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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Achromatic Telescope
From: George Huxtable
Date: 2003 Oct 7, 01:36 +0100
From: George Huxtable
Date: 2003 Oct 7, 01:36 +0100
Andrew Corl asked- >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). Response from George. I'm presently looking into some of the latitude observations of Lewis ans Clark. I wonder whether Andrew, too, has a special interest in their celestial nav. Early telescopes suffered from chromatic defect. Ordinary glass lenses have a focal length that depends on the colour of the light, so white light, being a mix of all colours from red to blue, can't all be focussed at the same spot. This muddies the sharpness of an image. By using clever combinations of crown glass and flint glass (which both show refraction changing with colour, but to differing extent) it's possible to make composite lenses which balance out this smearing, to a large extent. These are achromatic lenses. All modern telescopes will have achromatic lenses (some more so than others). Perhaps better, if you choose a reflector telescope, the mirrors are by their nature achromatic, and it's just the eyepiece that has an achromtic requirement. My guess is that all such eyepieces are achromatic nowadays. For Andrew's needs (seeing Jupiter's moons), I suggest he will find a reflector to be shorter, lighter, and cheaper that the corresponding refractor. The only real snag I can think of is that with most reflectors you have to look into them in an unnatural direction, whereas usually you look through a reflector toward the general direction of the star. George, ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================
