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
-- 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 test.
-- 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
-- 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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