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Safe to leave scope out when it's dewey?


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I've always read that it's best to take the telescope outside a couple of hours before viewing to allow it to aclimatise but I seem to remember reading that it's not a good idea if there's a lot of moisure in the air and it's dewey. Is this correct?

The reason I ask is that it's pretty clear tonight so I was planning on spending some time out later with my scope but it's really dewey outside. At the minute I've left my scope in the rear porch so it'll still be cooling down but it's not exposed to the elements fully.

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I just leave mine to cool down outside. A porch will not allow the scope to aclimatise, in fact it might make the mirror more dewey, once you've taken it outside? Have you thought about making a dew shield? http://stargazerslounge.com/beginners-help-advice/159085-making-dew-shield.html

It important that your mirror be at the same temperature as the evening sky. If it’s not, you’ll have difficulty focusing the image because the mirror will be minutely flexed due to thermal differences. If the mirror is colder than the ambient air temperature, you may get dewing on the mirror like steam on the bathroom mirror after a shower. Either one of these will result in bad images through the eyepiece.

Edited by Telrad
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Found I couldn't focus at all tonight. I think perhaps as it is colder and the house was quite warm; it was taking much longer than in the past to cool.

It's good to know this as you often start to think there is something wrong with your scope...

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Remember that it's not just the mirrors that need to be at ambient temperature, your eyepieces do too. A big 2 inch EP can take a while to cool, and can give not-so-good views until it has.

Anyway, dew won't hurt a scope, but if you are using a mains powered rig then be extremely sure that you use it on it's own trip, and don't get any moisture on the 240v stuff....you want to be looking up at the stars, not down from them :)

Rob

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Yes and no. Yes, as long as you don't try to wipe the dew from the optical surfaces. No if there are pollutants in the moist air that could eat into the exposed surfaces of mirrors. So it depends...

When you take the scope out to cool down, cover the exposed openings. Cooling down will take longer but worth the effort in perserving the delicate optic surfaces. Of course, once observing, dew can be a problem and if you can't control it with heat or dew shields, give up and try another night.

Once the scope's optical surfaces are covered with dew, DON'T try to wipe it off. Bring the scope inside and let the dew evaporate in the dry air.

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and don't get any moisture on the 240v stuff....you want to be looking up at the stars, not down from them :)Rob

Lol, that made me chuckle. :)

Edited by Brent
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My kit was dripping wet when I put it away just now, but the mirror stayed clear throughout tonight's session. Everything worked just find despite the heavy dew... apart from the eye piece that I had to bring inside to clear. I find leaving them on the scope can be a problem.

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Cheers, everyone. I did leave all my gear out for a couple of hours before I went out and it seemed ok. I was, however, concerned when I came back in because there was a lot of moisture on the outside of the tube so I was worried about any moisture inside the tube.

I think I had focusing issues too, to be honest. My target last night was Gamma Andromedae (Almaak (Gamma Andromedae)) but I couldn't separate the stars and I was wondering if that was a focusing issue. I was planning on making a post about it, actually.

EDIT: And I did (http://stargazerslounge.com/beginners-help-advice/164665-couldnt-separate-gamma-andromedae.html)

Edited by redneon
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Remember that it's not just the mirrors that need to be at ambient temperature, your eyepieces do too. A big 2 inch EP can take a while to cool, and can give not-so-good views until it has.

Would you just take all your eyepieces out (or the ones you plan on using at least) either in their travel box or just leave them on the accessory tray?

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Hi all,

I've been following the thread and seem to have had similar problems, just thought I see if anyone had any solutions. Put scope out last night to cool down, primary and secondary mirror were fine but EPs fogged up as did the finder scope . They just seemed to soak up the dew like a sponge. Is there a solution to this problem??

Ps not my intention to highjack the thread.

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what i do if it look like dew will attack is pit a old sheet over it the ones of a kids single bed minus the quilt in side and drape it over the scope ,ref eps keep them in ya pocket,and not in the scope,i keep a thermometer in my shed and if it gets below the dew point normally between 4/7c here i leave it on 10 mins before take it ofbe carefull with wet heavy tubes

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I tend to keep my eye pieces in an outside coat pocket, they just end up like the bathroom mirror if I leave them in the scope at the moment.

My red dot finder fogs up too, but it's still usable. I guess you could try covering the finder with a towel or something, as suggested before.

Once the weather starts to get colder the dew should be less of a problem. Then we just need to worry about freezing - thermals :)

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