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Cleaning optics in telescopes


Ajohn

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I recently bought a used Meade 8" LX90. I  tried it terrestrially at silly magnifications and wasn't that impressed. Good but hoped for better then I looked at the glass with  a torch and thought oh dear. After cleaning using window cleaner for volume and then what Meade recommend which is basically the same thing I tried it again and found I could now get a good clean focus with a 3mm eyepiece. The corrector and mirror were coated in something almost greasy leaving me wondering if a new fashion had started - using anti fogging agents on telescopes. Not sure how old the scope is. Could be US, China or Mexico made. I think they did make early ones on the goto alt ax fork mount in the US.

Feeling Xmassy I noticed a used 6" 1/10 wave Orion Newtonian for sale so bought that and probably paid too much for it as the finder was removed. 2 years old and following a check the mirror was much the same as the SCT so cleaned this too. No test certificate so I wanted to get the mirror out anyway to check the engravings. That focuses cleanly with a 2x barlow and the 3mm eyepiece, little iffy but the collimation is out - just set with a laser gizmo. An eyepiece less optics showed how much it was out easily. I have a collimating eyepiece on order as mine went years ago.

Neither scope showed spotting from dew so I wonder where the muck comes from and thought the subject might be of general interest.

Bought because of interest in using cameras rather than aperture for a better view and ease of moving about. A lot to sort out.  Sort of thing that should be started at the start of summer really.

John

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I'm not sure what you mean by window cleaner but I don't like the sound of it!  Distilled water with isopropyl alcohol and a tiny trace of detergent is what is usually recommended. However, coatings on glass are very hard. Be aware, though, that coatings on mirrors are very soft and easily scratched. Perhaps you were, in reality, very careful but the process as you describe it comes across as a bit too gung-ho for comfort. If I can avoid actually touching a mirror when cleaning it then that's what I do.

Olly

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Yes I suppose it does sound gung ho. Window cleaner in spray bottles is the mix you mention probably with slightly more detergent. I used clear. Some are blue which might mean they use meths rather than ipa as that type of alcohol has to be dyed. Pure ethanol might be better but can't easily be obtained in the uk

I carefully tested the coatings on the mirror as I have come across uncoated ones. In some cases the reflective coating on these is actually silver which has to be over coated again so that it doesn't tarnish. So called enhanced high reflectance mirrors. I am not sure what Orion used so was even more careful. I have also cleaned a Meade diagonal in a similar but some what drier way without problem.

What must be avoided is rubbing dust around on either as that may well scratch. It's also a good idea to clean very wet and lightly to be sure when that is possible - it probably isn't on an APO or refractor etc as it may get into the cell or even between the elements. Very wet is why I used window cleaner first. I have used it many times on microscope optics and know of one famous pro that never uses anything else for those. I do have a small bottle of specialised cleaner that should be used on these - triple distilled water and a wetting agent and nothing else.

Meade suggest making your own and using biodegradable dish washer liquid as a wetting agent. I couldn't find any so have recently bought some photographic wetting agent. Very very little of either is needed. Maybe a few DROPS per ltr.

However having said all of this it wouldn't be a good idea to clean optics often. Only if it is really needed. I am at a loss in that respect and have no idea how they accumulated so much muck. None of my own scopes ever have. Perhaps it's from where they were stored - regularly dewing up internally is the only thing I can think of.

Just in case some one tried this and thinks de ionised water - don't. It's strange stuff and doesn't like being that pure. Distilled is more stable.

John

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I can't comment on the state of the Newtonian mirror other than it might have been kept in a smokers environment. The corrector plate on elderly SCT's often displays a foggy deposit of condensed material from the egassing of the solvent used in the internal anti-reflection paint.  :smiley:

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Many window-cleaning mixes, like Windex® in the USA, contain ammonia of various concentrations. Heaven only knows what this does to optical coatings used on mirrors and corrector-plates. At least you skipped over using the hydrofluoric acid. That would turn your expensive glass into a cloud of silicon hexafluoride gas - which you could merrily chase as it went wafting down the street......

If using water, make sure the final rinse is with distilled water. You don't want the residual minerals from your city's plumbing drying on your lenses or mirror either.

Dave

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Its odd but ammonia based solutions where often recomended for military coated optics, the one downside of alcohol is that although it disolves some contaniments it tends to evaporate and leaves smears that require a lot of rubbing to remove.

I tend to stick to non chemical methods like a lens pen for lenses or water and the mild green stuff for mirrors with a final rinse in distilled water.

Alan

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That is a big catch with alcohol solvents and those in general - they evaporate and leave some of what was removed behind. In fact I get the impression that IPA may sometimes have something a bit oily in it.

Hydrofluoric acid - well I can read what is written on the side of bottles and have cleaned lots and lots of optics of varying sorts but not reflecting telescopes so I just used my loaf - an expression we sometimes use over here. Actually I don't think ammonia in it would cause me much concern but if any one does anything like this there is obviously a need to try a small area. I once tried to clean a mirror in  a variable magnifying head for a microscope and found it was uncoated - impossible job. Some people on a forum who lived in the USA laughed when I moaned about the lack of coating and pointed out that the part had been made there and not in Japan as the rest of the scope was. They over coat all mirrors so that they can be cleaned.

The best way I have heard of cleaning really dirty mirror say caked in dust is to put it under the shower set at a tepid temperature. I sold one once like that very cheaply and the buyer took great glee in telling me how he cleaned it. I though he would have to have it recoated. Extreme but worth trying if needed. I believe most if not all telescope mirrors are over coated with something - silicone dioxide at least if I remember correctly. Very high end might even use quartz. I have a flat mirror coated with that out of an industrial shadow graph. Even sulphuric wont shift it - I wanted to use it as a mirror blank

I have wondered about Alan's soap and water but am always concerned about the additives in soaps  :grin: but a mirror needs to be shiny. Pure soft soap seems to be thin on the ground.

Smoking - no.  Afraid I do that and it has never had that effect but I don't leave scopes just lying around. All in boxes or cases as are any bits and pieces. The mirror was literally a bit greasy anyway. It could be easily smeared with a finger. That's why I wondered about anti fogging compounds, Maybe it was steam etc from cooking but there is a cap for each end.

The LX90 had been stored in a steel cabinet in a garage. Way more of what ever it was than I would expect from paint drying but an interesting idea.  :smiley: Anyway it's good that they can be cleaned but the torch test shows that they don't do an incredible job of polishing the corrector - Texereau would have a fit but given the errors the ripple could introduce it doesn't really matter anyway. It's just noticeable due to the coating and the torch being held at various angles. Meade do warn about using this test so don't confuse this with grease etc.

John

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Hi all i have always used Reverse Osmosis water from the tropical fish dept in my local garden center,very reasonably priced. This is used as the second and final rinsing agent. luke warm shower water used first of all to soak and soften, Mirror is placed on a towel in bath and flooded,after a good shower off ,fill mirror concave with r/o water and one drop of pure soap,using natural cotton wool ,soaked in same solution swirl the cotton wool with circular motions from, center to the edge, new bit of cotton and repeat,rinse with a copious amount of r/o and the one final rinse with r/o ,stand the mirror on its edge to drain off excess transfer  without touching edges and stand on a clean towel and blow off any remaining droplets with a hairdryer on cold with a nozzle to direct flow.. This is how Obsession user s are advised to clean their mirrors and then only when necessary, i have to date had no cause for worry. Just my take on a subjective process.

Regards

Mike

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Some people advise using a drop or two of washing up liquid. It is a very bad idea, as many makes contain salt. Salt in any concentration will partially corrode the silvering (aluminium) coating. It may take a long time but it still happens. Pure isopropyl alcohol or methanol has no additives. Getting pure methanol is expensive. There is one supplier in the USA that I know of and you have to buy about $400 worth to get it. It is something like 99.99% pure. Over here the best you can get seems to be about 99.5%. The chemists supply is about 75% the rest being water, possibly deionised and distilled, but not guaranteed.

Derek

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Hi,

Just for info, AR grade methanol from http://webscientific.co.uk/methanol-analytical-grade-99-pure-1l.html.

£16 per litre.

And my favourite bugbear - there is no practical difference between deionised water and distilled water. Deionised water can contain traces of non-ionised solutes - organic compounds usually - but at a level that isn't significant for mirror  / lens cleaning. The purest water used in industry is the water that is turned to steam in a 'once through' super-critical boiler. This is always purified by deionising using mixed-bed deionisers.

Regards, Hugh

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I'd avoid de ionised water but it probably depends on the degree. I used to work in a place with lots of industrial chemists around and also a large laboratory with an analytic chemist in it, Every time I went in there the distiller was running producing double distilled water. They also had a real de ioniser so I asked why they didn't just use that so they showed me. He ran some off and stuck some litmus paper in it. It went rather acidic very quickly. CO2 being taken out of the air. He then poured some distilled water out and tested that and it was more or less neutral. Basically he said that de ionised water was too reactive with all sorts of things to use for his tests. He even included taking mercury out of peoples fillings if they were stupid enough to make their coffee with it - pass but apparently some had done that and suffered as a result.

Talking to another chemist later he explained in a different way. Water is rather corrosive in and odd sort of way and will dissolve small quantities of all sorts of things and just doesn't like being fully de ionised so is even more "corrosive" and there is no saying what it might take  up.  He also pointed out that this characteristic - the ability to dissolve tiny amounts of all sorts of things is what basically makes life possible. It also capable of dissolving small quantities of things that may do us harm - lead water pipes for instance.

This was with a resin deioniser though. The membrane types may not be as bad. depending on what is actually in it. Osmosis is sometimes used for large scale. I did keep discus some years ago and used a small none resin deioniser to fill the tanks. It took ages. The water generally went acidic but more neutral after things were put in to harden it a little. Some breeders will admit that the water they keep the fish in doesn't do them much good.

:grin: Me well I will just buy distilled. It isn't that expensive.

Labs use a detergent to clean glass ware and wouldn't be very happy if it left traces behind but they do rinse with distilled usually before use if critical. Tepol is popular. They seem to have branched out into several areas so it looks like Tepol L is the correct one. It can be bought mail order. I used to be able to get small quantities for free but it's all gone now. It can also be used as a wetting agent.

John

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Hydrofluoric Acid :eek: :eek: :eek: AVOID at all costs. It dissolves silicon dioxide ( aka Quartz in it's lump form ), the common protective coat on mirrors. It will also cause severe burns if it it contacts skin, burns that go right down to the bone.  It cannot be kept in glass bottles. Even it's ammonium salt, ammonium difluoride, can be used to make those etched glass panels for windows and screens..

Also to be avoided is the cotton wool that contains artificial fibres as these can scratch mirrors. So be VERY gentle if using cotton wool as the use of artificial fibres is not stated on packaging.

De-ionised water should not contain any ionic contaminants but will contain any non-ionic contaminants ( including any oily stuff ) present in the water prior to de-ionisation. It will also depend on how thoroughly the process has been done, Plenty of fresh de-ionising resin, or old, tired resin at the end of it's life?

Distilled water will be somewhat purer than De-ionised as all the ionic contaminants and some of the non-ionic contaminants will be removed.

Reverse Osmosis water should, in theory, be the best provided that it is done with an undamaged membrane.

Personally, I use distilled water, even that from dehumidifiers should be satisfactory. If necessary add some detergent when plain water proves to be inadequate. If drops of water remain on the surface when the mirror/lens is on edge then just touching the drops with the corner of a clean soft tissue will remove them. Failing that then Iso-Propyl alcohol can be tried. IPA will be the first choice for removing the oily film on the eye lens of eyepieces.

Nigel

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All I know about de-ionised and distilled water was gleaned from working at Newcastle University during my PhD in atomic research. All the water we used was distilled and deionised, the water was treated on campus in our research labs but not entirely sure how, (by which method). We could not allow and contaminants into the vacuum chambers. All of our aluminium,  stainless steel and glass components including fused silica lenses, were washed with it after fist  being cleaned in acetone then research grade methanol.

Degassing afterwards could take up to 10 days to get to the required vacuum pressure in the chamber (10-6  to 10-7 torr). Contaminants caused havoc with results to the point we had to start all over again if the pressure could not be accomplished. Even then I could tell by the results if all was well.

Hydrofluoric Acid, was a scary substance. Astrobits (Nigel)  is quite correct, and in as far as I know it can cause bone cancer unless an antidote is administered very quickly by injection. (that is only hearsay).

Basically I have stuck by tried and tested methods ever since.

Derek

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