ASTRO 801
Planets, Stars, Galaxies, and the Universe

Visible Light Telescopes

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Additional Reading from www.astronomynotes.com


Start Here!

The Hubble Space Telescope Science Institute team has put together an excellent resource on the history of telescopes called "Telescopes from the Ground Up." It covers much of what I'm going to present below, so I recommend spending some time looking through the material provided under the link "Get to the root of it: Basic Science Concepts." Many of the links in the material below take you to pages from this excellent resource.

The last step in studying the light from astronomical objects is detecting it when the light arrives here on Earth. The standard instrument that astronomers use to detect light is a telescope, which collects the light and brings it to a focus, and a camera to record the light from the object. Telescopes have three main functions:

  1. to gather as much light from an object as possible,
  2. to focus the light into a sharp image, and
  3. to magnify the image.

Magnification is probably the most familiar function of a telescope. Here is a comparison of two different familiar astronomical objects. One appears as seen with the naked eye, and the other is magnified:

This animation allows you to click through slides that demonstrate the magnification you can experience with a telescope or binoculars. The first frame shows a naked eye view of the constellation Orion. The second frame shows an image of what you might see with a pair of binoculars of a magnified area of the first frame. The third frame shows the additional magnification you would get of the Horsehead nebula (part of the nebula in the second frame) with a small telescope. The next sequence shows a naked eye view of the constellation Hercules, centered on the star cluster M13, a simulated magnified view of M13 for binoculars, and finally, a simulated magnified view of M13 from a small telescope.

Credit: Penn State Astronomy & Astrophysics

As you'll see at the Hubblesite pages about "What do telescopes do?" and "What makes a good telescope?," though, magnification is the least important property of telescopes.

The most important property is a telescope's light gathering power. The larger the aperture (the opening at the top of the telescope tube), the more light the telescope will gather. To understand this, picture a telescope as a “light bucket.” If you want to collect as much rain as possible in a short time, you would go out during a storm with a wide-mouthed bucket instead of a drinking glass, because the opening in the bucket will collect more raindrops than the glass in the same amount of time. Telescopes work the same way. As photons “rain” down on Earth, a telescope with a bigger aperture will collect more of them than a telescope with a smaller aperture. Thus, the light-gathering power (which measures how bright an object appears, or, alternatively, how faint an object is that is just barely detected) of a telescope is determined by the area of the opening at the front of the tube. For this reason, astronomers have built larger and larger telescopes since they were first invented four centuries ago.

Since most telescopes have circular apertures, the light gathering power is proportional to the area of the aperture, or π R 2 This equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers. . So, given two telescopes of different apertures (say one that is 1 meter across, and the second that is 4 meters across), the light gathering power of the 4 meter telescope is  ( π )( 2 2 ) / ( π )( 0.5 2 ) = 16 T timehis equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers. times better. You can translate that statement in the following way: you can say that if you look at the same object with the 1 meter and 4 meter telescopes, the object will look 16 times brighter through the 4 meter than it does through the 1 meter. Or, the 4 meter telescope will be able to observe objects 16 times fainter than the limit of the 1 meter telescope.

Try this!

You can get an idea for how much more powerful a modern research telescope is than the human eye. That is, how much fainter an object can you observe with the Keck telescope than with just your eye?

Repeat the calculation above, but use 10 meters for the diameter of the Keck, and 5 mm for the diameter of the pupil of your eye.

The earliest telescopes were simple—they had an objective lens at the end of an empty tube. The lens was shaped so that it would take parallel beams of light and focus them to an image inside the tube. An eyepiece (another lens at the other end of the tube), allowed you to magnify the small image in the telescope tube so that it appeared larger to your eye. Refractors (telescopes that use lenses to focus light) create sharp images; however, they suffer from chromatic aberration. That is, different colors of light get focused to different points (recall how glass prisms disperse white light into its component colors), so the blue light from a star is not focused as well as the red light, causing stars to be surrounded by a blue halo. For this (and other reasons described below), refracting telescopes were eventually superseded by reflecting telescopes (which use mirrors, not lenses).

Reflecting telescopes are empty tubes with curved mirrors fixed to the bottom of the tube. The mirror will focus parallel rays of light to a point inside the tube, near the top of the tube. Since the primary mirror is fixed at the bottom of the tube, a second, smaller mirror is set in the top of the tube to direct light out through the side of the tube into an eyepiece. This is called a Newtonian reflector. If the second mirror instead sends light back down the tube and through a hole in the bottom of the tube, that is called a Cassegrain reflector. The images created by reflectors are not usually as sharp as those created by refractors, because it is difficult to get curved mirrors to bring all of the light from a distant object to a common focus (an effect called spherical aberration).

In their article on buying your first telescope, Sky & Telescope magazine compares and contrasts these two types of telescopes and also has cutaways of the interior of a refractor and reflector, showing the path light takes to the eyepiece.

The largest refractor still in use today is the Yerkes refractor, which has a 40” (one meter) aperture. There are several reasons why larger refractors have not been built:

  1. As lenses get larger, they begin to sag under their own weight, degrading their ability to accurately focus light.
  2. The largest lenses need to be incredibly thick, but they can only be supported along their edges, where the glass is thinnest.
  3. Lenses need to be completely transparent and allow as much light as possible to penetrate. Thick, massive lenses are expensive to polish on both sides and absorb light.

Want to learn more?

The folks at Yerkes describe this in more detail at their page titled "Why are most research telescopes reflectors?"

Reflectors don’t suffer from most of these problems. Mirrors can be supported along the entire back side, correcting for the sagging problems of large refractors. Only the front of a mirror needs to be polished, and if the coating degrades and begins to reflect less light, it can be recoated to restore its reflection efficiency.

A 2.5 meter (100 inch) reflector has been in use at Mount Wilson in California since 1917, a 5 meter (200 inch reflector) has been in use at Mount Palomar in California since 1948, and the twin Keck 10 meter (400 inch) telescopes have been operated on Mauna Kea in Hawaii since the 1990s. All of the largest telescopes in the world are reflectors, and the next generation of large telescopes include proposed instruments with mirrors of 25 - 40 meters in diameter like the Thirty Meter Telescope, The Giant Magellan Telescope, and the Extremely Large Telescope.