Dew .. is it bad for telescopes?

[Draco]

Hi

I have a Meade LX90 and because I wanted to stay more than 30minutes outside during winter, invested in a dew cap. This has been the best $60 I have spent for a while since now I can stay for hours without getting the lens clouded with dew.

However, something new has turned up. Tonight after I finished my observing, I took the dew cap off and checked the lens. There was no dew on it, so I put the lens cap on (the lens cap was cold and dewy though).

I brought the telescope inside, and as you could imagine, it is much warmer inside than outside. Anyways, I took the lens cap off and checked the lens again. To my dismay, there was dew all over the lens!

From high school science class I know that if I took out a cold bottle of water from the fridge during summer and placed it on the bench, soon enough there will be droplets of water on its outside.. condensation as they are called. So now back to my telescope. Since outside it was cold and though the dew cap prevented the dew from forming on the lens, when I brought the telescope inside, since it was warmer inside (though there is no heater or anything of that sort turned on), does that mean that the moisture in the house condensed on the telescope lens? How do I get rid of it, apart from using a hair dryer?

Also, is dew bad for the telescope lens? Do I need to dry it out? Does it leave behind water marks, that will affect the light catching capability of the telescope?

Thank you for your help

[astro744]

I find leaving the cap off the corrector when bringing the telescope in works for me. Place the cap on when both telescope and cap have slowly acclimatised.

You just have to be carefull the telescope is not exposed to being damaged by foreign objects whilst the corrector is unprotected.

[Larryp]

I just use a 12 volt hair dryer if this happens to my scope-clears it up beautifully and doesn't leave any water marks. I wouldn't use a 240 volt dryer, though-I worry about heating the optics too much.

[OzStarGazer]

That's what I would do too.

[Pinwheel]

By your post I'd say you have a good grasp of the problem, go with your instincts..

[Renato1]

I'm surprised your telescope lens hasn't dewed over even with the dew cap attached - where I live, that is quite common after a few hours.

When I was a bachelor, my house was somewhat dustier than now that I'm married. So I wouldn't use a hair dryer inside the house, or I'd heat the house first, but turn off the ducted heating when I finally bought the C8 in - figuring that I was only blowing dust onto the front lens. Though a 12 volt hairdryer outside the house was okay.

Eventually though, you will have to clean the front lens. My manual said to blow the dust off first and use either a drop of detergent in water, or a mix of iso-propyl alcohol (not the rubbing version) and distilled water. I very gently used the latter together with lens cleaning paper sheets, and always got good results.
Regards,
Renato

[Pinwheel]

A drop of detergent yes, Dish washing detergent NO.. it has caustic soda in it for breaking down fat. Caustic soda is a corrosive.