1. Radio Telescopes
Infrared and UV Telescopes
X- Ray telescopes
Two Equations
The Rayleigh Criterion for resolving power

2. Radio Waves have a much Longer Wavelength
Than Light.
Therefore for a radio telescope to have the same
Resolving power as an optical telescope, dish
Needs to be very large. (see Rayleigh Criterion,
Last slide)
Resolving power of a single radio telescope is
Worse than the eye therefore many needs to be
combined
An antenna is used as a detector at the focal
Point. There is no equivalent to the eye lens as in
Optical telescopes.
Most are manoeuvrable allowing source to be
Tracked. Telescope moves with the source as Earth
Rotates.

3. IR and UV telescopes both use parabolic mirrors to focus radiation onto a detector as in
Optical reflecting telescopes
The atmosphere of the Earth absorbs most of the wavelengths of light
Therefore Infrared and UV telescopes needs to be launched into the space and set
In orbit.
Light and Radio waves can pass the atmosphere but not infrared and UV
Both types use CCDs or special photographic paper as radiation detectors as in optical
Telescopes.
Longer the wavelength of radiation, less they are affected by imperfections in mirrors.
So IR telescopes don’t need to be perfectly shaped as in optical telescopes.
IR telescopes have a problem in that they produce their own infrared radiation because
Of their temperature. This is overcome by cooling them to very low temperature using
Liquid helium or refrigeration units.
IR and UV telescopes both use parabolic mirrors to
focus radiation onto a detector as in Optical reflecting
telescopes
Both types use CCDs or special photographic paper
as radiation detectors as in optical
Telescopes.
Longer the wavelength of radiation, less they are
affected by imperfections in mirrors.
So IR telescopes don’t need to be perfectly shaped
as in optical telescopes.

4. X-rays reflect if they just graze a mirrors surface. By using a series of nested mirrors, we
Can gradually change the direction just enough to bring them to a focus on a detector.
This type of telescope is called a grazing telescope.
X-rays don’t reflect off surfaces in same way as most other Electromagnetic radiation
Usually x-rays are either absorbed by a material or it passes straight through it
X-rays can be detected using a
modified Geiger counter or a fine wire
mesh.
X-rays don’t reflect off surfaces in
same way as most other
Electromagnetic radiation
Usually x-rays are either
absorbed by a material or it
passes straight through it
X-rays reflect if they just graze a
mirrors surface. By using a series of
nested mirrors, we
Can gradually change the direction just
enough to bring them to a focus on a
detector.
This type of telescope is called a
grazing telescope.

5. In general X-rays have a much smaller collecting power than other types of telescopes
Rayleigh criterion
Minimum angle that can be resolved ≈ Wavelength (λ)
(Ѳ) Diameter of objective lens (D)
A UV telescope has a much better resolving power than a radio telescope for the same
Size of dish.
The Collecting power is Proportional to the area:
Power ∝ Diameter2
For a radio, optical, UV or IR telescope, this is the area of the objective mirror or dish

6. This is an equation to work out
the resolution of a telescope
Two
stars
Telescope 1
The angle, θ, which the telescope can resolve
The angle, θ, which the telescope can resolve
Two
stars
Telescope 2
The equation θ (minimum angle that can be resolved ) = λ (wavelength of radiation)
D (Diameter of aperture )
If two stars are very close together, it can be difficult to see them using a telescope, as they appear
Could appear blurred.
For a telescope, D is the diameter of objective lens or objective mirror. Both λ and D are in metres.
Θ in radians. The smaller the value of θ, the better the resolving power (telescope 2 above has a
Higher resolution than telescope 1, because the angle which telescope 2 can resolve is smaller)!.
Therefore, increasing the value of D, gives you a better resolution, as θ gets smaller.