When you work in the thermal IR you spend a lot of time staring at images of pupils trying to make sure you've baffled out stray infrared light from everything warm. These two images show the pupil at 88 degrees elevation and pointing straight up. To our surprise, at zenith, the roof crane appears to intrude into our pupil. Fortunately, we are rarely pointed straight up.
Can you find the crane intruding into the pupil?
Pupil at 88 degrees elevation Pupil at Zenith
The primary and secondary appear to contribute 11% emissivity to the background. Looking at this pupil image, there is quite a bit that we can block out to reduce this value. The crane added an additional 1.3% emissivity at zenith.