[The_bluester (Paul)]

If you have the parameters set too tight it can lose the guide star while it is still there plainly visible. In the brain, try turning off star mass detection and see if that helps. If it does you can probably turn it back on but increase the percentage bit by bit until it reliably keeps the star.

I am not sure how it is calculated but it relates to the average brightness of the detected star over the pixels it is detected on, so seeing variations will change the star mass value frame by frame, if the tolerance is too tight it will loose the star. As far as I know it is intended to keep PHD from jumping to the wrong star if there is a bit of movement (Gust of wind etc) and a different star might creep into the search area.