This is the radiator from the Narrow Angle Camera (NAC) from the Lunar Reconnaissance Orbiter (LRO). It helps to prevent the camera from overheating during operation in space.
The NAC is a CCD camera attached to a powerful telescope with a 700mm focal length. It can take black and white images of the Moon with a resolution of less than a meter. It arrived at the Moon onboard the LRO spacecraft in 2009.
This flight spare instrument is identical to the flown instrument built by Malin Space Science Systems. NASA transferred the camera to the Museum in 2016.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.