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12 changes: 12 additions & 0 deletions back_focal_plane.html
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Expand Up @@ -40,6 +40,18 @@ <h2>Back focal plane</h2>
a focusing laser beam into the rear of the objective will produce an
output of minimal divergence when the focus is some distance past the
flange. Sometimes the only way to find out is to do this test and measure!

A specific method for measuring the position of the back focal plane
follows. Place a tube lens in line with the objective lens and shine
collimated light into the tube lens from where the camera would normally
go. Adjust their relative axial position until the divergence of the
light out of the objective (e.g. spot size on a far surface) is minimized.
The tube lens is now focused at the back focal plane; if you are
telecentrically aligning these lenses you are done. If you need to know
the location of the back focal plane relative to the objective, you can
remove the objective and determine where the beam is focused, for
instance using the visual speckle technique demonstrated at
https://www.youtube.com/watch?v=h5y1h_DWeEM.
</p>

<figure>
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