Section 2 - Under the Stars! Part 2The main thermal problem with air spaced lenses are the air spaces themselves. It is also a problem, that most air spaced triplet lenses have rather wide air spaces, but to understand the reason behind this, we have to take a look at some features of the air spaced doublet and triplet lenses.Today, in the case of modern optical materials used in APO lenses, the partial dispersions of the members are very nearly matching each other, so the main source of false color is not the longitudinal chromatic aberration anymore (as it was in the case of achromats and first generation ED APOs). Now the source of false color is mostly the sphero-chromatism, i.e. the variance of spherical correction by wavelength. Today’s lenses have nearly the same focal length at every wavelength, so there is a negligible amount of false color as a result of different focal lengths at different colors. But the spherical correction of the lens still varies with wavelength, i.e. the lens is seriously under-corrected at one end of the spectrum (usually in red), and it is still over-corrected at the opposite end (in blue). As spherical correction causes part of the energy to leave the Airy disk and "move" into the diffraction rings, the diffraction rings are brigher in violet and in deep red, and the observer detects this as false color. It is true, that false color caused by sphero-chromatism has similar visual appearance (but in the case of an APO, the amount of false color is much less, if detectable at all). It is known among optical designers, that by increasing the width of the air space, we can somehow reduce the spherochromatism of an APO lens, which actually results in a decreased amount of false color at the eyepiece. In the case of doublet lenses, designing the air space to be wider usually results in less than optimal correction for coma, so the designer has to make a choice here: either to sacrifice part of the coma correction and make the lens more color free on the optical axis by applying a wider air space, or making the lens practically perfectly coma corrected but with a little more false color. Some companies chose to keep the lens perfectly coma corrected (most famous brands) and some chose to widen the air spaces (some producers at the Far East) in order to make the lens more color free. But in the case of air spaced triplets, the lens has a much more symmetrical internal structure with more free parameters and it is possible to design perfectly coma corrected lenses even with wider air spaces. As in this case the improved color correction is "free of charge" (i.e. the designer does not have to sacrifice any other optical correction to improve sphero-chromatism), most air spaced triplet lenses feature rather wide (0.8-3mm) air spaces. | |
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