RandyA I've noticed the internal temperature sensor always reads in the 30 to 40 degree C range even when the ambient temperature is 20 degrees and below.
If I am not mistaken, when you do not plug in an external ambient thermistor, the EAF uses the temperature sensor that are built onto the microprocessor chip that is in the EAF.
As such, they are useless for judging temperature that affects focusing. They are pretty much measuring the microprocessor temperature (which also could be elevated when you move the focuser in and out a lot). The external thermistor can be taped to the tube in between the objective and the sensor, or somewhere that most affects the optical length as temperature changes.
With OTAs that can achieve very small spot diagrams (point spread function), even a 1ºC change in tube temperature can affect focus. I would recommend ignoring the sensor that is on the microprocessor.
I just tested one of my 5V EAF indoors at room temperature. Upon booting up the EAF, external probe (taped to a 85mm refractor) read 23.3ºC while the internal sensor read 26.3ºC.
Without exercising the stepper motor, the internal sensor rose to 30.2ºC after about 1 hour. That is obviously not the temperature of the optical tube, and completely masks any ambient temperature variations.
If you continuously exercise the stepper motor, I can imagine the temperature to eventually rise even more, due to the heat that is generated by the stepper motor and transistor drivers.
Chen
Edit: absolute temperature is probably not important. You just need to detect changes in temperature of the tube to determine when there is need to refocus.