Cleaning refractor lenses

[LewisM]

Picked up an old refractor yesterday, but it has promise. Lenses are great, BUT...

Between the 2 objectives, there is a foggy layer with lint - so someone has cleaned it.

I tried water and a little Sunlight dishsoap,but not much better. Seems like a fine machine oil or something over the lens. Was thinking trying isopropyl alcohol swabs/swipes?

How do they get an immaculate interface between lenses in the factory?

[gregbradley]

Bintel cleaning fluid is the best cleaner I have used. Get some cotton puffs and some white unscented tissues.

Use a hurricane blower to blow off loose grit and lint. Then daub fluid on the lens to get loose grit off. Now wipe with one tissue one 1.4 of the lens and turn over the tissue (never wipe twice) and then use another tissue. Once wiped it may have grit on it and a second wipe could scratch.

Wipe any minor streaks away and blow off any remaining tissue lint.

Greg.

[Omaroo]

What sort of scope is it? If it features oil-spaced elements then removing the oil isn't going to do it much good.

[Merlin66]

If it's a "standard" air-spaced double element achromat, then the lenses should be marked on their edges with alignment marks and also watch for the spacers!!

[gb_astro]

99% isopropyl alcohol from a chemist and
hypoallergenic tissues might be safer than the swabs.

gb.

[MattT]

I use a recipe from The Backyard Astronomers book. 50% isopropyl 50% distilled water and a few drops of dishwashing liquid with flat cotton face wipes, works a treat.
Another refractor?:astron: Got any photos?
Matt

[LewisM]

Matt, there's anything else BUT refractors? ;)

[PlanetMan]

Lewis

If the "old" refractor you mention is the Fluorite lens refractor you were thinking of buying in another thread DO NOT touch it but seek some professional advice.

Some time ago I was thinking of buying a genuine fluorite which had what appeared to be a "dirty" lens. I sought advice from several sources and you have to be very careful. Firstly, Bintel for example, will not clean any fluorite lens due to what they describe as 'risks' with such lens. Instead, they recommended a highly specialised camera repair store in the middle of Sydney (one of the remaining few) which largely only repairs and cleans cameras. Apparently, there is a very particular and apparently expensive fluid which is safe for use on fluorite crystal. I also consulted AstroOptical and they also mentioned that some problems/degrading can arise with fluorite lens which they described to me as a "cloudy" appearance in the glass. I haven't seen what this is supposed to look like for myself but if I can offer you a little advice from my own experience - if your refractor is an original fluorite crystal lens take it to a specialist optical repair store that works with fluorite lens/optics. Fluorite is apparently much better than standard ED glass but also requires a lot more care and is comparatively more perishable.

[Wavytone]

Lewis,

You're probably thinking glass is glass, right, so clean it like you would a window pane. Wrong. Your everyday experience is based on borosilicate crown glass, which is quite tough stuff.

i hate to break the bad news, but here's why you don't mess with fluorite objectives. You can see why Bintel don't want to touch them, either. Be very careful with ED glass too - while it is more robust than fluorite, many of the following comments do also apply to it to some extent. This applies to both the ED refractors as well as ED EYEPIECES such as the Vixen LV, LVW and quite a few from other manufacturers. IN SHORT - resist the temptation to take these apart as you will do more harm than good.



That "foggy layer" is a fluorite lens that is now completely ruined thanks to the well-intentioned but uninformed efforts of a first person, and now yourself.

#1. Fluorite is hydroscopic. In its anhydrous crystalline form, clear. But it will adsorb water if you try to clean it with liquids containing water, and the surface turns cloudy. Basically you can't fix this.

#2. Fluorite has a high thermal coefficient of expansion, and is soft and brittle. If you use "air in a can" or worse, solvents that cool rapidly (acetone comes to mind) and cause sudden cooling you will probably cause two things to happen. Firstly the antireflection layer will lose adhesion as its coefficient of expansion is different, and weeks later it will start to peel off in small flakes.

The second consequence can be the formation of micro-cracks in the fluorite that will eventually propagate and become a visible crack.

#3. Fluorite is soft, very soft. Ordinary cleaning materials you may think are safe on glass will easily abrade it - not polish it - producing the grey cloudy surface you see now. This includes tissues and cloth.

#4. Since it is much softer than an antireflection coating, once a pinhole forms in the coating, the more you "clean" it the worse it gets as you are abrading the fluorite underneath into a pit that expands.

#5. In refractor objectives fluorite was often the second element on the assumption that the front element would be the only one exposed to the outside night air, and dew; the inner element would be protected safely inside the tube and lens cell. Sometimes the second element had 1 or more UNCOATED surface, so washing this is basically a catastrophe. Similarly with ED eyepieces, the ED elements are inside, not the eyelens or field lens. They aren't intended to be dismantled and washed.

This is also one of the reasons why ES eyepieces are nitrogen-purged and sealed - they are waterproof.

Basically you now have a paperweight.

[bojan]

How about washing a hint of fungus in my "new" Canon 400mm F2.8L lens?
I think I killed it by exposing the glass to UV light (from tubes normally used for photoresist PCB exposure) and couple of hours of sunshine..

The first two optical elements are in fact plan-parallel protection plates, and I am tempted to remove them, to reduce internal reflections.

[Slawomir]

What about this one?: http://www.photoniccleaning.com

And a short video: https://m.youtube.com/watch?v=v_meK7cfs6U

[bojan]

According to this website (http://www.mir.com.my/rb/photography/companies/canon/fdresources/fdlenses/400mm.htm), Canon 400mm F2.8L has two UD glass elements.. so they are not fluorite.
So.. I suppose it is OK to dismantle the front group and wash the fungus with windex.. and not destroy the elements in the process..
Can anyone confirm this?

[Shiraz]

maybe you could optically test the flat elements to find their refractive index? - I think that fluorite has a different refractive index to most glasses, so it should be possible to determine what your elements are made of with reasonable confidence.

[gregbradley]

If you shine a green laser through the lens it won't show the beam in a fluorite lens but it will with any other glass including FPL53. When Canon etc say UD I doubt that is FPL53 but probably some lesser glass that is still high quality like FPL51 or less. Undiluted Windex sounds a bit harsh, it probably should be watered down.

Greg.

[bojan]

Good idea, guys.. I will do it tonight.

[bojan]

I did experiment with green laser..
According to Greg, it must be UD glass then.
On the attached image it is visible that beam is passing through 6 glass elements.
So, washing with windex (diluted..) is safe :)

BTW.. does anyone know how to dismantle the built-in hood? It is badly damaged and I want to replace it with light plastic tube.
It seems I need to unscrew the cell with UD elements, to remove the hood, but I am not 100% sure this is the case.

[Shiraz]

now the question is, why have they put two protective plates up the front? - maybe you have a delicate element in the triplet that is either physically soft or possibly susceptible to moisture - or maybe even slightly toxic in some way (eg lanthanum glass can be slightly radioactive)? Perhaps when you disassemble, try to keep the room air as dry as possible using an aircon in cool mode and don't handle the elements without disposable gloves?

[bojan]

Hmmm..

According to this doc (http://www.micro-tools.com/pdf/Canon/EF%20400%202.8%20L%20USM.PDF.. it is not the same lens, but the opical design is the same/similar), both protective plates are the same part number.

It could be just an additional insurance - in case one surface was damaged, it could be removed with no cost involved.. or am I here assuming too much :question:

There must be a better reason for that.. but unfortunately, not much about this lens is available on web.
As for potential radioactivity, this could be checked easily with Geiger.. will do asap.