Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Outside the Optical: Other Kinds of Telescopes

Homework for next class

Astronomers started to investigate portions of the electromagnetic spectrum outside the optical in the 1930s. Advances in radar and rocket technology during World War II gave this new research a big push, and it has continued to grow ever since.

Note that the optical region is just a teeny, tiny portion of the entire electromagnetic spectrum:

                Wavelength (m)        Frequency (Hz)          Energy (J)
                             -1                    9                       -24
Radio                > 1 x 10              < 3 x 10                < 2 x 10

                  -3         -1          9         11          -24         -22
Microwave   1 x 10   - 1 x 10      3 x 10  - 3 x 10      2 x 10    - 2 x 10   

                  -7         -3          11        14          -22         -19
Infrared    7 x 10   - 1 x 10      3 x 10  - 4 x 10      2 x 10    - 3 x 10

                  -7         -7          14        14          -19         -19
Optical     4 x 10   - 7 x 10      4 x 10  - 8 x 10      3 x 10    - 5 x 10

                  -8         -7          14        16         -19          -17
UV          1 x 10   - 4 x 10      8 x 10  - 3 x 10      5 x 10    - 2 x 10

                  -11        -8          16        19         -17          -14
X-ray       1 x 10   - 1 x 10      3 x 10  - 3 x 10      2 x 10    - 2 x 10

                            -11                    19                      -14
Gamma-ray            < 1 x 10              > 3 x 10                > 2 x 10

In musical terms, the optical region corresponds to a single key on the keyboard of light:

Let's look at some representative telescopes for these other regions of the spectrum. Remember that the primary job of all telescopes is to gather light; if possible, it's good to focus the light, too.



The near-infrared region is similar to the optical in many ways. One can use telescopes much like optical telescopes -- with mirrors made of glass -- but one does need to switch to IR-sensitive detectors. The big problem is that the Earth's atmosphere blocks most of the infrared radiation ...

These data, produced using the program IRTRANS4, were obtained from the UKIRT worldwide web pages.

... so, in order to get a clear view, one must climb above most of the atmosphere.


X-rays are blocked completely by the Earth's atmosphere (and a good thing, too), so one must place X-ray telescopes in orbit. Another problem with X-rays is that they penetrate ordinary matter. That means that they tend to go through mirrors instead of bouncing off them. The only way to focus X-rays is to design a telescope so that the incoming light always grazes the mirrors at a small angle.


Gamma rays are even more energetic than X-rays, and they, too, are blocked by the Earth's atmosphere. The conventional way to observe them is to place a telescope in orbit ... but it turns out that one can observe them indirectly from the ground, too.

"Telescopes" for detecting particles, not electromagnetic radiation


Neutrinos are ghostly particles which almost never interact with matter. They can (and do) pass through an entire planet untouched. Every second, over ten trillion neutrinos produced in the core of the sun zip through your body!

So, how can one detect these neutrinos? One needs to put a big chunk o' matter in their path, monitor the matter very carefully ... and be prepared to wait patiently.

Solar wind particles

The Genesis Space Mission sent a spacecraft far from the Earth's magnetic field to collect particles of the solar wind. The basic idea was simple: expose plates of pure metal

to the Sun, and let solar wind particles smash into them and stick.

Some scientists are especially interested in the number and type of oxygen atoms in the solar wind. Even though the Genesis spacecraft was designed to spend over two years collecting particles, they realized that it might not be long enough to build up a statistically significant sample of oxygen (which is much less common in the solar wind than hydrogen or helium). Therefore, they designed one special collector which could concentrate particles, putting many more than usual into a small spot on one collector. You can see this collector at the center of the spacecraft:

This concentrator is just a telescope -- but it uses electric fields (instead of polished glass) to reflect charged particles (instead of light rays). Here's a closeup view of it:

When active, the back surface of the collector, which is shaped like a parabola, is given a large positive electric charge:

Here's a side view:

If we apply a positive charge to the "solid electrode" at the base of the concentrator, then a positive ion which flies into the concentrator will "reflect" off that positive charge and end up on the target:


  1. The Expanded Very Large Array (EVLA) is an upgrade to the big radio interferometer mentioned above.
    1. What is the highest frequency this telescope can observe?
    2. Convert this highest frequency to a wavelength -- what wavelength is it?
    3. Using this wavelength, and a diameter of D = 35 km, calculate the angular resolution of the EVLA.
    4. How does it compare to the resolution of the HST?
  2. The Spitzer Telescope looks at infrared light.
    1. What is the diameter of its mirror?
    2. What is the longest wavelength of light detected by Spitzer's main camera, the InfraRed Array Camera (IRAC)?
    3. Calculate the angular resolution of Spitzer at this wavelength.
    4. How does it compare to the resolution of the HST?
  3. The Compton Gamma Ray Observatory looked at very high energy radiation from space. You can find the URL of its web site in the text above. There is a table of instrument properties available from that URL; it's a bit hard to read, so you might also look at a local copy.
    1. What was the effective collecting area of BATSE in "Large Area" mode, for light of energy 0.1 MeV?
    2. Convert the collecting area into a rough diameter of the telescope, assuming a circular shape.
    3. Gamma rays of energy 0.1 MeV have a very short wavelength, about 1.2 x 10^(-11) meters. If BATSE were limited by diffraction, what would its angular resolution be?
    4. Look in the instrument properties table for the "Position Localization" of strong sources with BATSE. What was the actual angular resolution of BATSE? (It turns out that one cannot focus gamma rays with mirrors or lenses, so there are other problems besides diffraction which limit the resolution of BATSE).

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Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.