Section 2 - Cooling and its effects

And unfortunately, this is the point when the optically most perfect design starts to contradict the performance of a lens under the stars. The problem is that air is a very bad heat conductor (about 1000x worse than oil), and especially if the air space is wide, the internal member of the triplet lens is separated from the environment thermally. Furhermore practically all fluoro-crown material based triplet APOs have the fluoro-crown material in the center of the lens, and the center thickness of this member is much larger than its thickness at the edge. When we take such a lens out to the cooler environment, the center member can lose only very limited amount of energy through the air spaces, but its edge is in contact with the metal cell of the lens, which cools down quickly. This way, the center of the middle element remains warm for a long time, while the edge of this middle element start to cools down suddenly. The result is undercorrection, and this remains there for a long time, because the center member loses a significant amount of energy at the edge over the entire cooling process.

On the contrary, oil spaced lenses are good heat conductors internally, so their center elements can lose a lot of heat over the front and back surfaces of the lens. So, they cool down much faster, and also, during the cooling process, the internal heat distribution of the lens is much more homogenous compared to an air spaced lens, as most of the heat is lost through the front surface and less at the lens egde. So, these optics can better and quicker adapt to the changing temperature, thus they can deliver high resolution images sooner than their air spaced brothers.

It is also true, that in the case of oil spaced lenses, the designer can not decrease the inherent spherochromatism of the lens (by widening the not existing air spaces :-), so usually an oil spaced lens features less optimal comtuper simulation data, spot diagrams and other fancy looking graphs compared to an air spaced lens. But the difference is little, and much less important than the difference of the performance of well cooled lens compared to a not-so-well-cooled system.

In applications where the most critical high resolution images are expected from a telescope (e.g. planetary observing), we believe oil spaced lenses are clearly the best choice, and the market success and excellent user reviews that similar products receive all the time do prove this theory.

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