PISCO is the ESO Polarimeter with Instrumental and Sky COmpensation, a two--channel filter polarimeter using photon counting and offered on the ESO/MPI 2.2-m telescope. A description of its design and operation can be found in ESO Operating Manual No. 13 and references therein. Appendix A of that manual also discusses in detail the mathematical principles around which the MIDAS reduction program for PISCO data has been written.