Here are some answers to questions re PRINTRBOT.
What is the OPTO for?
Any mechanical device with a motor of any kind controlled by a computer of any fashion needs to have a reference to where the axis starts. This reference is the OPTO. When you start a printer in this case the computer will run all motors twards the 0-point. The 0-point is where X, Y and Z detectors tell the computer that this is as far as we can go on the axis this way. The rest of the dimensional awareness is basically setup in your slicer program. That is why you need to have correct data for the software. The 3D printer in it self does not nessecery need to know how long an axis is, only where it begins.
Where is the PTFE tubing?
The printerbot design does not use a PTFE tube at all! The heat barrier implemented is the PEEK.
Why such small NTC or thermistor ?
The reason is simple! A small body reacts immediatley to changes in the enviroment where as a bigger body or sensor is much slower. So accuracy is the same as small in this case.