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Some triplets are air spaced like the Tak TSA models and the AP160. I think some APM lenses were air spaced. Oiled triplets are more common as it gets rid of the need to correct the inside faces of the lenses like the need to be for air spaced. So air spaced would be harder to make.
I suppose there may be some other aspects with air spaced like thermal issues - not sure. AP155 versus AP160 (oiled versus air spaced). There are plenty of top grade AP155 images around. I haven't seen a matching AP160 image but perhaps there are. AP160 is a rare scope.
With the TEC180 its more about the huge fluorite lens. Fluorite is amazing and whilst FPL53 (there is now an FPL55 which is essentially the same colourwise but better for grinding) is much the same the difference is in less light scatter and easier to achieve better colour correction.
The scope is now sold to a US buyer.
Greg.
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