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Author Topic: Can a parabolic mirror focus UV?  (Read 5003 times)

kishjjrjj

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Can a parabolic mirror focus UV?
« on: 01/09/2008 18:46:31 »
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?


 

Offline Bored chemist

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Can a parabolic mirror focus UV?
« Reply #1 on: 01/09/2008 19:05:46 »
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.
 

lyner

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Can a parabolic mirror focus UV?
« Reply #2 on: 01/09/2008 23:38:55 »
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.
 

Offline Bored chemist

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Can a parabolic mirror focus UV?
« Reply #3 on: 02/09/2008 06:56:51 »
At the other end of the scale, satellite dishes work with microwaves, and radio telescopes with even longer wavelengths.
 

Offline Pumblechook

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Can a parabolic mirror focus UV?
« Reply #4 on: 02/09/2008 12:32:45 »
 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. 
 

Offline kishjeff

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Can a parabolic mirror focus UV?
« Reply #5 on: 03/09/2008 02:07:28 »
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?

 

Offline rich42

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Can a parabolic mirror focus UV?
« Reply #6 on: 05/09/2008 22:26:39 »
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 



 

lyner

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Can a parabolic mirror focus UV?
« Reply #7 on: 05/09/2008 23:27:46 »
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.
 

Offline Bored chemist

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Can a parabolic mirror focus UV?
« Reply #8 on: 06/09/2008 16:44:09 »
Are you sure that's degrees not radians?
 

Offline Pumblechook

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Can a parabolic mirror focus UV?
« Reply #9 on: 09/09/2008 15:58:18 »
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.
 

Online syhprum

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Can a parabolic mirror focus UV?
« Reply #10 on: 09/09/2008 18:28:06 »
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
 

Offline Pumblechook

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Can a parabolic mirror focus UV?
« Reply #11 on: 09/09/2008 19:39:16 »
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).
 

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Can a parabolic mirror focus UV?
« Reply #11 on: 09/09/2008 19:39:16 »

 

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