Extending mainpage example

The 3 was an arbitrary choice. I run it with the median bkg first, which left me with a few undetected sources. In this image the fluctuations are not big, but large scale variations can be very common. If part the image has lower background, and all the faint sources get undetected with a high threshold. Maybe a better parametrization is to have a very low threshold but higher S/N.
Estimating the background can be quite tricky, here is the approach I'm most familiar with as a user: http://casu.ast.cam.ac.uk/surveys-projects/software-release/imcore