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 together. (RIGHT)

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 mount.

Go to the previous page ---- Removing the Corrector Plate
Go to the next page -------- Adjusting Mirror Collimation
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