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An airspaced doublet has two more degrees of freedom to play with, and in principal the designer could achieve a better design (correction) and/or a slightly wider corrected field of view. The variables are an extra radius - since the two inner surfaces no longer have to mate - and the airspace between them.
The one risk with cemented doublets is that over a long time the cement can fail, producing a spidery pattern between the lenses. At first it seems insignificant but it propagates and eventually the lenses will separate which may mean the end of the doublet if it can't be repaired.
Cemented doublets are typically used in binoculars, finderscope objectives and eyepieces, where this isn't a big issue (easily replaced) - but if this happens in a valuable premium refractor objective... it can spell the end of the scope.
The other approach sometimes used is oil-spaced doublets, where an oil with a refractive index between that of the two glass types is used. This can be excellent, but its messy and requires periodic maintenance.
Last edited by Wavytone; 07-06-2019 at 01:30 PM.
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