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Speed of light (In our time)

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Astroman

Astroman

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Triskellion is right. The density of the material changes the speed of light a known amount. It's called the index of refraction. It's used to determine the maximum amount light will bend, or refract, when it passes through a material. So, for a refractor lens, the amount light can be bent, and thus the focal length of the instrument, is determined not so much from the curve on the surfaces as in a reflector, but by the actual composition and density of the glass itself. That's why refractors are so expensive-their focal length is determined by the glass. You can't make an F/15 lens with F/6 glass.

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The Warthog

The Warthog

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Triskellion is right. The density of the material changes the speed of light a known amount. It's called the index of refraction. It's used to determine the maximum amount light will bend, or refract, when it passes through a material. So, for a refractor lens, the amount light can be bent, and thus the focal length of the instrument, is determined not so much from the curve on the surfaces as in a reflector, but by the actual composition and density of the glass itself. That's why refractors are so expensive-their focal length is determined by the glass. You can't make an F/15 lens with F/6 glass.

But...but...but, isn't the f-number determined by the curve polished into the lens, and the amount of CA by the types of glass and how they are combined to get every wavelength coming to the same focal plane??

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Astroman

Astroman

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That's more the case with reflectors, WH. I know, I had the same objections, but a local glass man, who figured 3 mirrors for me, btw, told me different, and lent me a book on glass, er, LENS making, which is a whole nother smoke, compared to mirror making. Some of the figure is determined by the curve of the glass, but not as much as the refractive characteristics of the glass itself. My head hurts.... Besides, I said the maximum amount light can be bent, which determines the minimum focal length. So. I may have the last sentence wrong, but not by much. The glass will allow only a certain amount of correction within a range not as wide as F/9, as I intimated.

Also, (one last edit here), the CA is done by the doublet or triplet-two or three separate lenses, of different curves and different refractive indeces, brings all the colors together.

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Astroman

Astroman

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Everything's fine through a vacuum. It's when the light encounters some material of different density that the trouble starts. Supernovae actually do radiate in all different colors, if you will, everything from microwave energy on up to gamma rays. The spectra we measure are an indication of the energy of the burst itself, and also the distance and energies of the materials intervening. It's just a matter of density. Interstellar dust is barely noticable, so its density is extremely low. Silicon glass is very dense indeed. Then you have quartz glass and even diamond glass or crystal, so it gets very complicated. How you'd make a diamond refractor lens is beyond me, but I suppose it'd be a very short refractor.

Seems to me we've had this discussion before....I'm getting deja-vu....

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