Field Notes — astrocam v3
Peter, on first seeing it: "that is so beautiful."
Peter: "the glass cover for the camera is very thin and flat. probably the cause of the newton's rings."
A camera that knows nothing about the sky can still be told where it is pointing and how sensitive it is, by the stars in its own picture. It works in two steps: first work out which stars these are, then look up how bright they are already known to be. Do both on one frame and the camera has calibrated itself against the sky it was photographing.
Looking at light pollution maps Surbiton is on the fringe of urban/suburban London, quite near to Heathrow Airport, M25 and M4 so a provisional brightness figure of 19 is reasonable. However, we can measure the actual brightness by identifying stars, looking up their measured magnitudes (brightnesses) and using that to calibrate our measurements. We also need to know the background darkness. We found that the brightness was closer to 18 than 19, 18.1 being a reasonable figure.