Checking Mirror Collimation
The mirror collimation process begins by using an accurate ruler to find the center of the corrector plate
to within 1/32 inch. Try out a black felt tip pen on a windowpane to make sure you can wipe it off glass
with a damp cloth, and then use it to put a little dot in the center of the corrector plate. This is easier to
do if you place a piece of white paper on top of the spider assembly so you can see what you are doing
on the corrector plate.
Once the black dot marks the center of the corrector, lay a 1/2-inch wide paper arrow (basically shaped
like a cutout of a house, like a square with a triangle attached to form the "roof") on the corrector so the
point is right on the black dot.
Shine a light down the tube and you will see an inverted reflection of the
arrow opposite the center of the corrector. If the mirror is collimated, the tip of the reflected arrow will
also line up with the black dot. (LEFT) If the mirror is not collimated, the two arrow tips will never line up
If you look exactly down the optical axis the reflection will disappear, as the film holder magnet will hide
it. You have to look slightly off axis down the tube so the tip of the arrow is just out of eclipse with the
film holder magnet.
Look on one side, then the opposite, then rotate 90 degrees around the tube and
look from both sides again. Keep nudging the paper arrow until it lines up the best from all four viewing
angles. On one side of the tube, it may look slightly displaced in one direction, and then slightly
displaced the other direction from the other side of the tube. This shift is caused by changes in
perspective of the viewing angle. Nudging the paper arrow into adjustment with the tip of your finger
may be too clumsy for the minute steps required. It may be more convenient to move the arrow by
using the point of a needle.
An alternative, but less accurate, method for checking mirror collimation is to stretch two white threads
across the front of the corrector so they make a crosshair pattern centered on the middle of the corrector.
Shine a light down the tube and view the reflections of the threads on the bottom of the corrector. The
reflection and the threads should coincide with the reflection hidden completely behind the threads.
At the same time, as viewed from the center axis of the optical tube, the center of the crosshairs should
be centered on the middle of the film holder.
The crosshair method of collimation testing is not as accurate as the paper arrow trick discussed above,
but may be easier to apply in the field for a quick collimation check while the camera is installed in its
Go to the previous page ---- Removing the Corrector Plate
Go to the next page -------- Adjusting Mirror Collimation
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