Rock Salt & Refractor Lens

[Profiler]

I was asked about an old trick today for the care of refractor optics that instead of various desiccant satchels a small thin sealed cloth bag filled with rock salt crystals would do the same job.

Initially I thought this was a fine idea but then I wondered whether the salt might potentially also have some sort of corroding effect on the metal components of the telescope.

Consequently, while the salt might absorb ambient moisture which is good for the lens might the salt also potentially have some detrimental effect upon the metal components?

Any thoughts on this old idea would be happily received.

[multiweb]

Rice is the best.

[Profiler]

Yes - come to think about it I do recall hearing of rice as well.

However, is there any potential harm with using salt?

[anj026]

Don't know about the rock salt, I would be concerned without some further confirmation.

I have read that in order to reduce the risk of fungus causing damage to a lens surface it pays to expose the glass to sunlight every now and then. Of course you do not look through the telescope at the sun without proper filtration. The UV rays will kill any fungus spores that can lead to surface etching especially if the scope is stored long term and there is any moisture present.

I read it in this book which I found in the local library;

http://www.amazon.com/Care-Astronomical-Telescopes-Accessories-Telescope/dp/185233715X/ref=sr_1_1?s=books&ie=UTF8&qid=1367046966&sr=1-1

[DavidU]

NaCl is very corrosive to metals, the lens is usually coated with anti reflective metal coatings. I personally wound not put salt any where near a scope.
http://photo.net/leica-rangefinders-forum/00ENSM

[Profiler]

Yes - that was my main concern as well given the corrosive properties of salt (aka NaCl)

Okay - following the rice theme - any particular type or whatever is in the pantry will do the trick:rofl:

[lazjen]

I'd say any plain rice will probably be ok - long grain white, for example. I'd avoid the flavoured/scented ones though.

[astroboy]

Yes and uncooked is even better .

[OzEclipse]

Sodium Chloride does not exist as a vapour at room temperatures therefore if the salt is physically constrained within a fabric bag, it can't corrode nearby metal.

Lenses are usually coated with metal halides eg MgF2 which will not react with or corrode in the presence of salt. The reference quoted refers to salt spray on sailboats and near the ocean and then cleaning off the abrasive dried residue - a completely different transport and damage mechanism.

Desiccants
While I doubt that salt will cause corrosion, I don't think it is a particularly effective desiccant. If you must use a desiccant, why not just use silica gel? The reason it is used so widely is because it is a very efficient desiccant. You can buy it sealed in sachets, or just keep it when it's supplied with all sorts of electronic equipment and other products. Some types are able to be regenerated in an oven. Silica Gel itsel clear bead form is not toxic. It is chemically identical to beach sand but structurally very different which gives it its hygroscopic properties. Note: Sand won't work!

If you want to regenerate it in a food oven - regular or microwave, be careful to use the clear beads and not to use the indicating types. The type that is blue and turns pink when saturated is Cobalt(II) Chloride which is both toxic and carcinogenic and has been banned in many countries. The type that changes from Orange to Violet is Methyl Violet which is also toxic and carcinogenic albeit less than Cobalt(II) Chloride.

A cup sized volume of silica gel in my labs in Canberra needs to be regenerated about once a month when desiccating a container similar in volume to a 4" refractor that get opened once a day. My labs have a humidity that varies from 50%-80%.

Desiccant free method
The easiest and simplest solution for refractors or Catadioptric systems is probably to forget silica gel and leave the eyepiece holder open so dry warm daytime air can circulate.

This may not work in the far north where humidity is high for extended periods but should be ok anywhere south of the tropic of Capricorn.

If you are concerned about insects crawling in cover it with a piece of mosquito netting and a rubber band.

If you want to drive water out after an especially dewy observing session, put on a halogen work light and gently heat the tubes for an hour or two after observing with the eyepiece end open or covered with mesh. Add a timer switch so you can go to bed. This method can work on tube left mounted on an observatory mounted instrument or tubes placed on a shelf in the garage. Depending on the size of the tube(s) and distance to them use a 150W or 500W halogen light. You only need to heat the tubes so they are luke warm to the touch - 10-20C above ambient for an hour or two.

Bringing a cold tube back into a warm and humid house especially in winter is the worst thing you can do. Houses are much more humid than outdoors and so the cold metal condenses much more water inside the tube. A garage or tool shed shelf with a halogen heater is the best way to keep the tubes dry.

These same types of lamps can be used to heat and regenerate desiccant rather than putting them in the oven with the Sunday roast.

Joe

[Wavytone]

The original question concerned refractor optics, not reflectors.

In some respects salt would be OK as far as the lenses are concerned, however putting salt anywhere near the metal bits of the OTA will ruin them very quickly, especially aluminium or brass parts, regardless of whether they are machined, cast, painted or anodised.

And before anyone chimes in about aluminium boats in seawater, they survive the salt only thanks to a sacrificial electrode (dissimilar metal) creating an electrolytic battery that protect the aluminium.

But don't let me stop you trying, and destroying your scope in the process...

[space oddity]

A short term fix is a tea bag or two. Silica gel should not be left too long without being recharged-it starts releasing the moisture back! Bit of a reservation on the salt- what if it gets saturated and leaks the liquid? Chlorine is deadly to metals.

[Profiler]

I would be interested to hear some suggestions on how long silica should be used before it should be freshened up in the oven?

[Wavytone]

That's a "How long is a piece of string?" question.

Only way to know is to use silica that changes colour when its done, or dry it (in an oven) regularly each month.