I have seen refractors advertised as achromatics with a doublet lens and Apos advertised with a doublet (such as my TV102). So what in the lens design makes (or doesn't make) a doublet an APO? (Or am I just confused about the term - I'm assuming it means two lenses, right?)
What Makes a Doublet an APO?
Started by davidcolton, 02/14/2006 07:38PM
Posted 02/14/2006 07:38PM
| Edited 02/14/2006 07:39PM
Opening Post
Posted 02/14/2006 08:13PM
#2
There are two threads going on right now that are somewhat related. This thread, and the one about ED and Apos are confusing construction with results.
Achromatic and Apochromatic are measures of how much of a light comes to a focus at one point. With a normal lens different light comes to focus at different places. With Achromatic, two major colors focus at the same point, and with an apo, all three major colors focus together.
I have made an apochromatic telescope. Newts are generally speaking, apochromatic. They use neither ED nor Flourite, nor are doublet (meaning two lenses) or triplet (meaning three). However, by the nature of their optics, all colors come to the same point of focus--which makes them apochromatic.
To make a lens system apochromatic is more difficult. The refraction of one lens is generally corrected by the refraction of a different lens. And there is often a third lens that corrects the results of the first two. The net result is a cancellation of errors--and all three colors come to the same focus point.
The lensmaker can use shapes and materials to change refraction of the light. When the lensmaker uses glass with Flourite or other special materials, the lensmaker will emphasize that because some view that as a plus.......
In short--don't confuse the material (flourite, or Extra-low Dispersion (ED)) with the result (where the light focuses.
Alex
Achromatic and Apochromatic are measures of how much of a light comes to a focus at one point. With a normal lens different light comes to focus at different places. With Achromatic, two major colors focus at the same point, and with an apo, all three major colors focus together.
I have made an apochromatic telescope. Newts are generally speaking, apochromatic. They use neither ED nor Flourite, nor are doublet (meaning two lenses) or triplet (meaning three). However, by the nature of their optics, all colors come to the same point of focus--which makes them apochromatic.
To make a lens system apochromatic is more difficult. The refraction of one lens is generally corrected by the refraction of a different lens. And there is often a third lens that corrects the results of the first two. The net result is a cancellation of errors--and all three colors come to the same focus point.
The lensmaker can use shapes and materials to change refraction of the light. When the lensmaker uses glass with Flourite or other special materials, the lensmaker will emphasize that because some view that as a plus.......
In short--don't confuse the material (flourite, or Extra-low Dispersion (ED)) with the result (where the light focuses.
Alex
Funding Member
