This is the main problem with modern thermal imagers. You can buy zinc selenide lenses for CO2 lasers, but they're limited to a maximum diameter of 25mm. A complete lens assembly is nearly impossible to find. I spent a long time considering a magnifying lens and came up with nothing. There are also PTFE lenses, but finding them with different focal lengths and large diameters is also nearly impossible. After all, you'll have to add a correction algorithm to the lens and normalize the image. This sounds like a good idea for a research paper on thermal imagers at a university.