Telescope Reviews: Fingerprint smudge on corrector plate

Hmmm ... interesting information!

On the one hand:

>> ... strong multicoatings. (these coatings are actually thin layers of metal after all).

Yes the newer coating are stronger than they used to be. But metals are very susceptible to damage by acids and organic compounds such as are in fingerprints. And they are only a few atomic layers thick. Strong is a relative term and as you pointed out, you should not clean very often because of potential damage.

>> If you can let windex touch all of your optics, greasy fingerprints or eyelash oils wont do squat.

Windex is a mild solvent that is specifically designed to clean glass and has a very different chemical composition than fingerprints so I don't think the comparison is really meaningful. Also, the Windex is removed during cleaning and any small remaining amount will evaporate. Fingerprints, on the other hand, will linger for a very long time allowing any potential damage to occur over time.

>> and the consensus between all the telescope professionals Ive spoken to, is to clean your optics almost never...

I absolutely agree. This sentiment is echoed in the article I referenced by Dr. Clay of the Arkansas Sky Observatory. But as he and others point out, there are times when you do need to clean. Fingerprints are one example, pollen is another - again because of the claimed damage to the coatings. I recently cleaned mine because of a significant layer of pollen. This was not having an effect on my viewing, but I wanted to protect the coatings. Using the extremely careful techniques outlined by Dr. Clay, it worked very well.

On the other hand:

>> I called meade tech support several time ...

One would think that the people at Meade and other scope vendors should know what they're talking about so this certainly lends credence to your statements.

>> Dan i think that is an old wives tale, or urban legend that no longer applies to modern coatings.

It is true that rumors and old wives tales can be around for so long that everyone just considers them to be "common knowledge" without any facts to back them up. I spent some considerable time on the internet investigating this after reading your note. I found several reputable sources, optics manufacturers, observatories, etc. that warn about fingerprints containing acids or organic compounds that can damage coatings. But, that still doesn't mean that they have actually done the research to back that up. It's entirely possible, in fact likely, that they are just repeating what is "common knowledge".

I was completely unable to find any research done on the direct effects of fingerprints on modern (or even older) optical coatings.

The information I found is more general in nature. First, what exactly is in a fingerprint when people refer to acids or organic compounds? It seems that it's basically a mixture of sweat and sebum, the fatty substance that is produced by the sebaceous glands. Since sweat is mostly water, when it evaporates it leaves behind small amounts of organic substances such as chlorides, ammonia, uric acid and urea. In fingerprints, the fats provide for a relatively stable substance that will not evaporate and will resist cleaning except with solvents. It also serves to hold the other organic compounds mixed with it in place where they can do damage over time. And it provides a substrate for bacterial growth (that's why sweat smells) which can also damage surfaces.

Another source of information is my wife. She is a metallurgist that spends her days looking at damaged and corroded surfaces using optical and electron microscopes, and analyzing contaminants with and x-ray analyzer and Fourrier Transform IR Spectrometer.. One of the primary sources of surface corrosion that she sees is compounds containing chlorides such as can be found in fingerprints. She is always complaining about people providing her samples that are covered with fingerprints since that contaminates and damages the samples. These samples are primarily metals such as are found in the optical coatings we're discussing.

So does this mean fingerprints are a problem? Not necessarily. If the coatings are specifically designed to resist the particular compounds in fingerprints, then they should be alright. But I would guess that it's difficult enough to engineer coatings that have the desired optical characteristics without having to include resistance to organic compounds as an additional requirement.

This is a really interesting topic to me (although I'm probably boring everyone else :-) and I'd love to see some real research on the topic if someone can dig some up.

For me, I'm still planning on keeping fingerprints off my optics as much as possible, and cleaning them (very carefully and very infrequently) if they accidentally get on there.

-Dan

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Scopes: Celestron NexStar 8, Orion EON 72mm ED/APO on Astroview mount (EQ3)
Eyepieces: Celestron 40mm, 25mm, Baader Hyperion 13mm, 8mm, 5mm
Other: Telrad, 2x Barlow, 0.63x Focal Reducer, Dew-not strips, DewBuster controller