For those of you who wonder about the design of professional supplementary lenses that cost many thousands of Dollars new..... I have a couple of supplemental lenses from FLIR That represent the top end of such accessories for thermal imaging. I also obtained the technical drawings for these lenses so you can see the type of lens elements used in them
I attach some pictures of the two lenses and the technical drawings.
The PM series cameras for which these supplemental lenses were designed has a Fixed 24 Degree HFOV Lens.
The X2 magnification Telephoto supplemental lens provides a 12 Degree HFOV with minimal transmission losses. Just look at the size of that Germanium objective ! The size of the Germanium lens elements is a part of the reason for these lenses high cost. It is due to the large objective lens used on the FLIR PM series industrial cameras. This lens contains a positive meniscus lens element followed by a negative meniscus lens.
The x0.53 magnification wide angle supplemental lens provides a 45 Degree HFOV. This lens contains a negative meniscus lens element followed by a positive meniscus lens.
These two lenses also work perfectly well when placed in front of other thermal imaging cameras.
Fraser