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Dear Andrew,
the functionment of a lens is based on the physical principle of refraction, i.e. the deviation a ray of lay shows whan it passes between two different mediums such as air and glass. Refraction is a function of wavelenght and this is the reason why a single lens is unable to focus the entire range of visible wavelenghts in a single point (this is the so called "chromatic aberration"). As a result, the image is surrounded by colored halos. The simplest way to try to cure the problem is to couple two lenses (to form a so called "doublet") made of different glass qualities (typically "crown" and "flint" glass) that should correct each other's chromatic aberration. This is what is referrred to as typical achromatic telescope. In reality, an achromatic just bring the two ends of visible spectrum (red and violet) into the same focus, while for the other wavelenghts the problem still exists. Many other, more sophisticated designs were developed to solve the problems, and now we have the so called "apochromatic" telescopes, which use many lens made of exotic glasses and are really able to practically null the chromatic aberration.
Hope this information will prove useful. If you have any doubt, ask me.
Bests, Federico
----- Original Message -----
From: Andrew Corl
To:
Sent: Monday, October 06, 2003 11:46 PM
Subject: Achromatic Telescope
I was recently reading the journals of Lewis and Clark. They mention taking several telescopes with them on their voyage. Further research told me that one of the telescopes was an "Achromatic Telescope." Could anyone tell me (or refer me to a website) what this type of telescope is. If there are plans available for this I would appreciate that link as well. I am mainly interested in being able to see the moons of Jupiter (gives you an idea of the magnification I am looking for).
Andrew
