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(meteorobs) Re: [ASTRO] Dew prevention on lens with soap



I
claudio<< Interesting. The obvious questions are what the process does to the
 existing lens coatings over time and if any discernible change occurs to
 the image.<<

I don't know what it will do to the lens coatings over time. I highly doubt
there will be very much change occuring to the image. If there is, I expect
there will be much less than a solid dew coating would cause. I'm mainly
interested in catching usable meteor images for measurements. If the quality
of the photograph is not of sky and telescope quality, I'm not that much
concerned as long as measurements can still be made. Since the f/stop of most
night time photography is either wide open or near wide open, the depth of
field is relatively short near the area at the front of the lens. I recall
years ago of sticking a short piece of black tape onto the lens to reduce
bright highlighted areas during daytime photographs. The tape wasn't even
noticeable with a wide open f/stop. If there is concern about destruction to a
lens coating, perhaps one can coat the outside of a UV filter instead of the
lens? I'd prepare for this during the day or another day by attaching the
filter during a period of very low humidity. So if any dew might want to form
between the filter and lens, there will be very little moisture for it to
happen with. Anyhow, individuals should experiment with this idea. 

Last night I made another all night test with a piece of glass. The dew stayed
off for a few hours again. But by morning there was a layer of moisture
covering the treated area. However it looked different than the non treated
area. Despite being wet, the moisture looked clear as the glass to look thru.
The untreated area had the dew quite extensively beaded up and you couldn't
see anything thru it. So, it appears that even if it gets moist, useable light
will probably still pass thru and show the stars and meteors quite nicely. I
will eventually give it a test run at my observatory.
George Zay
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