Service Note #7   Focus Stiffeners

Author:  Tom Droege            
Date:    Jan 11, 2003

This service note provides instructions for installing the focus stiffener 
bars.

Materials:

2 ea.   Short Stiffener Bar
2 ea.   Long Stiffener Bar
12 ea.  set screws
6 ea.   #10 32 cap head screws.

Viewing a camera from the side, remover the collars from the ends of the 
focus rods.  Undo the set screw that holds the focus screw at the camera 
support bar.  The whole camera system can not slide back without 
limit.  You may have to remove the connectors to get enough movement.

Slide the camera back until the bars come out of the most forward 
bearings.  You should now be able to slip a stiffener bar on each camera 
support bar.  Install one of each.  Note the collar set screws may have 
marred the focus rods.  The stiffener bore hole is intended to be a snug 
fit.  You may have to take emery paper and smooth off the collar mar spot 
to get the stiffener bars on the focus rods.  Now move the camera forward 
so that the bars return to their front bearings.  You can now replace the 
collars.  Position the camera at about the correct focus, and tighten down 
the set screw on the focus screw.

You should now be able to slide the stiffener bars back and forth on the 
focus rods.  Position them about the center of the declination axis.  Here 
is an opportunity to use the stiffener bars to make a fine adjustment on 
the declination balance.   You can now screw the two stiffener bars 
together using 3 ea. #10-32 cap screws.  The original took kit contains the 
allen wrench that you need.  Now install 3 set screws to clamp each end of 
the stiffener bar to the focus rods.

When you tighten the set screws, make sure the camera support bars are 
approximately square to the lens axis.  If you have trouble reaching the 
set screws, you may have to un-mount the electronics boards.  Then the set 
screws can be reached easily.

Now get a veneer caliper or some similar instrument.  Measure the distance 
from the camera support bar to the bar which supports the rear bearing for 
the focus rods.  The distance should be equal for the two sides of the 
camera support bars next to the focus rods.  You can loosen the set screw 
holding the focus rod in the camera support bar to make an 
adjustment.  Since 0.010" is roughly a mr (milliradian) for this 
configuration, we have an f/4 system, and there are 2000 pixels, one mr is 
1/4 pixel growth in the spot size if we look at the center compared to the 
edges.  You can make your own choice, but I see little reason to get the 
alignment closer than a mr.

Note that if my knowledge of Statics is correct (it has been a long time 
since I considered the problems of building bridges) the camera support 
bar, the bar closest to the camera with the bearings in it, and the focus 
rods form a parallelogram.  If we consider that the parts are pinned 
together (they are not, but are to a first approximation of how things 
distort in a structure) then the camera will stay perpendicular to the lens 
axis as it droops with different declination positions.  Since the movement 
is small, the small angle approximation means that the focus will not move 
much with camera droop.  So it should both stay in focus and stay 
perpendicular to the lens axis  with gravity loads.