We present a deep multi-colour CCD mosaic of the Coma cluster ( Abell 1656 ) , covering 5.2 deg ^ { 2 } in the B and r bands , and 1.3 deg ^ { 2 } in the U band .
This large , homogeneous data set provides a valuable low redshift comparison sample for studies of galaxies in distant clusters .
In this paper we present our survey , and study the dependence of the galaxy luminosity function ( LF ) on passband and radial distance from the cluster centre .
The U , B and r band LFs of the complete sample can not be represented by single Schechter functions .
For the central area , r < 245 ~ { } h _ { 100 } ^ { -1 } kpc , we find best-fitting Schechter parameters of M ^ { * } _ { U } = -18.60 ^ { +0.13 } _ { -0.18 } and \alpha _ { U } = -1.32 ^ { +0.018 } _ { -0.028 } , M ^ { * } _ { B } = -19.79 ^ { +0.18 } _ { -0.17 } and \alpha _ { B } = -1.37 ^ { +0.024 } _ { -0.016 } and M ^ { * } _ { r } = -20.87 ^ { +0.12 } _ { -0.17 } and \alpha _ { r } = -1.16 ^ { +0.012 } _ { -0.019 } .
The LF becomes steeper at larger radial distance from the cluster centre .
The effect is most pronounced in the U band .
This result is consistent with the presence of a star forming dwarf population at large distance from the cluster centre , which may be in the process of being accreted by the cluster .
The shapes of the LFs of the NGC 4839 group support a scenario in which the group has already passed through the centre .