[Emily]

Emily asked the Naked Scientists:
   
What are the advantages and disadvantages between optical (reflective, refractive and catadioptrics), radio, microwave, infra-red, ultra violet, x-ray and gamma ray telescopes. How do they each work?

What do you think?

[LeeE]

It is not so much that there are advantages or disadvantages to the different types of telescope, rather than they look at different ranges of the electromagnetic spectrum; you can't see optical stuff with a radio telescope and visa versa.

Infra red, visual-optical and UV telescopes are all 'optical' and use either lenses (refractive), mirrors (reflectors) or a combination of both (catadioptic).

Refractor telescopes are long and unwieldy, suffer from chromatic aberration (because different frequencies of light are refracted to differing degrees) and are limited in their maximum aperture, the aperture diameter being a function of the focal length.

Reflector telescopes do not suffer from chromatic aberration, as all frequencies are equally reflected, and reduce the physical length of the telescope by folding the light path.  The major drawback is that the diameter of the telescope must be greater for a given aperture, because part of the collecting area is forfeit for one of the reflecting mirrors.  However, this is greatly offset by the reduction in length over a refractor.  Reflector telescopes usually have their optical surfaces open to the ambient air and are susceptible to airborne dust.

Catadioptic telescopes use the reflector design but augment it with refraction, from a relatively small number of lenses when compared with a true refractor, to further reduce the length of the telescope.  Solid glass catadioptic lenses have been made (with the outer surfaces of a single solid lens being mirrored) which reduce the number of interfaces that the light has to travel through and which are also both very rugged and very short for their focal length - I've seen a solid catadioptic 600mm focal length lens that was only about 100mm long.  Having lenses at each end, catadioptic telescopes are less prone to dust penetration but are more difficult to clean when it does occur (this is another plus point to the solid catadioptic lenses, as there're fewer air cavities where dust can collect - there's usually an air cavity or two in the small final stages but the main and most voluminous stage is solid).

In optical telescopes the lack of length is a plus factor.  The length of mobile reflectors makes them relatively heavier and more difficult to physically transport than a reflector type of similar focal length and aperture, and in large reflector telescopes there are the problems of thermal expansion and bending due to gravity.

I'll let others comment on the other types.

[chris]

Here's an interview with Peter Quinn from Perth, WA, one of the team behind the Square Kilometre Array (SKA), a radio telescope.

[syhprum]

Telescopes for X Rays pose difficulties because materials will only function as reflectors at very low angles of incidence.