To make images with as much angular information as possible
To gather as much light as possible to allow study of faint things
Where you put the telescope can also be important because
It’s better to have nice weather (or no weather)
It’s better to have as little absorption by atmospheric blocks
It’s better to have very stable (or no) air to minimize blurring
Light-Gathering Power A telescope it can gather is proportional tois like a “light bucket”. The amount of light the area of its opening or “aperture”. That in turn is proportional to the square of the diameter: L.G.P.~D 2 The larger the telescope, the fainter one can see things (although detector efficiency and exposure time also play an important role).
Angular Resolution A small resolution is better, as it allows more closely spaced features to be distinguished. A fundamental limit on how fine the detail a telescope can resolve is its “diffraction limit”. The resolution of a telescope is given by:
Examples of Resolution Angular units are given as degrees, arc minutes, arc seconds. An arcsec is about the angular size of a quarter seen 5 km away. The resolution of your eye (which has a diameter of 2.5mm) at the wavelength of visible light (which is 500 nm) is therefore: 2x10 -3 x 5x10 2 /2.5x10 -3 =400arcsec=6.5arcmin (the Moon has an angular diameter of 30 arcmin). To reach the practical limit of what can be seen (~1 arcsec) through our atmosphere, you need a telescope with 400 times that diameter, or 1 meter. Telescopes larger than that only gain you light gathering power, unless you do something to reduce the blurring by the atmosphere. Radio telescopes, which operate at wavelengths 10,000 times longer, will have far worse resolution. The real equation for resolution is:
Astronomical “Seeing” It’s best to find a site where the air layers above are very stable (or flow smoothly). You also need to make sure that the telescope and dome does not give off heat.
Refracting telescopes The telescope at Chabot Space and Science Center You can use a lens to gather the light and bring it to a focus. The magnifying power of the telescope depends on the focal length, but mostly on the eyepiece you use to magnify the image at the focus. It is hard to make lenses really big.
Reflecting Telescopes One generally has a “secondary” mirror to take the light to the focus; this blocks some of the light from the primary. It is much easier to make a large mirror, because you can support it from behind. All large telescopes are reflectors. You can take the light to various foci, some of which are better for placing heavy instruments.
Mirrors of the World
The Quest for Light: The Biggest Telescopes Twin 10-meter Telescopes On Mauna Kea The Keck Observatory: the largest in the world (UC, Caltech, NASA).
The European VLT (very large telescopes) Four 8-meter mirrors on Paranal in Chile (gives coverage of the Southern Hemisphere). Different instruments for each telescope, but they can also work together.
Active Mirrors and Adaptive Optics You can constantly monitor the focus of a mirror and improve the image (removes flexing and thermal problems; still seeing limited). The alternative to segmented mirrors.
Observatories like to be high and dry
… so they can see more of the EM spectrum On the ground, getting above water gives you the near infrared and sub-mm regions (good for studying star formation and distant galaxies). But in space, ahhh….! Not only do you get the whole spectrum, but also no seeing problems Of course, it costs a LOT more…
The “Great Observatories” Space Program
The Hubble Space Telescope
Other Space Telescopes SIRTF : infrared Chandra : X-rays HESSI : Gamma Rays
Radio Telescopes Because radio wavelengths are much bigger, it is easy to build radio telescopes bigger (but they don’t have better resolution).
The Quest for Resolution – Adaptive Optics Wouldn’t it be great if you could analyse exactly how the atmosphere is distorting the light waves coming in, and make a correction with a flexible mirror that was fast enough (100 times per second) to keep up with the turbulence. Amazingly, we can now begin to do that! The technique is called “adaptive optics”. You use a small “rubber mirror” near the focus (not the big telescope mirror). You may need several hundred little pistons to correct a large telescope.
Make your own correction star… Using “Star Wars” technology, today we are trying to make ground-based telescopes have sharper vision than Hubble (but only over a tiny patch of sky), along with their superior light-gathering power.
… or maybe you could do even better
Interferometry Must be combined “in phase” (preserving the character it would have had if it came off the same larger mirror). This is easy at radio wavelengths, but hard at optical wavelengths. How about making 2 separate telescopes behave as though they were 2 pieces of one much larger telescope. You get the larger one’s resolution, but not its light-gathering power. The light from the units
The “Very Large Array”
Optical Interferometers That’s why the new observatories come in matched sets…