Chapter 5: Astronomical Instruments

Since we can't travel to distant stars and galaxies to study them, we study them by observing radiation they emit that has travelled to us. We've classified radiation as different types, from radio waves to gamma rays, and we wish to study all the types possible.

5.1 Telescopes

First, let's discuss some properties of telescopes. The ability of a telescope to see distant objects comes from its power to collect light (or other EM radiation) over a large area and focus it onto the eye, or photographic film, or electronic sensor. The light gathering ability of a telescope is characterized by its aperture. The larger the aperture, the more light that can be collected, and the greater the power of the telescope to see distant objects. For astronomy, the magnification of the telescope is irrelevant.

Telescopes come in two types, refracting and reflecting.

There are two methods for bringing the light out of a reflecting telescope for viewing (see Fig. 5.5). Both methods require a mirror be placed along the central axis of the telescope. In a Newtonian telescope, the mirror is oriented at a 45° angle to the axis and reflects the light out the side of the telescope. This is common in smaller, hobbyist reflectors. In a Cassegrain telescope, the mirror is perpendicular to the axis and reflects the light out a hole in the center of the main mirror. This is common in large observatory telescopes.

Again, the ability to see faint, and more distant objects is determined by how much light can be collected. More light can be collected by building a telescope with a large aperture. Also, more light can be collected if the telescope can remain pointed at the object for a longer period of time, and film, or an electronic detector can integrate all the light arriving in the period. But, the Earth is rotating. To keep focused on the same point for a long time, the telescope must be constantly moving as the Earth rotates.

Many of these concepts are applied to telescopes that "view" wavelengths other than visible: radio, infrared, x-ray, and gamma ray.

5.2 Optical Detectors and Instruments

The three uses for telescopes:

All of these benefit from longer integration times to collect more light.

After the telescope, the next big advance in astronomy came with the use of photographic film to record images. First, images are recorded for later inspection or reinspection. Second, the integration time can be minutes or hours, allowing more faint objects to be seen. And third, photographic film is much more reliable than human observation.

A modern advance is the use of charge-coupled devices (CCD's). They record the amount of light incident on them, like photographic film, but they are more sensitive, and are readout electronically, ready for computer analysis. CCD's are the devices used in digital cameras, and many hand-held video cameras.

5.3 Optical and Infrared Observatories

Ground based optical and infrared observatories are what people normally associate with telescopes. The telescopes themselves are often housed in the familiar domes to protect them from the elements. To get the clearest pictures, modern telescopes are located on remote mountain tops, far from large cities.

The atmosphere affects the choice of sites in four ways:

  1. Weather: Clouds, wind, and rain limit observing time. Find sites where the weather is often clear.
  2. Water vapor: Water vapor in the atmosphere filters out infrared wavelengths. Places where the air is dry are preferrable.
  3. Darkness: City lights can overwhelm faint stars. Remote locations are darker, and better for viewing.
  4. Turbulence: Turbulent air blurs images. Calm conditions offer better "seeing".
The most modern telescopes are located in the Andes mountains of Chile, desert peaks of Arizona, the Canary Islands in the Atlantic Ocean, an Mauna Kea in Hawaii. [Show pictures of telescopes in these locations] The locations are often crowded with telescopes.

Modern telescopes use large segmented lenses, adaptive optics, and interferometry. Interferometry is a process where light from two (or more) telescopes is combined into one signal. This achieves a resolution equal to a telescope much larger than either telescope alone, and is much cheaper to build than the larger telescope.

5.4 Radio Telescopes

Radio telescopes are telescopes that use radio waves rather than visible or infrared light. The first radio telescope was built at Bell Labs in 1932. The largest, and perhaps most famous radio telescope is the 1000 ft. Arecibo dish built into a mountain valley in Puerto Rico. But many others exist. Major observatories exist in West Virginia and New Mexico.

Radio telescopes can operate during the day or night -- the Sun is not a major source of radio waves. The real problem for radio telescopes is radio waves generated by modern society, not just the obvious radios, TV, and cell phone transmitters, but also vehicles -- spark plugs put out noise in the radio spectrum.

5.5 Observations Outside the Earth's Atmosphere

Viewing in many wavelengths requires that we put detectors above the Earth's atmosphere. Even for visible light, there are advantages to being above the atmosphere (hence the Hubble space telescope).

Airborne and Space Infrared Telescopes

A small amount of infrared light penetrates the Earth's atmosphere and can be viewed with ground based telescopes. This is primarily infrared wavelengths very near visible red light wavelengths. To see the full infrared spectrum we must put a telescope on a high flying airplane or on a satellite.

Hubble Space Telescope

The Hubble space telescope is the first large optical telescope placed in Earth orbit. The advantage of putting a telescope in orbit will be clear later in the course when you see some of the amazing pictures that have been produced.

High Energy Observatories

A recent and interesting addition to our observatories are satellites that can detect higher energy radiation that is not able to penetrate our atmosphere. We have satellites that can view in the ultraviolet, x-ray, and gamma ray spectra. The x-ray and gamma ray observations are very interesting, revealing some rather unexpected phenomena related to some of the most bizarre objects in the universe.

© Robert Harr 2001