The Focal Plane Assembly

Here's a fun one. The focal plane assembly has a filter mask in front of it with 4 different coloured windows.

These windows are separated by black masks which are designed to prevent cross-talk - that is light bouncing around from one field to another. In this way we keep the light in each filter bandpass as pure as possible. But of course we have to test it.
A laser was fired at a filter window which should be opaque to that laser wavelength. And indeed it is. The semi-circles you can in the image here come from the diffraction pattern of the laser. The main beam of the laser is almost completely masked out (if you look really closely you just see it - the filter is not perfect although pretty close). You can also see the black mask. There is almost NO cross-talk. The scheme works. NB. The broad band of light in the middle of the frame comes from the light in the lab.