HP 5501A Operating And Service Manual page 80

The cosine error is purely a function of the angle 8 and the displacement
cu
of the return mea-
surement beam is a function of the angle 8 and the distance traveled. I n addition, a cube-
corner retroreflector such a s the 10703A or 10704A has the property of doubling the displace-
ment of the laser beam (i.e.,
if
the incoming laser beam to the retroreflector displaces 1 mm,
the total separation between the incoming and outgoing beam is increased by 2 mm). Over
short travel it is possible to maintain measurement signal even though there is considerable
cosine error.
NOTE
The presence of measurement signal through the total length of
travel does not guarantee that the measurement axis is aligned for
minimum cosine error. Also, any angular misalignment of the laser
beam to the direction of travel causes a decrease in the measure-
ment signal.
2.41
ALIGNMENT TECHNIQUES
There are two basic alignment techniques used with the Laser Transducer System: Visual
alignment, which
is
a very satisfactory method in applications involving relatively long
travel; and autoreflection, which is used for short travel applications and measurements
where cosine error must be reduced to the absolute minimum possible. I n general, regardless
of the techniques used, alignments are performed with
all
optical components in place, and
since the alignment procedures require adjustment of the optical components, provisions
must be made for this when the components are installed on the machine.
2.42
Alignment Principles
Prior to beginning any alignment procedure, a basic understanding of what you are trying to
accomplish will make the procedure easier to perform. The following information i s intended
as a concise summary of the various factors that affect the optical alignment of the Laser
Transducer System. As you are performing the alignment procedure keep the following
points in mind:
a. The laser beam is the measurement standard. In order to achieve maximum accuracy,
the beam must remain parallel to the path of travel.
b. The angular direction of the beam can be changed by moving the laser head; using a
beam splitter (reflected beam only); using a beam bender; or, using the interfero-
meter to bend the beam.
c. The angular direction of the beam is not changed when using
a
retroreflector or for
the transmitted beam going through a beam splitter or interferometer.
There is a 20-arcminute displacement of the beam when pass-
ing through the interferometer (see Figure 2-77).
d. To rough align the optics, you should start by aligning the laser head and the first
moveable optical component. After this alignment is done move out one component
at a time until the last component in that leg is aligned and the laser beam impinges
on the receiver aperture.
e. The cube corners do not change the angular direction of the beam. However, they do
displace the beam and reverse the direction. The laser beam remains parallel to its
original path. I n the case of the 10705A Single Beam l nterferometer reference cube-
corner and the 10704A Retroreflector the displacement is zero (i.e., the beam is re-
flected back on it's original path).