  MAGIC is a system of two imaging atmospheric Cherenkov telescopes (IACTs)
located on the Canary island of La Palma. Each telescope's imaging camera
consists of 1039 photomultiplier tubes (PMTs). We developed three detector
modules based on silicon photomultipliers (SiPMs) of seven pixels each that are
mechanically and electronically compatible with those used in the MAGIC camera.
These prototype modules are installed next to the PMTs in the imaging camera
and are operated in parallel. To achieve a similar active area per pixel we
used seven to nine SiPMs for producing a composite pixel. The SiPM signals
within one such pixel are actively summed up for retaining the fast signal
pulse shapes. Two different PCB designs are tested for thermal performance. We
present our simulations of Cherenkov and light of the night sky (LoNS)
responses. Based on those we calculate the signal-to-noise ratio (SNR) for this
imaging application. We compare our expectations with the measurements of one
of the SiPM-based detector modules.
