fixtures included the whole bearing assembly weighs 67n to the astronomer an important performance parameter is the stability of the bearing in order to determine this a mirror was mounted on mount 122 and directed a laser beam to be reflected from the mirror onto an x y position sensor the position of the beam on the x y position sensor was monitored for continuous time periods of different lengths from 8 ms to 32 s traces of the laser spot on the sensor are reproduced in fig1 a 14 e for the various time periods of 8 ms 64 ms 0 5 s 8 s and 32 s the accumulated spot size defined by the standard deviation of the angular position from the average over the entire observation period is plotted as a function of the observation period in fig1 the spot size increases from about 0 97 u03bcrad of arc at 8 ms of time to around 19 4 u03bcrad of arc after 32 s of time for observations from the moon an accuracy of pointing to 0 48 u03bcrad of arc is desirable the angular measures were converted from the position on the x y position sensor by dividing over the distance 3 m from the mirror to the x y position sensor it is to be.