something I thought about tonight. Why no refractor objectives made from lanthanum glass? How would such an objective compare abberation wise with flourite crystal, ED glass, and traditional crown/flint designs? How would such an objective compare price wise?
refractor design 'simple?' question...
Started by inbred, 11/08/2002 03:08AM
Posted 11/08/2002 03:08AM
Opening Post
Posted 11/08/2002 05:33AM
#1
Hi Andrew,
(Warning, hazy recollections of optics theory ahead, read at your own risk)
Although my copy of "Telescope Optics" is out of reach at the moment, from what I remember, one needs two glasses with quite different dispersion, or the ability of glass to act like a prism and spread white light into different colors. The key item was that if you had a low dispersion glass such as "ED" or fluorite, you could use almost any other common glass and get good color correction by adjusting the curves of each element. By carefully selecting the second glass, one could get really good color correction. If I recall, Lanthanum had some special properties (high refractive index?), but low dispersion was not one of them. So it may be useful to compensate for eyepiece aberations, but I don't think it would get you very far as a main element of the objective as compared to much cheaper glasses.
David Simons
(flame suit on)
(Warning, hazy recollections of optics theory ahead, read at your own risk)
Although my copy of "Telescope Optics" is out of reach at the moment, from what I remember, one needs two glasses with quite different dispersion, or the ability of glass to act like a prism and spread white light into different colors. The key item was that if you had a low dispersion glass such as "ED" or fluorite, you could use almost any other common glass and get good color correction by adjusting the curves of each element. By carefully selecting the second glass, one could get really good color correction. If I recall, Lanthanum had some special properties (high refractive index?), but low dispersion was not one of them. So it may be useful to compensate for eyepiece aberations, but I don't think it would get you very far as a main element of the objective as compared to much cheaper glasses.
David Simons
(flame suit on)
Funding Member
