Webster IRP 4100 Installation Manual page 28

a If the projected image is wider at the top of the screen, lower
the front of the projector using the height adjustment knob
on the projector.
b If the projected image is wider at the bottom of the screen,
raise the front of the projector slightly.
Refer to the documentation that came with your projector for
specific instructions on how to adjust the projector height.
9 Repeat steps five through eight as many times as may be
necessary to get both sides of the projected image square and
located within 0.2" (5mm) of the edges of the IRP screen at both
the top and the bottom of the image.
10 If necessary, eliminate horizontal keystoning as follows. If not,
go to step twelve.
a Locate the horizontal keystone knob on the projector plate
at the rear of the console. The knob is labeled as shown
below).
b To unlock the plate that supports the projector, rotate the
knob counter-clockwise.
c If the projected image is wider on the left, push the front of
the projector slightly to the left.
d If the projected image is wider on the right, push the front of
the projector plate slightly to the right.
26 INSTALLING THE IRP 4500