To get away from prismatic aberration, which would happen out at the edges with just 1 lens. All fine lenses are multiple lenses, no matter what the application - telescopes, microscopes, binoculars... The best ones are internally coated to suppress internal reflections, but we have no such luck on our medium priced sets.
There are typically 28 optical surfaces in a CRT RPTV - one mirror, 3 lens barrels with 4 lenses each inside, plus the CRT coolant covers. To keep your set looking brand new at all times, all of them need to stay crystal clear, which is impossible without external intervention when HV is involved.
10 of these surfaces typically get dirty over the years due to the 30KV inherent in CRT use, causing all airborne particulates, no matter how small, to be statically attracted to those optical surfaces. They are literally sucked out of the air and onto those surfaces, every minute the set is powered up.
What usually needs to be cleaned on Hitachis is the mirror, the tops and bottoms of all 3 lens barrels, and the 3 coolant covers of the guns themselves. The internal lens surfaces of the barrels will have individual particulates on them, but what needs to be remedied is the blanketing of the surface, not individual particles. Everything in there is out of focus at that part of the light path, so individual particulates don't impede the performance of the lenses.
To see which surfaces are really dirty, shine a powerful flashlight onto the surface in question from a very steep angle, in the dark.
If you have strong high light content video on and you elect to draw a happy face in those lens tops to really get it, be sure and lick your finger first! Those are plastic surfaces and being highly vulnerable and easily scratched by dust (permanent damage), they need to be handled very carefully.
The bottoms of the lenses, which face the coolant covers, often don't need it, tho they should be thoroughly inspected anyway with that flashlight. Probably won't find dust, but only smoke, on those, but that smoke can be pretty thick, and definitely an impediment to crystal clear viewing. When they do need it and there is a lens striping bar in there, there's often a black mark in the position of that bar, where the smoke could not get to the bottom of the lens to adhere (but adhered to the stripe instead). Once the lens has been removed, it's easy to see that black exception mark in the smokiness covering the bottom lens of the barrel, if the lens is very smoky.
The deeper optics can be tested by removing the lens - usually the green is the most telling, tho they are all 3 identically dirty - and lick your thumb and put a fresh wet mark in the top 1/2" area of the cover farthest away from you, then play really bright video material, so it will be backlit. If that thumbprint is black while the rest of the cover is gray, you have your answer. If there's really no difference, you can probably leave the coolant covers alone.
Edited by Mr Bob - 7/21/13 at 11:49am