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Model
SS28A
Appendix
A:
Principles
of Operation
A-a
Straightness
Measurements
In the following di scussion
,
it
will
help to
visualize the Straightness
Reflector
as
a
straightedge
supported
at one
end
,
and
the
Straightness
Interferometer
as an indicator
that runs
along
this
straightedge.
Initia lly
,
the two path
s
from the Interferometer
to
the
Reflecto
r
(fl
and
f2)
have
so
me length
relative to
each other.
If
either
the
Straightness
Interferometer
or
the
Straightness
Reflector
is
moved
side-to-side (across
the reference
Sraightedge ),
the
relative
length
s o
f th
e
two
beams
will
change.
The
change in
accumu lated fringe counts will
be
-
20
sin
(0/2),
where
-
D
=
the
distance
of
the
offset, and
-
0
""
the angle
between the two
beams
leaving the
Interferometer.
o
is
about
1.5
deg
rees
for
the
Short-Range Interferometer, and about 0.15 degree
for the long
Range
Interferometer.
To provide the
correct
readout
of
the
offset distance, the E lectronic
hardware
automatically
mu
ltiplies
the
fringe
count by
the reciprocal
of 2 sin
(012).
Small
pitch,
yaw, or
roll motions of the Interferometer do
not
create
a path
differen
ce
and
therefore do not
affect
the measurement
accuracy.
The two
return
beams from
the
Straightness Reflector
combine
in the
prism,
at the
same
point
where
Ihe beam from the laser Head was
sp
lil. The
combined
beam is returned
10
the laser Head
along the
same
path
as
the laser
Head's
eKit beam.
A
beam bender
al
the Laser Head
's
Exit port
,
switched
into pOSition
by setting
the laser Head's turret ring to "STRA IGHT",
deflects
the
incoming
beam
from Straightness
Interferometer
down to
the photodetector behind the
laser
Head
's
normal Return port
.
(Thi
s
offset
in the return path
is
very
mu ch like thai in
a
perisco
pe
or a
pair
of
prism binocu
lars.
)
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