PHD2 is the second version of the very popular guiding application Push Here Dummy. The name was I think intended to give the idea that there was no real need to spend lots of time setting up this application or configuring lots of settings to get good results. Default settings for most options are a great starting point for people making their first attempts at using a guide camera for improving their astrophotography.

The practice of using a guide camera is meant to reduce the effect of imperfections in the mechanical performance of telescope mounts, as well as compensate for image drift due to imperfect polar alignment. During the long exposures normally used for deep sky image capture it is possible for the mount to move slightly off target due to either of these issues. To address this, a second camera attached either to a separate guide scope or to the main scope through an Off Axis Guider takes short exposures (usually of around 1 - 3 seconds) and the guide software issues adjustment commands to the mount to keep a selected star anchored to the same pixels on the guide camera sensor. When this is working well it results in much sharper images with no sign of trailling or blurring of the object being photographed.

More information about PHD2 and its capabilities can be found on its website.