Reducing available images of NGC 2770 I went into some troubles (data seems not homogeneous on multiple night scale). There is probably some systematic shift in the data. Need to explore it more. But combining Stephan's quintet 68 minute exposures I got very nice image. It seems that limiting magnitude is 20.5-21.0 mag (pm 0.5) which is very good in Brno and consistent with previous experiments. I'm using usno stars to get an estimate of CCD image limit.
68 minutes, R band. Faintest stars are 20-21 mag faint ;)