  Microwave Kinetic Inductance Detectors (MKIDs) are superconducting detectors
capable of counting single photons and measuring their energy in the UV,
optical, and near-IR. MKIDs feature intrinsic frequency domain multiplexing
(FDM) at microwave frequencies, allowing the construction and readout of large
arrays. Due to the microwave FDM, MKIDs do not require the complex cryogenic
multiplexing electronics used for similar detectors, such as Transition Edge
Sensors (TESs), but instead transfer this complexity to room temperature
electronics where they present a formidable signal processing challenge. In
this paper we describe the first successful effort to build a readout for a
photon counting optical/near-IR astronomical instrument, the ARray Camera for
Optical to Near-infrared Spectrophotometry (ARCONS). This readout is based on
open source hardware developed by the Collaboration for Astronomy Signal
Processing and Electronics Research (CASPER). Designed principally for radio
telescope backends, it is flexible enough to be used for a variety of signal
processing applications.
