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UV-C radiation and Optical Coatings

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#1 HydrogenAlpha

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Posted 16 August 2019 - 05:00 AM

I have recently bought a UV-C steriliser which I intend to use for periodic irradiation of my refractor optics. I live in a high humidity environment, but I often have my telescope outside of the dry cabinet for extended periods of time, which is why I got the UV-C lamp as an added measure. 

 

I know that such ionising radiation can cause chemical changes in certain (usually organic) compounds, but I am wondering if it will have any effects on the optical elements/coatings?



#2 happylimpet

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Posted 16 August 2019 - 05:24 AM

I would eat my hat if it did, inorganic substances like MgF laugh at long wavelength UV (ie compared to the almost X-ray end). I assume this is to kill of Fungus? Good idea.



#3 MikeTahtib

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Posted 16 August 2019 - 05:58 AM

I don't know for sure, but I'll throw this anecdote out there.  I used to antique shop a lot, and found people selling purple glass doorknobs.  They were rare, and quite expensive, until I found one seller at a flea market who always had lots of them.  I learned that they started out as regular clear glass doorknobs, but over time, exposure to sunlight made them turn purple.  This one vendor allegedly was allegedly buying ordinary clear glass doorknobs, and autoclaving them under UV light to turn them purple.  Of course this story is full of rumor, and unknowns about the type of glass required, so take with a big grain of salt, but it does make me think glass isn't always impervious to UV.



#4 Spikey131

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Posted 16 August 2019 - 07:31 AM

We use refractors all the time for H-alpha solar viewing with no objective filter.  Lots of UV-C in that sunlight.

 

I had a Tele Vue refractor once that had some mold on one of the lenses.  TV cleaned it with no ill effects.  They told me that regular unfiltered exposure to sunlight would kill mold.



#5 HydrogenAlpha

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Posted 16 August 2019 - 08:19 AM

I would eat my hat if it did, inorganic substances like MgF laugh at long wavelength UV (ie compared to the almost X-ray end). I assume this is to kill of Fungus? Good idea.

If MgF the main component used for coatings? I have zero idea of the composition of optical coatings, which is why I am asking. And yes, it is to kill fungus. 

 

We use refractors all the time for H-alpha solar viewing with no objective filter.  Lots of UV-C in that sunlight.

 

I had a Tele Vue refractor once that had some mold on one of the lenses.  TV cleaned it with no ill effects.  They told me that regular unfiltered exposure to sunlight would kill mold.

I believe sunlight is mostly UV-A and (to a smaller extent) UV-B, but I don't think the distinction matters too much. I was under the impression that refractors usually require full aperture filtering, but now I do indeed recall that many people can use smaller refractors without the ERF. 

 

I don't know for sure, but I'll throw this anecdote out there.  I used to antique shop a lot, and found people selling purple glass doorknobs.  They were rare, and quite expensive, until I found one seller at a flea market who always had lots of them.  I learned that they started out as regular clear glass doorknobs, but over time, exposure to sunlight made them turn purple.  This one vendor allegedly was allegedly buying ordinary clear glass doorknobs, and autoclaving them under UV light to turn them purple.  Of course this story is full of rumor, and unknowns about the type of glass required, so take with a big grain of salt, but it does make me think glass isn't always impervious to UV.

I did a bit of a search online, and it appears that the purple colouring is due to oxidation of elemental manganese: https://sciencing.co...le-7183982.html

 

I don't suppose refractor glass contains that? Or at least I hope not. 


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#6 joe4702

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Posted 16 August 2019 - 09:14 AM

Minor point but I don't think any wavelength of UV is considered ionizing.

#7 HydrogenAlpha

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Posted 16 August 2019 - 09:45 AM

Minor point but I don't think any wavelength of UV is considered ionizing.

Higher energy wavelengths of UV are ionising. That's how emission nebulae work. 



#8 joe4702

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Posted 16 August 2019 - 11:03 AM

Higher energy wavelengths of UV are ionising. That's how emission nebulae work.


Interesting and a bit scary! Thanks.I always thought only x-rays and above were ionizing.

#9 EJN

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Posted 16 August 2019 - 01:08 PM

Higher energy wavelengths of UV are ionising. That's how emission nebulae work.


It's also the cause of most skin cancers.

#10 Spikey131

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Posted 16 August 2019 - 01:16 PM

My bad, and I stand corrected about UV-C. It does not penetrate the atmosphere.

So did those Hasselblads they took to the moon have special coatings?

#11 Spikey131

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Posted 16 August 2019 - 01:31 PM

A brief internet search reveals that this is a complex issue which has been thoroughly researched for the use of optical coatings in space. Here is a quote:

Effects on Optical Coatings
Among the first coatings used in space in 1958 were solar cells to produce power, and emission control tiles to minimize temperature extremes. Both of the first satellites were spherical with reflective aluminum coatings and solar cells. Thick glass tiles with a silver coating on their rear faces were used to control the temperature of the spacecraft. Glass absorbs infrared and re-emits at longer wavelengths providing cooling. The silver mirror doubled the absorption path and thus the emittance while reflecting shorter wavelengths. The first space optical coatings used for band-pass filters were constructed of thermally-evaporated soft materials such as ZnS and MgF2. Exposure to the space environment containing ionizing radiation, solar UV, atomic oxygen and high vacuum revealed the unstable operation of those coatings. In addition to humid-vacuum shifts in wavelength properties, filters, anti-reflective (AR) coatings and other coatings suffered radiation-induced transmission loss that was especially pronounced at short wavelengths.”

from here: https://materion.com...00D7D3B554A5842

You might want to check with the manufacturer of the sterilizer about this. The devil will likely be in the details: eg, how much energy does your unit emit? What exactly are the coatings made of? Etc..

#12 KLWalsh

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Posted 16 August 2019 - 08:55 PM

My bad, and I stand corrected about UV-C. It does not penetrate the atmosphere.

So did those Hasselblads they took to the moon have special coatings?


Most all types of optical grade glass used for lenses absorb All wavelengths of UV, for any reasonable thickness of glass.
Quartz ‘glass’ does transmit some UV.

#13 Spikey131

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Posted 17 August 2019 - 10:26 AM

But do you think it would affect the glass? I would be concerned about the coatings. And given the fact that UV-C does not naturally occur on the surface of the earth, lens makers might not concern themselves with it.

#14 HydrogenAlpha

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Posted 25 August 2019 - 07:01 AM

Just to close the question, I have contacted APM, which made an inquiry to LZOS, and they confirmed that there will not be any problems doing so. Thanks to everyone's input as well. Cheers! 


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