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Frost on the mirror

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#1 hokkaido53

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Posted 29 November 2013 - 09:06 PM

Hi, everyone,

Over the past few nights, my 18” mirror has been collecting frost while I’m observing. This isn’t too surprising, since the temperature has gone down to 15 degrees at night. Because I live in the desert, there is no industrial pollution, so I’m hoping that the mirror’s coating hasn’t been harmed.

Last week, I washed and rinsed the mirror with distilled water, as per Dave’s instructions, but I don’t want to make that a weekly chore.

My question is: Would just frost alone damage the coatings?

Thanks,

Roy in Taos

#2 aatt

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Posted 29 November 2013 - 09:33 PM

I hope not-I have stored my scope in the shed and it dews up some days-never at night.You don't have to "obsessively" sorry for the pun, wash the mirror very often. I did not wash my 6" mirror for 7 or 8 years and it still put up good views prior to cleaning and after 15 years it still works very well. It is getting close to a recoat though. Dew/frost evidently will damage coatings in the long run as there is still air pollution even where you are, just not as much. Dust and soil tend to be negatively charged and charged will always oxidize enough to cause damage in the long run. It may take sometime depending on what type of coating you have perhaps, but the alternative is not to use it.I did last spring refrain from observing one night when my white pine trees were pumping so much pollen into the air that it looked like a dust storm in my flashlight.For me not doing astronomy to avoid condensation issues is not going to happen! Hope some better informed folks will chime in soon.

#3 precaud

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Posted 30 November 2013 - 01:24 AM

Roy, I'm just south of you and had the same experience a couple nights ago. The temperature alone isn't the cause. The problem has been the unusually high humidity - dew point within a degree or two of the temperature. It's been getting slowly better the last couple evenings, as the recent snow melts off, from 88% RH to 75% last nite to 69% tonite. All much higher than we're accustomed to up here.

I highly doubt that a couple nights of frost would damage the coatings.

#4 hokkaido53

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Posted 30 November 2013 - 09:21 AM

Roy, I'm just south of you and had the same experience a couple nights ago. The temperature alone isn't the cause. The problem has been the unusually high humidity - dew point within a degree or two of the temperature. It's been getting slowly better the last couple evenings, as the recent snow melts off, from 88% RH to 75% last nite to 69% tonite. All much higher than we're accustomed to up here.

I highly doubt that a couple nights of frost would damage the coatings.

Yes, the humidity is unusual. The skies have been hazy, too. I think the frozen droplets in the upper atmosphere lowers the transparency.

#5 precaud

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Posted 30 November 2013 - 09:38 AM

Absolutely. Seeing has been good (relatively still air) but the humidity has really taken a toll on transparency, scattering light from any source and whitening the background. Not fun for DSO hunting... :(

#6 GlennLeDrew

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Posted 30 November 2013 - 11:55 AM

If there are 'frozen droplets' (ice crystals) in the upper atmosphere, we call it cirrus cloud. If you see no cloud, then the water is necessarily in the vapour state. Excluding, of course, any water molecules attached to solid particulates.

#7 precaud

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Posted 30 November 2013 - 12:53 PM

Glenn, what I'm referring to is near the surface. Not dense enough to be fog (though we've had that in the morning) but enough to cause visible domes around light sources, including ones not too far away, such as the school building 500 ft. to my north.

#8 GlennLeDrew

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Posted 30 November 2013 - 01:35 PM

For ice crystals to form in air, the air temperature would have to get down to well below -20C, or several degrees below 0F. Warmer than this and you have water droplet mist, or fog when dense enough. In either case, a GLP beam would scintillate with a veritable blizzard of quite brilliant sparkles.

#9 hokkaido53

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Posted 30 November 2013 - 02:11 PM

For ice crystals to form in air, the air temperature would have to get down to well below -20C, or several degrees below 0F. Warmer than this and you have water droplet mist, or fog when dense enough. In either case, a GLP beam would scintillate with a veritable blizzard of quite brilliant sparkles.

Correct, but I'm talking about ice crystals forming in the upper atmosphere - maybe 60,000 feet altitude. If you are trying to observe through this layer of frozen droplets, then things become fuzzy or opaque. There doesn't need to be an actual cloud formation, or so I've read.

- Roy

#10 Scanning4Comets

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Posted 30 November 2013 - 03:23 PM

My question is: Would just frost alone damage the coatings?

Thanks,

Roy in Taos


He's not looking for a lesson in weather or Physics, he just wants to know if it will damage the mirror or not. ;)

I've had it form on my mirror dozens of times, and my mirror still gives excellent images of planets, globs etc.. Just leave the top cap off of your mirror for at least 4 hours and then re-cap it.

The only thing I would be concerned about is repeated condensation from "acid dew".....If it gets really bad over the course of so many years, get a re-coat or a new mirror.

Weather eventually takes it's toll on many things.

Take a good look at my mirror. I got it this way through a trade and I have still been using it for my third year and images are still really good. (It has been cleaned by me 2 times. The original owner used the wrong Alcohol type which left blue streaks...or the coating it failing).

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#11 Scanning4Comets

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Posted 30 November 2013 - 03:30 PM

Here it is without using the flash....

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#12 Mirzam

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Posted 30 November 2013 - 08:54 PM

If you are not already using a shroud doing so may help avoid frost. Another option would be to use bubble foil insulation (reflectrix) to wrap around the lower part of the scope. This stuff has low emissivity and helps reduce chilling from exposure to the cold sky.

JimC

#13 hokkaido53

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Posted 02 December 2013 - 02:53 AM

My question is: Would just frost alone damage the coatings?

Thanks,

Roy in Taos


He's not looking for a lesson in weather or Physics, he just wants to know if it will damage the mirror or not. ;)

I've had it form on my mirror dozens of times, and my mirror still gives excellent images of planets, globs etc.. Just leave the top cap off of your mirror for at least 4 hours and then re-cap it.

The only thing I would be concerned about is repeated condensation from "acid dew".....If it gets really bad over the course of so many years, get a re-coat or a new mirror.

Weather eventually takes it's toll on many things.

Take a good look at my mirror. I got it this way through a trade and I have still been using it for my third year and images are still really good. (It has been cleaned by me 2 times. The original owner used the wrong Alcohol type which left blue streaks...or the coating it failing).


Thanks, Markus.

- Roy

#14 hokkaido53

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Posted 02 December 2013 - 02:58 AM

Absolutely. Seeing has been good (relatively still air) but the humidity has really taken a toll on transparency, scattering light from any source and whitening the background. Not fun for DSO hunting... :(

I just got inside from a 2-hour galaxy-observing session. I began with NGC891, which still looked fuzzier than usual, although better than a few nights ago. Other galaxies in the Perseus and Pisces clusters were clearer, but they were also nearer the zenith. Galaxies closer to the horizon, like NGC1232 and even M33 were hazy.

- Roy, again.

#15 northernontario

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Posted 02 December 2013 - 06:48 PM

My 16 inch has been in the shed for a few years now. And we go from plus 30 Celsius to minus 30 Celsius through out the year.

No problems at all.

jake

#16 precaud

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Posted 02 December 2013 - 08:43 PM

I just got inside from a 2-hour galaxy-observing session. I began with NGC891, which still looked fuzzier than usual, although better than a few nights ago. Other galaxies in the Perseus and Pisces clusters were clearer, but they were also nearer the zenith. Galaxies closer to the horizon, like NGC1232 and even M33 were hazy.


Yeah, I was out last night too, not as late as you were, and it was hazy as you say. It's kinda windy tonight, but I'll be out, mostly because we have weather coming in and this will be the last chance for a few days...






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