Minolta 500/8 RF: Cleaning mirror lenses?

[Andreas Thaler]

A Minolta 500/8 RF had already seen clearer times.

The front primary mirror is evenly clouded. I assume that there is fog that should be cleared up.

I suspect that the coating on a mirror can be compared to that of a deflection mirror on an SLR.

The rule here is that cleaning can be done, but only with extreme caution.

Now a mirror lens is an extremely sensitive matter.

Aside from the delicate coating, mirror tension plays a role, the lens needs to be opened, which may already affect the conditions inside, and whether isopropyl alcohol on microfiber is the right cleaning medium - I don't know.

What do the experts think about this?


The current situation in comparison

I have versions II (left) and III of the 500/8 RF:

Version II has even fogging.

Here for comparison - not optimally presented, but should work - Version II:

and version III, where the mirror is perfect:

Even if that isn't a reason to open now, I would like to be able to estimate what to expect if things get worse.

Both copies have not yet been used.

Since I only shoot on film, testing is a bit complicated because there is a lot of preparatory work.

 

[Mark J]

I haven't been able to get a proper cross-section of the lens with rays, however this is something :

Minolta MD 500mm 1:8.0 RF tests and review - Lens QA Works

Minolta MD 500mm 1:8.0 RF (MD III) lens review: full description; sharpness, resolution, aberrations tests; comparisons; technical details; historical notes

lens.ws

I think it's very likely that the primary mirror is a Mangin , ie. the reflection and the reflective coating are on the back of the mirror.
This will probably be vac-coated silver with a protective overcoat of paint.
If this is breaking down them it will need recoating, not cleaning.
However your photo shows a haze that may just be the AR coating on the front glass surface of the primary.

If you are going to open up the lens, then be very careful to note any alignment markings or shims in the front section. The tolerances on centering of the primary and secondary mirrors to each other will be tight, of the order of 25µm .

 

[Andreas Thaler]

The tolerances on centering of the primary and secondary mirrors to each other will be tight, of the order of 25µm .

I have definitely no assignment there

 

[_T_]

I have cleaned first surface mirrors with gentle wiping with disposable wipes designed for optics (pec pads). Fortunately for me the problem was only dust and was easily picked up by the cloth. I would be afraid to try any kind of cleaning solution.

 

[reddesert]

Amateur astronomers have to routinely clean first surface mirrors. They are concerned about doing it right, of course, but they aren't as dear-God-no afraid of it as photographers are for some reason. You can search for how astronomers recommend cleaning mirrors (suggestions include PEC-pads, distilled water, alcohol, very weak dilutions of dish soap). You still have to be careful, of course. Some amount of the care is to avoid leaving streaks or cleaning marks.

Telescope mirrors are usually coated with aluminum, which forms a thin layer of protective aluminum oxide when exposed to air. We often refer to photo mirrors as "silvered," but if they were actually silver-coated, they must have some kind of clear overcoat to prevent tarnish. I think aluminum is likely more common on modern mirrors.

My pet theory about why photographers are afraid to clean mirrors is that it's conflation with the half-"silvered" mirrors found on rangefinder beamsplitters/mirrors and occasionally on part of a reflex mirror. Half-silvered coatings have to be very thin, and I think much easier to damage than a fully reflective first-surface mirror coating.