Lecture 11: More on Telescopes: X-ray and Radio Telescopes, Telescopes in Space
Because of angular resolution problems at radio wavelengths (recall the equation for resolving power from Lecture 10), radio telescopes are typically very large compared to visible light telescopes.
For example to resolve the star and planet discussed in Lecture 10, a radio telescope operating at a wavelength of 10 centimeters would have to have a diameter of
Clearly a single telescope of this size cannot be built for finite sums of money!
Radio astronomers instead have adopted a strategy of building interferometers where practical size telescopes are built and spaced at distances as large as the diameter of the Earth. The outputs of the telescopes are combined to synthesize the angular resolution of a much larger telescope. The separation of the telescopes is equivalent to the diameter of a single telescope and is frequently referred to as the "baseline". Note that this interferometric style of telescope can vastly improve the angular resolution of a telescope but the light gathering power is just the sum of the areas of the individual telescopes.
The VLA . Very Large Array, in Socorro, New Mexico, is an example of a radio telescope built on interferometric principles.
The VLBA = Very Large Baseline Array couples telescopes spread across nearly the earth's diameter:
So far we have seen that 1) reflecting telescopes are used over a much broader range of wavelengths than refractors, and 2) radio telescopes are reflectors but are generally much larger than visible light telescopes because of the longer wavelength of radio waves.
Underlying much of the discussion of different types of telescopes is the value of observing objects at different wavelengths. Each regime of the electromagnetic spectrum can contribute to understanding a particular object, but we can also categorize various parts of the electromagnetic spectrum as being the optimum range for studying particular phenomena:
|-ray, x-ray||10 keV (=0.12 nm)||T~106 ░ K, accretion disks around black holes|
hot stars, interstellar medium, ground state transitions of hydrogen
most stars, galaxies
cool stars, dust-enshrouded objects, planets
|Radio||21 cm (=2.1x108nm)||cloud of atomic H,
Notice the importance of temperature in the above table -- relation between wavelength of maximum output and temperature is the governing principle!
Telescopes built to operate in the ultraviolet or at infrared wavelengths are usually similar to visible light telescopes in style. However, ultraviolet telescopes must be sent on spacecraft above the Earth's atmosphere. Much of the infrared must be done above the atmosphere also, and even at some wavelengths where the atmosphere is transparent, space is preferred because of being able to cool the telescope.
Infrared and telescope cooling:
The Earth's atmosphere is transparent at 10µm, a wavelength that corresponds to a blackbody with T=290° K which is the typical temperature on the Earth's surface. Cooling to ~30° K will reduce the telescope emission to a level that will permit detection of faint astronomical sources.
X-ray and g -ray telescopes have to be built in very different ways from UV-visible-infrared telescopes because of the ability of these high-energy photons to penetrate matter. X-rays can be focussed by grazing incidence mirrors:
Note that it is difficult to build an x-ray telescope with a lot of light gathering power. The angular resolution of the best x-ray telescopes is ~0.5".
Sampling of Space Telescopes
Compton GRO (GRO = Gamma-Ray Observatory)
Launched in 1991.
ROSAT (Roentgen Satellite)
X-ray satellite launched in 1991.
Chandra = AXAF (Advanced X-ray Astronomy Facility)
Launched July,1999, operates from ~0.1 to 10 keV, great results already.
Hubble Space Telescope (HST)
Ultraviolet, visible, and near-infrared wavelengths
Launched in 1990, repaired in 1993. Should remain operational until ~2010.
U of Az built a Near-Infrared Camera and Multiobject Spectrometer, NICMOS, for HST which operated from March, 1997, to Dec. 1998 when its solid nitrogen coolant ran out. A new refrigerator will be installed on the Feb 28, 2002, Space Shuttle mission which should revive NICMOS.
IRAS (Infrared Astronomical Satellite)
Worked from 12µm to 100µm.
Launched in 1983 and operated for 9 months until its liquid helium coolant ran out.
SIRTF (Space Infrared Telescope Facility)
Will work from 3µm to 160µm with the ability to make true images.
To be launched in Jan 2003.
(U of Az building one of the three instruments)
Detecting Radio Waves
Radio telescopes use detectors that are the same in principle as any radio -- the radio sets up an oscillation in the antenna = radio telescope which is sensed by an electronic circuit tuned to oscillate at the same frequency. Because of this tuning, radio telescopes should be thought of as detecting waves rather than photons. Because of this, radio telescopes also naturally provide information on the phase of the wave making it relatively easy to combine the outputs of several radio telescopes to behave as a single, larger telescope (e.g., as an interferometer). The detectors used at shorter wavelengths lose this phase information making construction of interferometric telescopes much harder.