Is there a way to prevent chromatic aberration without using an achromatic lens in refracting telescopes or in any other instruments...?
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$\begingroup$ I suppose it would be mean to suggest an all-reflective design -- or installing a narrowband filter. $\endgroup$– Carl WitthoftOct 21, 2019 at 15:36
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3 Answers
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It's probably impossible to prevent chromatic aberration, since there is no such thing as a perfectly achromatic lens. but you can certainly minimize it.
As you have mentioned, for a given spectral range, the best way is to choose a lens that is said to be "achromatic" even though the correction is never perfect.
But if you want to use a lens that is not achromatic, then you can try the following to reduce the chromatic aberration:
- Use a lens that has a very long focal length, for more on that see How does making a refracting telescope very long reduce the chromatic aberration of an uncorrected lens? I figured out the answer there and will try to post an answer soon!
- Minimize your spectral range with a filter, or at least cut out the short wavelengths (e.g. blue). The dispersion of glass is stronger at the shorter wavelengths.
- If you are doing color astrophotography with a color sensor, then there may be a way to shoot the three color images separately, bringing each color into focus separately. One problem is that for color sensors used to reproduce human color perception, the red channel often has a blue bandpass to replicate the way color receptors in our eye work. See answers to Filter for RGB separation and its effect on the image and the image below. So if you are trying this, then it might be helpful to add a blue filter, at least to cut below say 425 nm.
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$\begingroup$ There's a recent research paper which discusses a very oddly-surfaced singlet which has no chromatic, but that's not likely to be of general use. $\endgroup$ Oct 21, 2019 at 15:36
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1$\begingroup$ @CarlWitthoftm, do you happen to have a link to said paper? Searching the web for "very oddly-surfaced singlet which has no chromatic" is not getting me anywhere ;-) $\endgroup$– GhanimaOct 21, 2019 at 18:29
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$\begingroup$ @mskfisher ok, so a meta material not a "traditional" lens with some sort of oddly-shaped surface. Thanks. $\endgroup$– GhanimaOct 21, 2019 at 20:04
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Canon camera has another method to reduce or cancel chromatic abberation they call Diffractive Optics. The idea is to use a lens with a diffraction pattern in it that causes the chromatic aberration to bend the opposite way from normal lenses. So if blue is bending the most through the normal lens, then it will bend the least when going through the diffractive lens element. If you place these two lenses together, a diffractive and normal lens, you can cancel out the chromatic aberration.
They have a description of the idea along with diagrams here: Multi-layer Diffractive Optical Element and photos of the lenses on page 2
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+1I've heard about using diffractive optics to reduce chromatic aberration; because diffraction is so strongly dispersive they can be very weak and still do the job. But I didn't know there was a commercial product using this technology. Great answer! $\endgroup$– uhohOct 22, 2019 at 3:07
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A fouth approach to add to @Uhoh's 3 others, is to make the lens of extra low dispersion glass. Dispersion is a measure of how refractive index varies with colour. However such glass is relatively difficult to make, in the quality required for lenses, and so rather expensive.
Further information about dispersion can be found on this wikipedia page https://en.wikipedia.org/wiki/Abbe_number
Here's a plot of Abbe numbers and indices of refraction for various glasses. The ultra-dense flint glass types have the lowest Abbe number. Source
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1$\begingroup$ I totally forgot about this, but it's really important; excellent answer! $\endgroup$– uhohOct 22, 2019 at 3:27