Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: kishjjrjj on 01/09/2008 18:46:31
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kishjjrjj asked the Naked Scientists:
I recently listened to your podcast as you punted down the Cam.
It was quite interesting as usual, however I did have a question
related to your fire... you used a parabolic dish to collect and
direct sunlight and infra-red I believe. Is it possible to focus
ultra violet in a similar manner, and if so what materials are used,?
thanks again, you are quite good.
Jeff Kish, USA
What do you think?
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Yes, and the even neater thing is that a mirror (unlike a lens) has the same focal length for any radiation it reflects.
Mirrors are used in optical instruments like spectrometers, for just this reason, for both UV and infra red (IR) work.
Most metals reflect UV reasonably well (gold and copper are conspicuous exceptions). Aluminium is often used all the way from the UV through the visible and into the IR.
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You can even focus X rays with a metal mirror. You have to use it with the mirror at a near-grazing angle but X ray telescopes actually work. They have good resolution, for their size, as well - because the wavelength is so short.
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At the other end of the scale, satellite dishes work with microwaves, and radio telescopes with even longer wavelengths.
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I gather highly polished aluminium with a thin layer of protective quartz makes a good reflector for UV.
I would think a normal (domestic) mirror with the reflective surface on the back of a piece of glass would be poor
For radio waves it is considered that a parabolic reflector has to be at least 10 wavelengths in diameter otherwise other types of aerial or array will be more effective.
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ah, that is quite interesting. How does the width of the parabolic relate to the wavelength with respect to err reflect-a-bility, and is it a pretty smooth transition i.e. as wavelength changes say, delta, does the width need to change K * delta where K is some constant?
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For radio waves it is considered that a parabolic reflector has to be at least 10 wavelengths in diameter otherwise other types of aerial or array will be more effective.
...which is why radio dishes are huge, as they have to process wavelengths on the scale of a metre. One point which should be noted is that we can only capture optical and long microwave/radio signals from the surface of the earth, as the atmosphere is opaque to all other wavelengths. As you go higher in energy, it becomes steadily more difficult to get good spatial resolution on images because of the difficulty in focusing incident photons.
As Sophie mentioned above, a series of nestled parabolic mirrors are required to focus X-rays and gamma rays as any photon arriving with an angle greater than a couple of degrees will pass through the interatomic spaces in the mirror. Nevertheless, great discoveries are being made all the time with the latest observatories at these wavelengths in orbit, XMM-Newton (the one I work with, hence my enthuasiasm for this subject) and Chandra (also good!)
Richard
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The actual performance of a dish depends on a lot of factors but a good approximation for the beam width of a dish of diameter D, for a wavelength λ is λ/D degrees (both dimensions need to be in the same units of course).
It's a very simple rule of thumb but it's a good start. It shows that a bigger dish or smaller wavelength gives a narrower beam.
The beam width, btw, refers to the angle of the beam between the points where its response is 3dB or half power. Out beyond this, the power gathered drops off fairly sharply.
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Are you sure that's degrees not radians?
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A 3 metre dish with have the same beamwidth (and gain) at 1000 MHz as a 30 cm dish at 10,000 MHz. Scaled by the wavelength. Generally you would consider a 3 metre dish to be borderline whether it should be usec or not instead of an array of yagis.
It is interesting that satellite up-links use large dishes and high power transmitters which are not really really needed if you do the calculations. I am guessing it is prevent some oik from trying to jam the things. Any nation or rich organisation could do it do but they could afford to do it.
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Not all up links use large dishes, there are plenty of mobile systems that use ones of about 1M.
I think the large dishes are a hangover from an earlier age before the satellites had directional receiving antenna
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Obviously mobile links can't use big dishes. Sort of proves the point that big dishes are not really needed. I think you find most Earth stations use large dishes with large transmitter powers (available if not used all the time).