A device for in-situ thermal imager calibration having a rotatable lens curtain with an aperture for the lens of a thermal imager to observe a scene. The lens curtain includes a balancing weight and blocking portion having a thermally uniform interior calibration surface that is rotatably disposed such that when the UAS is in level flight, the lens views the scene through the aperture. When the UAS is laterally rotated about its flight path, the lens curtain maintains its absolute position relative to the Earth by virtue of the balance weight and the lens rotates within the lens curtain for viewing a thermally uniform interior surface for calibration.