Lessons Learned While Adjusting Camera Focus
Now that both Schmidt cameras have been reassembled and adjusted, there are
a number of "lessons learned" that I need to add to the previous pages where
repair techniques were discussed. However, I do not have time to sort
through everyting right now, so I will simply list them on this last page.
-- To adjust the collimation of the mirror while performing the "paper
arrow on the corrector plate trick", add shims (a spot cut from typing
paper) between the mirror and the cast metal pad in the mirror cell under
the Invar rod. To adjust the arrow toward the rod, add the shim under the
pad for that rod. The thickness of a piece of typing paper will move arrow
about 1/32-inch on the corrector plate.
-- After gluing the mirror in place, leave back off camera for several days
while the RTV is curing.
-- Be careful to keep the telescope used for bench focusing at a stable
temperature to avoid a focus shift from thermal expansion.
-- Use fresh a piece of film in the film holder for each bench focus test to
avoid the film buckling from heat and humidity.
-- Use a remote spotlight, such as a bright Maglite or Streamlight
flashlight, to illuminate the film holder during bench focusing. Placing a
100-watt light in the camera door causes heat problems that can affect the
film in the holder.
-- Use the coarsest-grained film possible as a target when bench focusing.
You can actually look through the upper layer of the film and confuse the
focus. Lightly scuff the film emulsion surface with very fine grit polishing
paper so the surface layer is visible through the focusing telescope.
-- Use a paper scale taped to the eyepiece barrel and graduated in
one-millimeter increments to gauge the focus shift between infinity and the
focus of the camera. This will allow easier calculation of how far the nuts
on the Invar rods need to be moved.
-- Recheck the infinity focus on the focusing telescope after each bench
test. If it is not the same, refocus the telescope on infinity and redo the
-- Bench testing will get the focus to within several thousandths of an
inch, but likely will not "nail" the focus. The final focus adjustments must
be done photographically. 30-second exposures on unhypered Technical Pan
will show stars beyond the 10th magnitude without bloating the star images
to where they confuse the focus.
-- Devise a way to mark the focus test film so its orientation in the camera
is known in order to note the needed corrections.
-- And lastly, I can't say enough about being careful to move the nuts on
the Invar rods in tiny increments. If any of the nuts suddenly break free
while loosening them, you have automatically overshot the focus point. The
nuts are adjusted with a small 3/8-inch open-end wrench and it is not easy
twisting your wrist through the door to access the forward nuts. Don't be
surprised that after carefully centering the spider assembly that moving the
Invar rod nuts will also move the spider ring. On my cameras, the six points
on the nuts rubbed on the inner wall of the tube as they were rotated during
focus adjustments and I had to tweak the centering of the spider assembly.
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