Pryor is good everywhere but in the outer parts where our CCD star counts are clearly above those of [@King68]. This difference is probably due to our better estimate of the background star contamination. In the plot we have shown also the raw star counts (crosses) prior to the background star subtraction: it can be clearly seen that our star counts go well beyond the tidal radius, $r_t=977''$, published by [@Trager95]. This allow us to estimate in a better way than in the past the stellar background contribution. The background star counts show a small radial gradient: we will discuss this point in greater detail in the next Section. Here, the minimum value has been taken as an estimate of the background level.

We point out that the differences present in the central zones of the cluster could be in part due to some residual incompleteness of our star counts, to the absence in the starcounts of the brightest saturated stars, and to the difficulties in finding the center of the cluster. We searched for the center using a variant of the mirror autocorrelation technique developed by [@DJ88]. In the case of M55 we encountered some problems due to a 