Complex sentence: The origin of Van Maanen's measurement errors where that Van Maanen used a stereo blink comparator comparing new plates with plates some 10--20 years old, and by blinking between those two plates he could detect small discrepancies on the positions of the objects.
Split into simpler sentences: The origin of van Maanen's measurement errors emerged from his stereo blink comparator, which van Maanaan used to compare new plates with plates some 10--20 years old. By blinking between those two plates he could detect small discrepancies on the positions of the objects.