daytime dew

[Shiraz]

I normally stagger out to cover the scope (Newtonian) at around dawn, turn everything off, roll over the roof and go back inside for a cup of coffee. Today I did the normal procedure, but went out again at about 10am to check on a slight collimation issue - and found the mount to be dripping wet and the mirror with rivulets of water on it!!!! How come, since I don't have dew trouble when imaging and it was dry 3 hours ago.

What seems to have happened - and possibly has been happening for months without me being aware - is that dew from the previous night had settled all over the interior and floor of the obs and then evaporated as the sun warmed the walls and roof. This airborne water then condensed on the nearest cold things - the mount and the mirror, both of which were still at night temperatures.

I think that I will have to install fans to blow ambient air across the mount and scope to warm it up more quickly as the air in the obs warms - and to exhaust the humid air as well. I already have chloride absorbers (laundry style) in the obs, but they did not do the job in this case. Any other ideas?

thanks, Ray

[Paul Haese]

Hi Ray,

I have had this problem at Clayton every winter since I have had an observatory there.

Have you considered having some fans on your scope running all night? My RC has fans which run all night and that keeps the dew off the primary. The secondary has a dew heater.

I typically run the fans about a couple of hours prior to imaging and opening the scope to the night air.

This seems to work well in keeping the problem at bay.

[gregbradley]

Leave the roof open for longer until it warms up from the morning sun. Of course if the weather looks stable.

I haven't seen that in my observatory but you are a long way south of where I am so I assume cooler temps there.

Greg.

[Shiraz]

thanks Paul. I don't have dew problems when imaging - I also run fans and it seems to work well enough in keeping the optics dry while imaging. But it all dews up when the fans stop, the roof is closed and the sun comes up - getting dripping wet cannot be good for anything.


thanks Greg. Leaving the roof open is an option, but I am often away during the day, so there are times when it must be locked up early. I was surprised how wet it was at 10am, since I do not have a problem with dew while imaging. It didn't occur to me that dew might be problem during the day, when everything was shut down and the sun was up. Nightime temps have been 5-6C, so it isn't really cold - less than a km to the sea though, so lots of dew in the obs after a night of imaging.

[PRejto]

Hi Ray,

I've put two solar powered fans on my roll of "shed." Seems to work well and no problem with rain since they are venting to the sides and are under the overhang of the roof a bit.

http://www.pbase.com/prejto/image/155360723

Peter

[Paul Haese]

I still get that too. I was quite concerned at first but after 5 years in the roll off roof and now a couple of years in the dome too, I am not as concerned. The gear does not seem to have a problem. Still some fans in the obs might be a good idea as suggested by Peter.

[Shiraz]

Thanks Peter. I have a small solar panel and a couple of fans - looks like the next job.

thanks Paul, that's comforting. Mine has only been under the ROR for about 6 months and I only just noticed the problem.

[Merlin66]

Over the years in a tin shed observatory (UK/ Belgium etc) I used a "doggie blanket" to throw over the scope/ mount when finished observing.
This is a 12V low power (80w I think) electric blanket about 1.2 x 1.2m. Keeps everything dew free and warm.
I see there are now 240v "throw over blankets" available, again low power but with a 2hr built-in timer.
May be a solution?

[Paul Haese]

Also Ray, I have found this often also occurs when I have not had my observatories open for a week. An environment that lefts the moist air inside through the gaps will create condensation once the surfaces fall before the dew limit, same as a glass of beer once filled with cold beer.

[The_bluester]

Funny enough I was reading up on a similar topic recently, energy efficient home design rather than obs.

One of the condensation solutions they put up were one way membranes which were intended to allow moisture to pass through one way but not the other to prevent or reduce condensation in well sealed houses.

In context, one thing that was suggested was one of these membranes under condensation prone materials like metallic roofing, so moisture passes through the membrane from bottom to top and then drains along the top of the membrane to the roof edge rather than forming under the roof and dripping.

[Shiraz]

that's a very good idea, hadn't thought of a blanket - thanks Ken


Thanks Paul. I find that the chloride moisture removers work well enough long term, but they cannot do the job immediately after a night of imaging.

thanks very much for that tip Paul - had no idea such material existed. It has to be useful in this situation.

regards Ray

[multiweb]

I've had this issue a couple of times. Bits and pieces that dewed up were the bulky ones such as the mount head, dovetails, refractor tube, basically everything that had a higher thermal mass. I suspect that leaving the gear uncovered to allow it to radiate to ambient might help. On occasion I have dewed up when the gear was over-covered. So the mount was still freezing although it was warm in the tent and obviously the cold aluminium was dripping wet. Every time I had just put a thin piece of sheet over it, with good air circulation underneath I was surprised to see it completely dried by sunrise.