, the maximum size we can make them is set by their weight, which makes large ones sag out of shape. The limit seems to be a diameter of about 40 inches (roughly a metre). A telescope with a lens this size was successfully built at the Yerkes Observatory, U.S.A, at the end of the 19th Century. This remains the biggest astronomical telescope using an objective lens. Fortunately we have a solution; use a mirror as the objective.

In 1668 Isaac Newton demonstrated that we can use a concave mirror as a telescope objective. This has some tremendous advantages. First of all the light does not have to go through it, so we can have a really solid support structure. Secondly, the mirror, the heaviest component in the telescope, is at the bottom of the telescope rather than the top. .

Starting in the late 19th Century there started a procession of bigger and bigger reflecting telescopes. In 1948 the biggest telescope in the world was the 200-inch (5.1m) Hale Telescope, located on Mount Palomar, California. This remained the biggest working astronomical telescope in the world until 1993. The reason for this was the difficulty in keeping a massive mirror from distorting under its own weight. In 1993 an even bigger instrument appeared on the scene ñ the Keck Telescope, on top of Mauna Kea, Hawaii.

This telescope marked a radical departure in design. Instead of making the mirror thick, rigid, and heavy, the mirror is thinner, lighter, and corrected for sag by computer-controlled adjusters. The result is a pair of telescopes with 10-m mirrors! Soon after came the Gemini Telescopes, having 8.1-m mirrors. One of them is on Mauna Kea, and the other on a mountain in Chile. Canada is a partner in these instruments. This technical breakthrough has opened the floodgates to proposals to build telescopes with mirrors up to 42 metres in diameter. The 42m instrument would be called the Extremely Large Telescope. At this rate we will soon run out of superlatives before we run out of ideas for larger telescopes. It would be very unwise to name an instrument the Ultimately Large Telescope because there will be nothing to call the next one!

Jupiter, Mars, Venus and Mercury are all clustered at the moment, but for us in the Northern Hemisphere, lost in the sunrise glare. The only bright planet available is Saturn, which rises about 5 p.m and is well up in the eastern sky by dark. Look for a moderately bright, yellowish “star”. The Moon will be full on the 17th, and reach last quarter on the 24th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the DRAO.