HP 5501A Operating And Service Manual page 90

14. The laser beam now exits the interferometer and is reflected by the mirror back upon
itself into the interferometer. Angularly adjust the plane mirror interferometer until
the beam reflected from the mirror returns through the plane mirror interferometer
and back to the small aperture of the laser head. Slight, lateral adjustments of the plane
mirror interferometer may be required to ensure that the reference beam is still
centered on the receiver alignment target. If the distance between the mirror and the
laser head is at least 0.5 metres (20 inches) then the formula given in the section on
Visual Alignment will determine the cosine error based on the offset of the return
beam at the laser.
NOTE
For high accuracy alignment and installation where there i s
less than 0.5 metres (20 inches) between the laser and mirror,
perform steps 15 through 17.
15. Remove the receiver alignment target and plane mirror interferometer alignment
target and rotate the turret of the laser head to select the large aperture. Do not re-
move the plane mirror interferometer alignment aid on the output side of the plane
mirror interferometer.
16. With a fast responding voltmeter (preferably a meter type) attached to the receiver
test point, fine adjust the plane mirror interferometer angularly until a signal is
received on the receiver. (The voltmeter will suddenly jump to some value greater than
0.25 volts.) This is a critical adjustment and may initially require great care to achieve
the desired result.
'
Peak the voltmeter reading (which will be fluctuating) by fine adjusting the plane
mirror interferometer in both angular axes. This aligns the laser beam to within +I5
arcseconds to the mirror. If the mirror has been aligned to the direction of travel within
+15 arcseconds, the laser beam will be aligned to the direction of travel within k30
arcseconds, or approximately 0.04 parts per million. That is, 0.04 micrometre of travel
(0.04 microinches per inch) of cosine error.
18.
Lock down the plane mirror interferometer securely. Make sure the alignment is not
disturbed.
19. Remove the plane mirror interferometer alignment target and alignment aid. The
reference beam and the measurement beam must be centered on the receiver align-
ment target.
20. Remove the receiver alignment aids and rotate the turret on the laser head to the
large aperture. Verify the LED indicator on the receiver is illuminated and the voltage
at the receiver test point is between 0.6 and 1.5 Vdc.