Condensation

[tripleped]

Had my 10” skywatcher DOB (open tube truss design) outside about 3 hrs before observing on a recent cold night and still managed to get condensation on the secondary. Wondering if I should leave it in the garage all day (in my padded Orion bag to keep spiders out) to allow for better cooling. Live in St Louis MO metro area so winters pretty cold and garage not insulated so it gets quite chilly. Is there such a thing as it being too cold for the scope to sit all day like on frigid January days? 
 

Also- last night had condensation form on EP’s. (it did pretty quickly resolve however) So do I need to leave EP’s outside with covers off on cold nights to equilabrate? If so for how long? Appreciate any advice on this. 

[Seelive]

Condensation forms when the temperature of a surface falls below the air dew point temperature (which normally results from thermal radiation by the surface). Leaving the scope out to 'cool' (which is the right thing to do for 'seeing' reasons) therefore means that the mirror temperatures will be nearer to the dew point before you even start observing!  As soon as you uncover it, it won't belong before condensation starts to form unless you take other precautions.

[tripleped]

What other precautions specifically?

[Seelive]

I would guess that the secondary mirror is particularly problematic in an open tube reflector as it hasn't got the 'natural' dew shield formed by the telescope tube. I have heard of secondary mirror dew heaters but it's not something I've ever needed to worry about, and I'm sure their are many people on here that could advise you on a solution.

[John]

A light shroud can help with this and you can add a dew shield to the top of the tube as well.

 

[jonathan]

Some people use a portable hair dryer to keep their mirrors and other optical surfaces clear, never bothered with it myself though, I use a dew shield and dew heater equipment with my SCT.  If it gets really bad with dew then I just call it a night, my fingers are usually freezing by that time anyway.  Easy way to check for local dew conditions is look at your car windscreen.