Astrophysics 04 - Non-Optical Telescopes

How are radio telescopes similar to optical telescopes?

They use a parabolic objective mirror.

How do radio telescopes differ from optical telescopes?

They use a wire mesh for the mirror, rather than an optical mirror.

Waves are directly focused onto an antenna at the focal point.

How is a signal processed in a radio telescope?

Signals are detected by the antenna, then passed through a pre-amplifier (amplifies weak signals without adding too much noise) then through a high-gain amplifier.

The signal is then passed through a tuner to filter unwanted wavelengths, then digitised into a false-colour image.

How does the resolving power of radio telescopes differ to optical telescopes?

Radio wavelengths are ~ one million times longer than visible light, so the resolution is much lower.

How can the issue of radio telescopes’ resolving power be corrected?

Multiple smaller telescopes can be combined to produce a single image, with resolutions thousands of times better than optical telescopes.

Describe two benefits of radio telescopes.

Being able to make telescopes out of fine wire mesh makes their construction cheaper and easier than optical telescopes.

The longer the wavelength, the less it is impacted by imperfections in collecting dish/mirror. Spherical aberration is much less of an issue.

How are IR and UV telescopes similar to optical telescopes?

They use very similar constructions, with similar parabolic mirrors and CCDs or photographic paper.

How are the mirrors different in IR and UV telescopes versus optical telescopes?

As spherical aberration is affected by the wavelength of light, IR (with a longer wavelength) is less affected by this aberration, so mirrrors can be less perfect.

UV, in contrast, has to be more accurate due to its shorter wavelength.

How are IR telescopes affected differently to other telescopes?

All objects, including the telescope itself, emit IR radiation, so to stop the telescopes from detecting its own IR radiation, they must be cooled to very low temperatures using liquid helium or refrigeration units.

How does the penetrative ability of X-rays make constructing telescopes difficult, and how do we get around this?

X-rays only reflect at grazing angles.

We use a series of grazing mirrors to redirect the incident X-rays into a focal point.

How do we detect X-rays in X-ray telescopes?

They can be detected using a modified GM tube, or with a fine wire mesh.

Modern telescopes use highly sensitive X-ray CCDs.

How is the resolving power of a telescope limited by the Rayleigh Criterion?

Resolving pwer depends on wavelength of incident radiation and dish / aperture diameter. For the same size of dish, a UV telescope has a higher resolving power than a radio telescope.

How is the resolving power of a telescope limited by the quality of detector?

The closeness of detector units affects resolution. This could be the dimensions of CCD pixels, or by how fine a wire mesh is.

Define collecting power.

The amount of radiation a telescope can recieve; proportional to area. A telescope with a larger dish collects more energy, so images are more intense, and fainter objects can be observed.

Give the equation for collecting power.

Collecting power ∝ (dish/aperture diameter)²