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How could it dew up?!

emadmoussa

By emadmoussa
in The Astro Lounge

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emadmoussa

emadmoussa

Posted
Posted

I left the scope outside last night for a couple of hours to cool down while I was sorting out things around the house.

The shroud was fitted, and the primary and secondary were covered. I put thick waterproof cover and top of the scope.

I came back,  removed the covers  and started observing. However, only 10 minutes into the session I noticed that views were not that great. I figured the scope properly lost collimation, but no. I looked at the secondary and it was completely foggy. I took the telescope inside and to my surprise the primary was also foggy - but not as much. Still foggy enough though.

What I don't understand is, how could this happen so quickly (never experienced something similar before)? I could understand the secondary closer to open air - hence the dew - but why primary got foggy as well even with the shroud on?!

I'm aware humidity level was between 96% and 98% last night. Wasn't an exception, I did that several times before under similar circumstances without such a issue.

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astro mick

astro mick

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I,m not sure,but maybe the Humidity built up under the cover,you say you were away for a couple of hours. For a time last night the wind totally died down,and the air was very still. 

It may have occurred at this time.

Usually scopes require some sort of ventilation even when covered.

Someone else may have the answer.

Mick.

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knobby

knobby

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I may be wrong but from memory in physics at school (long time ago) warm air can hold more moisture than cold air... So by covering the scope you've trapped the warm air so when the mirrors cooled down the moisture only had one place to go... Towards the nice cool mirror, so out came the moisture.<br />

that's my theory anyway :-)

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emadmoussa

emadmoussa

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I may be wrong but from memory in physics at school (long time ago) warm air can hold more moisture than cold air... So by covering the scope you've trapped the warm air so when the mirrors cooled down the moisture only had one place to go... Towards the nice cool mirror, so out came the moisture.<br />

that's my theory anyway :-)

Makes sense.

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SnakeyJ

SnakeyJ

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Posted

+1 for the hairdryer Imad - I got a tescos value one for £5 which raised an eyebrow from my young lady as I'm somewhat balding. Also perhaps a fan to help circulate the air - this speeds up cool down and helps prevent tube currents. I don't use my dew heater tapes on the newt anymore, but do run heaters for the telrad, finder and guide scope.

typed on my mobile with Tapatalk

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leelee970

leelee970

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Emad, you can buy dew heater pads for both primary and secondary mirrors.

Myself I'm going to make one. I've already have them for the EP's and refractor plus diagonal.

When my mirrors fog up I switch on a bar heater and let the cooling fan draw in the warmer air - soon clears.

Lee

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emadmoussa

emadmoussa

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Emad, you can buy dew heater pads for both primary and secondary mirrors.

Myself I'm going to make one. I've already have them for the EP's and refractor plus diagonal.

When my mirrors fog up I switch on a bar heater and let the cooling fan draw in the warmer air - soon clears.

Lee

OK - you're talking to somebody who knows way more about astro-physics than simple DIY :D  What are heater pads?

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John

John

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I always wondered about the hair dryer, but wouldn't be loud at night?

Tele Vue make a silent one with specially modeled air currents which optimally clear fogging while actually increasing the resolution of the scope. It was designed by Al Nagler before his hair fell out.

It's around £899 I believe :wink:

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leelee970

leelee970

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OK - you're talking to somebody who knows way more about astro-physics than simple DIY :D  What are heater pads?

I've read somewhere that you can buy dew heater pads for prim and secondary mirrors.

or you could make one for the primary at least but soldering resisters which when connected to a dew heater controller will generate enough heat to keep the mirror clear.

http://homepage.ntlworld.com/frederick.higginbottom/docs/astronomy/dew_heater_and_controller.pdf

Lee

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emadmoussa

emadmoussa

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Tele Vue make a silent one with specially modeled air currents which optimally clear fogging while actually increasing the resolution of the scope. It was designed by Al Nagler before his hair fell out.

It's around £899 I believe :wink:

Again  - great products and a lot of snobbery. Why exactly is this price?!!

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