Dewing problem (though not the usual one)

[acey]

I don't have problems with dew in the field - my dewshields sort that. My problem is once I get my scope home, I find that the mirrors have become completely steamed up, and require tedious de-steaming before I can get to bed. My question is: how do I stop my mirrors misting up while I'm driving home in the car?

The scope is a SkyWatcher Flextube 300P. I've tried the semi-airtight approach, replacing all caps and covering tightly in a big plastic bag. I've tried the semi-open approach, putting on the tube caps but allowing air to circulate around the tube end. I've tried the cosy approach, turning on the car's heating full blast, and the hypothermia method, leaving the windows down and my hood up all the way home. But so far, whatever method I try, I find that the big primary, after being out in the cold for so long, just can't adjust to a warmer ambient temperature fast enough, or else can't be sealed off from it adequately. My old 8" wasn't such a headache - size definitely makes a difference here, and also perhaps the more open-tube design.

Has anyone else encountered this problem? Has anyone solved it?

Andrew

[roundycat]

put it away somewhere dry when you get home and do not cover it with anything at all. Let it dry naturally.

Dennis

[ollypenrice]

I'm with Dennis. You don't have a problem, you just have a wet mirror. It'll dry. Our big Dob looks like a bucket of water some mornings when I check the obsy but it has been fine over several years. It just dries.

Olly

[acey]

Cheers for this. I agree the dew itself is not a problem, but leaving dewed optics sealed will accelerate corrosion of the coating over time, and as it's a new scope, the mirror surface hasn't fully oxidised and hardened yet. So I shall continue my efforts to prevent the dew from forming in the first place. Think I'll try leaving the scope uncapped on the way home, and if it dews I'll leave it still uncapped in the garage to dry slowly before sealing up.

My last scope served me for 9 years without recoating but at the end of that time the primary was covered in pinholes, visible from the back when you held it up to the light. They were caused by spots of dew that had deposited salts on the mirror surface which ate through the coating. Can't be completely avoided, but it's worth minimising.

[ZOG]

I have the same scope and I find that the dew forms, not in the car, but at almost the instant that I take the scope into the house. Leaving the covers off for half an hour usually clears the mirrors.

[themos]

pinhole diameter: 0.1 mm gives area of 0.03 mm2

mirror diameter:300 mm gives area of 282,600 mm2

half a million pinholes will reduce your area by 15,000 mm2 or 5%.

To eliminate dew I guess you'd need a primary mirror heater.

[acey]

Fair point, Themos. I'm resisting the dew heater route, though. Since rigging up a camping mat dewshield for the top end and a shower-cap cover for the bottom (and keeping my eyepieces in a case when not in use) I've had no dewing at all while observing, and my 12V hairdryer has become redundant. If I can take that and its power source out of my set-up then I'm travelling just a little bit lighter, which I prefer. The only electrics I need worry about are my red lights.

Andrew

[themos]

I was thinking of a heater that you plug in only when you bring the mirror into the car, for the journey home. You might have to start the heater a few minutes before.

However, some people think that active cooling of the mirror before observing can have dramatic effects:

http://www.pha.jhu.edu/~atolea/WAS/thermal_management_newtonians.pdf

So, you might want to put up with some electrical power for your sessions.

[acey]

In-car heater might be something to try. I realise that we tend to do everything back-to-front with our mirror-warming antics, given the traditional notion that it should be all about cooling the mirror down - which is what tube ventilation, fans etc are supposed to be about. But I wonder if mirror-cooling is more of an issue in climates where there's a very big drop between day and night temperatures. For us Brits it's humidity more than temperature that seems to be the issue.

[themos]

If you take the plunge and install an airflow-over-the-mirror system, I think that would take care of the dewing up as well. You could just leave the airflow on for hours.

[themos]

Also, on the clear nights when you want to observe, the temperature drops faster than on those nights more typical of the British Isles.

Temperature drops to -18C as wintry weather grips Scotland - The Daily Record

[acey]

Only got down to -6 when I was out on Saturday but it certainly felt nippy!