Kepler Mission – First Light update for 280309 – With Jon Jenkins
The stray light inside the telescope was sunlight scattered through baffles at two gaps around the dust cover where the pins holding the dust cover to the sunshade are located. The stray light illuminated an “arc” around the edge of the dust cover opposite the entry points. (Interestingly enough, there are some dust particles on the field flattening lenses that functioned as inverse pin hole cameras, so we actually saw reverse images of the illumination pattern inside the sunshade.) The images we first obtained were very dark at the top of the focal plane array, and became gradually lighter towards the bottom of the focal plane. We aren’t talking about much light: at most, about 12,000 photoelectrons per read were falling on the brightest edge of the focal plane. When we tilted the photometer away from the sun, we almost completely eliminated the stray sun light getting through the simple baffles in these gaps (more complex baffles could prevent the dust cover from releasing cleanly). The maximum amount of light we measured on these final images was less than about 10 photoelectrons per readout, so the light was reduced by over a factor of 1000.
Keep in mind that these dark or bias frames were pictures only a parent could love: we were very happy to see the same intricate patterns in the dark frames matched those that we saw in preflight characterization tests. It was just like seeing the wizened face of a dear old friend or relative and noting that every wrinkle and freckle was in place and that no additional ones had appeared after launch.
We’re having a hard time waiting for the dust cover to be released so that we can finally see what the stars look like through Kepler’s eyes. I’m confident that we won’t have to wait too much longer.