There appears to be confusion here.
I believe Termal lee is trying to make a telescope with a large diameter objective lens.
Freaky is referring to the FL of the lens and not the diameter. Close up lenses made from ZnSe are usually less than 20mm in diameter and come in a range of FL distances. Common examples are 50mm, 63mm, 75mm, 100mm.
Larger diameter lenses than 20mm cost a lot more as they are not mass produced products like those made in China for CO2 laser work
GaAs lenses are also suitable for thermal camera close up lens s and are reasonably priced when bought for CO2 laser applications. They offer greater hardness, so do not scratch so easily, but are not as good as ZnSe when it comes to transmission.
As many know, GASIR is also available, and though it looks like Germanium it is not. It is an affordable option in cheaper cameras and so is common in sub $10K cameras. Germanium still remains the choice for professional applications but has eye watering costs associated with it.
Fraser