Archomatic Vs ??? Telescopes?

Question

Just curious to know what does it mean to have achromatic scopes versus perhaps "un-acromatic" or whatever the opposite is, which one is better?

And what does the terminology of being achromatic mean in practicality.

Thanks

Answer 1

Refractors are types of telescopes that use lenses only (as opposed to reflectors, which use a mirror). The problem is, a lens works by "bending" light but it doesn't bend all light the same.

An example of this is a prism. You've seen that prisms create a rainbow by seperating white light into it's individual componants. This happens because when the light hits the prism, red light bends the least and purple the most and the others in their respective order.

Well unfortunatly for astronomers, lenses do the exact same thing only to a lesser degree. Since all the colors of light bend to different degrees when they encounter the lens, even though the lens is directing the light to a particular place, it can't get all the colors, or wavelengths as they really are, to focus to the same exact point. This is called chromatic aberration. This can cause objects to have trange hues and not focus just right.

To fix this, a refractor can be made with special coatings and multiple lenses "glued" together.

An achromatic refractor is one in which this phenomena has been minimized somewhat using the above methods. An apochromatic refractor is a refractor in which this problem has almost been completely corrected for.

You can go to http://hometown.aol.com/siriusbc/telescopes.htm to see an illustration of chromatic aberration.

Answer 2

"Achromatic" means without color and refers to a type of refractor telescope (refractors are the ones with lenses, not mirrors). An achromatic objective lens has at least two different types of glass (flint and crown) to reduce the amount of color error (chromatic aberration) in the image. When achromats were introduced in the mid 1700's they were an important advance in telescope technology, but they are now the standard refractor lens.

However, the color correction of an achromat is less than perfect, and unless the focal ratio is very long, you will see a significant amount of purple haze whenever there is anything bright in the field of view. There is another level of color correction beyond achromatic, called apochromatic. This also means "without color", apparently with the added implication of "and we really mean it this time". The color correction in an apochromat is at least four times better than in an achromat, with other aberrations usually being better controlled as well. The better apochromats will show no false color whatsoever.

There is a stiff cost to be paid for this level of optical excellence, though - you can expect to pay four to ten times as much for an apochromat as for an achromat, with some recently introduced Chinese scopes at the low end of that price range. High-end apochromats also feature high-quality mechanical components.

It should be noted that Newtonian reflectors are inherently free of chromatic aberration, and catadioptrics (Schmidt-Cassegrain and Maksutov-Cassegrain telescopes, e.g.) have only a minuscule amount of color error.

Answer 3

Achromatic means the lenses are constructed to avoid chromatic abberration. A high power lens with chromatic abberation would be a bit of a mess and not worth having.

Answer 4

If your scope is listed as being achromatic, it means that the lens, mirrors, etc will process light of _all_ colors. It makes true observation a lot easier if one does not have to try to compensate for a tint in his optics.