Introduction

PHD2 is the second generation of Craig Stark's original PHD application.  PHD has become a fixture of the amateur astronomy community with more than a quarter million downloads.  From its inception, it has successfully embraced three seemingly conflicting objectives:

1. For the beginning or casual imager, to deliver ease of use and good guiding performance "out of the box" 
2. For the experienced imager, to deliver sophisticated guiding algorithms, extensive options for tuning, and broad support for imaging equipment 
3. For all users, to consistently exhibit a commercial level of quality while being available free of charge

In order to extend PHD to more platforms and further expand its capabilities, Craig released his program to the open-source community, and PHD2 is the direct result of that generosity.  It has been substantially restructured to make it more extensible and supportable going forward.  Moreover, the initial release of PHD2 already includes a substantial number of new features and refinements while retaining all the core strengths of the original.  Users of the new PHD2 can be confident it will remain committed to the three objectives that made the original application so successful.