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Am I putting my scope outside too early?

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Hi all,

Been doing this for a couple of months now, and I'm aware of the need to let a scope that's been kept indoors time to adjust to the ambient temp outside, so what I've been doing is putting my scope outside when I get home from work. At about 6.15 or so. A basically level the base and mount the scope in the daylight which makes things easier. I then leave it there with the lens caps on the scope and finder.

The trouble is that I'm now required to be a parent so it'll be sitting outside until about 10PM. A couple of times I've then gone out to discover that the body of the scope has a layer of ice over it. The optics seem fine but after a certain amount of time I start to get ice on the secondary mirror and then it's game over.

Would it be better for me to put it out for less time, to stop the scope getting too cold? By the way, it's a Skywatcher 130P.



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I'd suggest putting it out around 9pm - it'll be ready by 10pm and you'll get a couple of hours viewing before it ices over. Keep a hairdryer handy on an extension lead and you can keep going longer (only use it on light warm setting when necessary - not full blast or you'll crack the optics).

Failing that then dew shields and dew heaters might be worth investigation.

Hope that helps :)

Edited by brantuk
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Stuart I have made large dew shields for my scopes and finders which are made from neoprene. Even though the scope gets 'icey' the secondary is usually ok. I also use a hairdryer but its a 12 volt not mains (too risky). My scopes 10" Dob and the 6" frac were put out at 5.00pm today ready for the tonight's viewing. I am expecting its going to be ok even though frost is expected.


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Cheers for the tips guys. Getting dark now, oh, actually it is dark now, and scope still indoors. Stupidly didn't even think to take the mount out and level it in daylight. Hmmm.

Might look into these dew shields though. Are they cheap to make?

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I've seen dew shields made from all sorts including cardboard and plastic sheet. Some people use camping mats cut to size. Or you can buy them from astro suppliers ready sized for your scope (tends to be a tad dearer).

Main thing is it needs to be strong enough to hold shape, attach firmly to scope, and preferably water proof. :)

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An old trick used by many vehicle owners in North America to keep frost (and snow) off the windshield is to place a beach towel over it. When you come out to the car just lift the towel off and 'hey presto' no snow, ice or frost to scrape off.

Try it with your telescope.


If moisture gets between the plastic and metal or glass it will freeze the plastic to the metal or glass.

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The amount of time a scope needs to be out to cool depends on the size of the primary mirror but even a 10" mirror only needs an hour to 2 hrs cooling (if even 2 hrs). I have been told that while the scope is out cooling that it should be in a horizontal position.

My scope is only 5" (5.1" to be EXACT) and i have found that when i put it out to cool that it is ready to use by the time my eyes have become fully dark adapted (30-40mins). We have had many cold frosty nights since i bought the scope in mid December but i cant say that i have had a problem with mirrors fogging up and certainly no ice building up on optical tube.

My scope is an open truss Dob, so dew,frost etc should be a real problem. I dont as yet have a proper shroud for it but i put a small woolen blanket over the open truss. Keeps it snug and warm.

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