Outside the Optical: Other Kinds of Telescopes

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.

Radio

The Arecibo Telescope is the largest radio dish in the world, with a diameter of 305 meters (1000 feet)!

courtesy of the NAIC - Arecibo Observatory, a facility of the NSF and David Parker

Its primary mirror, made of wire mesh, reflects light upwards to the secondary and detectors, which hang high above:

courtesy of the NAIC - Arecibo Observatory, a facility of the NSF and Tony Acevedo

Now, despite its huge size, the Arecibo telescope can't take particularly sharp pictures of celestial objects. Astronomers describe the amount of fine detail visible in an image by the term resolution. Here's a low-resolution version of an image:

and here's a high-resolution version of the same image:

The best theoretical resolution of a telescope depends on its diameter and on the wavelength of the light it focuses:
```
                                      wavelength
                resolution goes like  ----------
                                       diameter  
        
```
The smaller this fraction, the smaller the details a telescope can distinguish.
Even though the diameter of the Arecibo telescope is much larger than that of, say, the Keck optical telescope, the wavelength of its light is much, much, MUCH longer. As a result, its resolution is much worse (i.e. the fraction above is much larger), and its pictures are blurry.
But there is a way for radio telescopes to take sharp pictures ....
The Very Large Array of radio telescopes in Soccoro, New Mexico, has 27 antennas of 25 meters diameter each, arranged in patterns up to 22 miles across.

courtesy of Dave Finley and National Radio Astronomy Observatory and Associated Universities, Inc.

If one properly combines the signals from the individual dishes, one can achieve the same resolution as a single enormous telescope with the same diameter as the array. Radio interferometers can achieve the same resolution as optical telescope, as this image shows (green is optical, blue is radio):

Infrared

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, so one must get as high as one can....

The SOFIA telescope has a 2.5-meter mirror

which will eventually ride in a 747 to avoid most of the Earth's atmosphere:

The SOFIA data will be processed here at RIT before being distributed to scientists around the world.

X-ray

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.

One of the biggest X-ray telescopes currently operating is Chandra, which used to be called AXAF. Joel Kastner, professor in the Imaging Sciences Department, worked on Chandra for many years, and is now analyzing some of its data. Here's a schematic of Chandra's mirror system, seen from the side:

So the mirrors are nested cylinders:

The Chandra mirrors were aligned right here in Rochester, by Kodak.
X-rays are produced by very energetic processes, which usually require high temperatures. Compare these two views of the Orion Nebula: first, optical:

And now in X-rays, as seen by Chandra:

Gamma-ray

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.

The Compton Gamma Ray Observatory orbited the Earth from April 5, 1991, until June 4, 2002, detecting high-energy photons with several different instruments.

It detected thousands of gamma ray bursts, but was only able to indicate the location of each one to about 5 degrees.
The Whipple Gamma Ray Observatory has a 10-meter dish made of multiple mirrors to detect Cerenkov light from gamma-rays striking the Earth's atmosphere.

Even though the gamma rays themselves don't make it to the ground, remnants of their violent collisions with air molecules do.
The Milagro Gamma-ray Observatory is a swimming pool lined with phototubes, and covered with an opaque sheet.

It detects the particles (not the photons) created by the collision of a gamma ray with the upper atmosphere.

"Telescopes" for detecting particles, not electromagnetic radiation

Neutrino

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.

AMANDA is a neutrino telescope which uses the Antarctic ice cap as its "primary detector". A very few neutrinos collide with the nucleus of an atom in the ice, producing a brief flash of Cerenkov radiation. In order to detect those brief flashes, scientists have bored holes more than 1 mile deep in the ice and suspended phototubes in the holes.

Here's the record of an actual event (muon, not neutrino) recorded by Amanda (click on the figure to watch a short animation).
The Sudbury Neutrino Observatory is a big plastic ball filled with very pure heavy water, lined with phototubes, placed in an old mine.

Photo courtesy of Ernest Orlando Lawrence Berkeley National Laboratory.
Scientists watch for simultaneous detections of flashes by many tubes all at once. A real neutrino event will appear as a ring on the side of the sphere opposite the place the neutrino entered:

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:

For more information:

Bill Keel has put together a Web site about The Telescope's He's Used (and what he's done with them).
Interested in interferometry? Play with a toy radio interferometer created by Nuria McKay, Derek McKay and Mark Wieringa.
Watch the SOFIA aircraft video to see how the telescope will work in flight.