When I was in the US Army, I spent four months at Fort Belvoir, VA training
to become a topographic instrument repairman. In other words, I was
trained, at much expense to US taxpayers, to repair surveying instruments
which included everything from simple levels to very sophisticated
theodolites. These instruments demanded perfectly clean internal and
external (coated) lens and mirrors. To clean these parts when they were
really dirty, we used lens tissue and denatured alcohol. After the lens
tissue and alcohol, we used a soft lens brush and a handheld air bellows
(like you can purchase in any camera store) to remove dust and lent.
Denatured alcohol is cheap and is available at any hardware or paint store.
Be certain to buy denature and not rubbing alcohol as rubbing alcohol
contains lanolin.
BTW, we were also taught that the easiest way to see dirt on internal and
external lens surfaces was to look through the instrument backwards. Same
is true for camera lens. This was the procedure used by instructors when
grading our work. Sometimes looking through an instrument backwards will
allow you to see imperfections in the glass if any exist.
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