I want to share a technique for re-polishing lens surfaces etched by fungus residues. In the best case, fungus can be removed with acetone, but usually fungus permanently etches the glass. If the damage is severe, it may be worth re-polishing the glass using same technique as was used in making the lens originally. That is, if the lens is no longer usable and you are willing to experiment.
The old school technique of polishing optical glass is by molding warm polishing pitch against the lens and using it as polishing pad with compounds like cerium oxide or optical rouge. The method is described in books like "Amateur Telescope Making" and the second "Advanced" edition of this book. Also, "How to Make a Telescope" by Jean Texereau. Old school optics were made by hand and can be made to exceed factory made optics in accuracy. So this is not something I just made up.
To polish old lenses:
Polishing pitch is melted and poured into a circular shape, then allowed to cool until it is no longer tacky. The lens is coated in polishing slurry (to prevent sticking) and pressed/molded against the still-soft pitch. When fully hardened, square channels are cut into the pitch to allow better recirculation of the polishing slurry.
Even at room temperature, the pitch behaves like a very slow flowing liquid, so it gradually conforms to the lens shape precisely. Slurry is applied on the pitch and used to polish and refigure the lens surface if required. Particles of polish become embedded in the pitch and work like micro abrasives, where the scratches produced are much smaller than a wavelength of light.
The polishing technique requires the lens and pitch tool to be rotated periodically in opposite directions to maintain a spherical surface. This will work on most old lenses, since majority of them are spherical. Modified shapes of the pitch pattern and special strokes can be used to refigure the lens into geometries other than a sphere, such as parabolas and hyperbolas.
The only issue I encountered is the hard anti-reflection coating that could not be polished off with cerium oxide initially (too soft), so I first had to use a one-micron alumina to polish through the coating. Alumina is harder than cerium oxide apparently. The lens coating will be stripped off with this process as a side effect. But what good is an anti-reflection coating on a fungus-etched lens that cannot be used any more?
The process requires a lot of patience, especially with the pitch wanting to stick to the glass when soft and chipping off when cold. There are many nuances to the process, such as that placing the lens on top of the tool will polish center of the lens more than edges, and vice versa if flipped. Too much pressure and uneven polishing can refigure the spherical surface into something like a parabola or an ellipse. Polishing too dry and with too much pressure will soften the pitch, etc. etc. So you have to decide if the lens if no longer usable due to damage and should be replaced or repaired with this method. Depending on the degree of fungus damage, type of coating, what the lens is worth, etc.
With all this in mind, I was able to polish out most of the fungus damage on an Apo-Ronar 360mm. Only a few small pits were left that went too deep into the glass, but they were not significant enough to affect image quality. The lens looked essentially like new at the end of the process.
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