Removing fungus from a primary mirror.

[sjastro]

How does one remove fungus from a mirror without damaging it?
It is impossible to remove by simple wiping.

Regards

Steven

[bojan]

Try mild detergent (dishwashing liquid).
If it doesn't work, I think vinegar should do the trick - it doesn't react with Al coating (IMO - please check elsewhere - I know it's OK on lenses, but haven't tried on Al coatings).

[Star Catcher (Ted Dobosz)]

Hi Steven

I removed fungus from an old orange C8 primary mirror once. The trick is to get to the spores, especially if the fungus/mildew has slightly etched the glass surface.

I covered the mirror with those strong absorbent paper towels, then poured isopropyl over the towels to saturation point. Then made sure to keep the towels moist and in contact with the mirror surface for about 30 minutes. Then removed and gave mirror a wash and dry. It came up well, with only very slight etching marks which has no effect on the image. I also gave the secondary mirror and corrector a quick wipe down with isopropyl, although they looked fine.

I then made up a mild bleach solution and wiped down as best as I could the inside OTA walls and secondary mirror holder for stray spores.

Others might have better or less radical ideas. My fungus/mildew however has not come back and it is 15 years since it was done.

Ted

[sjastro]

Thanks Bojan and Ted.

I'll look at the options next time I disassemble my BRC-250.

Regards

Steven

[OzEclipse (Joe Cali)]

Originally Posted by bojan

Try mild detergent (dishwashing liquid).
If it doesn't work, I think vinegar should do the trick - it doesn't react with Al coating (IMO - please check elsewhere - I know it's OK on lenses, but haven't tried on Al coatings).

Bojan's advice is usually spot on but this time he's wrongly extrapolated reactivity of unreactive lens coatings to very reactive mirror coatings.

The anti reflective coatings on lenses are Magnesium fluoride or similar compounds which probably will not react with the weak acetic acid in vinegar because the Magnesium-Fluorine bonds are quite strong. This should not be used as an indication that vinegar won't react with Aluminum which is a pure metal with available bonds and hence much more reactive.

Acetic acid is transported in bulk in aluminium tanks but that relies upon the acid forming an acid resistant layer of Aluminium Oxide which protects the rest of the tank from corrosion. An alumium coating on a mirror is only one atom thick and that mono-atomic coating will convert to aluminium oxide. A silicon monoxide over-coated mirror may provide some protection but overall I'd recommend against trying it.

Star catchers method sounds safer to me. The isopropyl alcohol will sterilize the spores, the wash will remove most of the residual material. You could also use metho. Level the mirror and just pool it over the mirror and keep topping it up or submerge the mirror in metho in a dish a bit bigger than the mirror and gently waft a big wad of cotton wool over the surface to dislodge the spores. Don't press and rub. Just gently drag the cotton wool over the surface letting it float in the metho. Wash in mild diute detergent and water and then rinse with demineralized water.

Joe