I have set my gear up inside my dome for the first time this last week.
It has been -3C here and colder over night. I am finding that there is quite a lot of condensation forming on the telescope and mount.
This condensation is only forming in the morning when the sun first hits the dome and heats up the air inside. It goes from -3C to about 2C within half an hour. The air inside is heating up far quicker than the metal components of the telescope, hence the condensation.
I have tried two thing so far to try and stop it.
First I covered the scope and mount in a quilt and sheet, this really didn't do anything. Any metal that is not touching the material still forms condensation.
Second I used a ducting fan to blow air around the mount and OTA. I thought the turbulent air would make it harder for the condensation to form.
It reduced the condensation and it dissipated a lot faster, but it still formed.
My next step will have to be some kind of heat straps that I can wrap around the metal surfaces of the mount and OTA. Setup with a timer that is programmed to come on before dawn and turn off an hour or two later.
I don't really want to go down this path it just adds extra wires and clutter. At the end of the night I want to close the shutter and go to bed, I don't want to be fiddling around with heat straps.
So I am wondering what do other people do to prevent this from happening inside their observatory's?
Second I used a ducting fan to blow air around the mount and OTA. I thought the turbulent air would make it harder for the condensation to form.
It reduced the condensation and it dissipated a lot faster, but it still formed.
Now i don't own an observatory, so im sure some others have come across this, but from reading what you've said;
Sounds like your observatory is sweating. As you've deduced, the heat inside the obs is heating up faster than outside which is causing the cold air to drop its moisture as it heats up.
My thoughts are that you need relieve this build up so there is not such a large difference in outside air temp.
I am not sure how you would, but having the fan blowing air out of the obs and having an inlet for air somewhere else (lower is better for inlet, higher better for outlet as heat rises) may reduce or remove condensation. Simply set up a timer to start it at 5:00am (or whenever) and turn off at 8:00am or until outside temperature heats up.
Preventing direct sun contact with the obs will reduce the heat build up rate as well. Throwing a sheet over it won't help, it needs to be in the shade so an umbrella would be cheapest i would think. Not very practical though.
Another option is to throw in a desiccant bucket. We have some around our house to prevent mold, there a bout 1-2L and normally contain bally desiccant bits. Not sure where you can find them, maybe your local shops, or even Bunnings.
Only problem with them is that they will need to be replaced/have their moisture removed periodically. The wetter the environment, the more they need to be replaced/cleaned. You may only get 1-2 weeks out of it if it moisture in the air is severe.
I'm sure someone more knowledgeable will be a long shortly
Phil if its well sealed you could get yourself a cheap de-humidifier and put it on a timer if its really bad. I thought of this option if it was needed but so far I have had no condensation at all - touch wood.
Now i don't own an observatory, so im sure some others have come across this, but from reading what you've said;
Sounds like your observatory is sweating. As you've deduced, the heat inside the obs is heating up faster than outside which is causing the cold air to drop its moisture as it heats up.
My thoughts are that you need relieve this build up so there is not such a large difference in outside air temp.
I am not sure how you would, but having the fan blowing air out of the obs and having an inlet for air somewhere else (lower is better for inlet, higher better for outlet as heat rises) may reduce or remove condensation. Simply set up a timer to start it at 5:00am (or whenever) and turn off at 8:00am or until outside temperature heats up.
Thanks Nathan for the suggestions. I will try placing the fan so it is sucking its air from underneath the dome, this air should be a lot cooler. I can't use desiccant because I have a perforated floor, it is not a sealed environment.
I would have thought the warm air would rise and escape out the top of the dome, (there is a small gap at the top ) intern sucking in cooler air from the floor. I was hoping for a natural ventilation system. I thought this would elevate the condensation problem. Like you said maybe I need to force the air out?
Quote:
Originally Posted by Marke
Phil if its well sealed you could get yourself a cheap de-humidifier and put it on a timer if its really bad. I thought of this option if it was needed but so far I have had no condensation at all - touch wood.
Thanks Mark, that is a great idea but as I said above I have perforated flooring so I don't think it would work. I could put some covering down on the floor to seal it up but it would ruin the natural ventilation effect that I am getting. I would like to have this for summer.
I will have to experiment and see what happens.
I hope you don't have to deal with the problem it is a real pain.
Phil maybe your getting too much air flow ? The inside temp is dropping to dew point and causing condensation Increased air flow would help it dry out faster later but not stop it happening first. Just a theory
For what it's worth, Phil, I have a (homemade) observatory of galvanised iron sitting in an open paddock. The concrete floor is 100mm above ground level and covered with 10mm thick rubber (secondhand conveyor belting), the walls and dome I have insulated with 25mm polystyrene.
We get frequent frosts here with minimums below zero. I have had no problems with condensation, but cover the 'scope (8" Newt) and mounting with a light plastic motorcycle cover just to be sure.
Hi phil, I think Kevin is on the right track regarding insulation. I did the same to my roll-off observatory. I approached the local schools and asked them to save any of the packing foam that's used for whiteboards and such. It's about 5mm thick and water-based liquid nails is the go to stick it on. A BBQ cover over the instruments also helps.
Cheers!
Phil, my Ob has a sealed floor but natural ventilation around the roof edge, no fans, and I don't get the issue. I think you are maybe getting too much evaporation chilling as the sun comes up. I would try blocking the perforated floor for a starter which might also stop ground moisture coming up during the night and permeating everything. Cheap test fix with an old plastic sheet might help. I have a blue plastic tarp down first, then a sheet of card board just to protect the sheet then carpet tiles on top ( free chuckouts during work upgrade). No problems and Auckland is renowned for dew.
Thanks guys for your responses, your suggestions and ideas it is very much appreciated.
I did a test this morning, I opened the shutter of the dome just before the sun got to it.
It was -2C at 8am inside, the temperature increased to 1.5C over about 1 hour.
There was some frozen condensation on the upper wall of the dome. This was quite fine and wasn't enough to drip.
As I was sitting watching I noticed vapour going out the top of the dome through the open slit.
I had a cover over the scope and mount, so I uncovered the rear of the OTA and the counter weights to see if condensation would form on them.
I waited for about 1 hour and nothing formed.
I think what is happening, condensation is forming on the wall of the dome. When the sun warms up the dome in the morning the condensation evaporates and raises the humidity inside. There is nowhere for the warm moist air to go so it re-condenses on the cold metal surfaces of the OTA and the mount. Oddly enough there is no condensation forming on the cold metal floor.
So you guys have given me some ideas to try out. I will look at all of them and use a process of elimination to get the best result.
I am looking for the most simple fix but I am sure I will have to line the wall with insulation, cover the floor, add forced air ventilation, dehumidifier
and a desiccant bucket.
Metal roofs need sarking/insulation to prevent condensation forming on the underside of the sheets. Metal is often colder than ambient. Add to that humid air and the temp dropping below the dew point and you get condensation.
Put some sarking/insulation under the dome sheets to reduce it at source.
You do get condensation on your gear on really humid nights when the air temp goes below the dew point. That will vary according to location but where I am it happens about one night in 20. That is with the roof open.
I lose part of night to dew on the mirror about one night in 30 and only for a small part of the time.
My observatory is on a slab, I run a fan at the base of my scope to reduce local seeing effects (not sure it does anything), I have air cell insulbreak insulation behind all walls and roof.
My roll off roof observatory is probably in a colder place than yours and regularly has ice on the roof by 10pm.
The scope and mount will often be dripping by the time I finish. The mirror is dry due it heat straps etc. I just throw a cotton sheet over the mount and close the roof. It drys the next day.
Why are you worried about it?
Dont look at it a dawn and it won't worry you.
Turn the electricity off when you are finished and let it dry off.
I did another test this morning just to confirm what I had observed yesterday was repeatable.
I opened the shutter before sunrise again. It was -5C inside the dome at 7.30 am and warmed to a toasty 1 degree by 9am
No condensation formend on the mount or the OTA.
So my next experiment will be to have the shutter closed and exhaust the air from sunrise to about 10am and see what happens.
Quote:
Originally Posted by gregbradley
Metal roofs need sarking/insulation to prevent condensation forming on the underside of the sheets. Metal is often colder than ambient. Add to that humid air and the temp dropping below the dew point and you get condensation.
Put some sarking/insulation under the dome sheets to reduce it at source.
Greg.
I do have a barrier on the under side of the shutter. It is a product called Form Shield. It is a 10mm polyurethane foam with a foil surface on one side . I had to use contact adhesive to stick it to the zinc. I put this on to stop condensation dripping onto the Scope. It works quite well.
I had the thought of using this product on the wall of the dome. This product has to be stuck on with contact adhesive, which is no problem but it will make it very difficult for me to disassemble and unbolt the panels of the dome if and when I want to relocate. ( Not insulating the wall I think is the main cause of my problems.)
So I want to do the experiment above to see if it will work, if it does it will save me quite a lot of extra work and future disassembly problems.
Quote:
Originally Posted by Terry B
Dont look at it a dawn and it won't worry you.
I think that would have been the easiest option.
I would prefer to keep my gear dry if possible. I don't like the idea of water sitting around on the equipment.
Being inside a dome I do not get any condensation or dew forming while I am using the telescope it is very sheltered. It is only getting wet after the sun comes up.
Thanks Greg and Terry for you input, much appreciated.
I hate to be picky Greg, but this violates at least the first law of thermodynamics, unless something is cooling the metal, it cannot be colder than the surrounding air. It will FEEL colder due to its greater thermal conductivity, but it won't be physically colder.
The advice given is quite valid nonetheless, but I think Phil's problem is lack of air circulating through the dome, once the sun comes up and starts to warm the exterior of the dome, then everything inside will be colder than the dome roof, hence it will all collect water if there's enough humidity.
My roll off roof observatory, without any insulation, but with timber walls, doesn't seem to suffer, but there's quite a good gap around the top of the walls which allows for heaps of ventilation. Good at this time of year for getting rid of unwanted moisture, bad when it's windy and dust gets blown in.
... unless something is cooling the metal, it cannot be colder than the surrounding air.
Whilst I agree that most of the effect of "coolness" is due to thermal conductivity, the system is unlikely to be a closed system in equilibrium, and radiative cooling of the OTA may occur faster than convective heat transfer from the surrounding air. However, I would expect any temperature difference to be small - a fraction of a degree perhaps.
I'll defer to more learned opinions if someone can summon the appropriate maths and show what's going on ... beyond my semi-lucid mumblings.
rat156, no thermodynamic laws are being broken for an object's temperature to fall below that of its surroundings! The heat is radiated away from surfaces like metal faster than it is radiated/conducted to the metal. If the radiation heads upwards, much of this longwave infrared radiation then ultimately escapes to space (far from the immediate surroundings of the telescope), though some is intercepted by CO2 and H2O molecules in the atmosphere as part of the greenhouse effect. Because the lost heat isn't returned so effectively through radiation or conduction by the warmer surrounding air, the object cools a little below its surroundings. This is the reason you get most visible frosts, especially on cars or telescopes, and also the reason meteorologists distinguish between air frost (air temperature below 0C) and ground frost (air temperature above 0C).
From Wiki: "Hoar frost (also called radiation frost or hoarfrost or pruina) refers to the white ice crystals, loosely deposited on the ground or exposed objects, that form on cold clear nights when heat losses into the open skies cause objects to become colder than the surrounding air."
"Other objects on which frost tends to form are those with low specific heat or high thermal emissivity, such as blackened metals; hence the accumulation of frost on the heads of rusty nails."
Yes any object that is exposed to a hemisphere of cold dark sky will cool down lower than ambient air temperature. That is why frost occurs even when the ambient air temperature is above zero C on anything out in the open. The so called dew shield on your scope only slows down this effect by limiting how much cold sky your telescope 'sees'.
The mechanism is quite simple it is due to heat flowing from a hot body to a colder one by radiation. It is called supercooling.
A clear night sky can have a temperature far lower than ambient air temperature at ground level. So anything exposed to it will equilibrate at a temperature lower than ambient and if that is below the dew point you get condensation.
An uninsulated observatory roof will also cool below ambient and then start to cool what is under it below ambient.
It has not alot to do with ambient air temperature just the dew point and the supercooling of any object due to radiative heat loss to it's environment be it a cold sky or a cold observatory roof or dome.
I used to thermostatically heat my 300mm lens to about 16C in winter and 20C in summer 24/7 to stop condensation and focus shift. I am currently heating my RH200 to 17.0 +- 0.1 C so focus does not change and there is no chance of condensation. You would be amazed what nasty compounds are in the air from plants, animals, your observatory structure and yes you.
That nice blue air haze in the mountains is due to compounds from all the gum trees. Some are resin like.
I'm thinking a 50w solar panel, regulator, 12v 7AH sealed battery, 2w resistor, 12v fan, 12v relay and a motorcycle cover.
When the sun comes up the 12v relay kicks in and the low mounted fan blows out the dense moisture laden air and the panel charges the battery. When the sun goes down the 12v relay kicks out and a 2w resistor turns on and runs all night under the cover keeping the gear warmer than ambient (maybe!). Might be overkill but I'm going to give it shot. Bought the bits and will let you know how it goes in a month or two.
Cheers
Stephen
Thanks again guys, the physics lesson was very informative.
For curiosity sake I will attach some thermometers to the dome and mount and measure their surface temperatures. It will be interesting to see the difference between the air temperature inside, outside and surfaces.
Quote:
Originally Posted by coldlegs
When the sun comes up the 12v relay kicks in and the low mounted fan blows out the dense moisture laden air and the panel charges the battery. When the sun goes down the 12v relay kicks out and a 2w resistor turns on and runs all night under the cover keeping the gear warmer than ambient (maybe!). Might be overkill but I'm going to give it shot. Bought the bits and will let you know how it goes in a month or two.
Cheers
Stephen
Thanks Stephen I will be very interested to hear how you go. Nice Obs by the way.
I am wondering how many other observatory's suffer from this but no one has noticed?
Quote:
Originally Posted by avandonk
I used to thermostatically heat my 300mm lens to about 16C in winter and 20C in summer 24/7 to stop condensation and focus shift. I am currently heating my RH200 to 17.0 +- 0.1 C so focus does not change and there is no chance of condensation.
Bert
That is interesting Bert, what are you using to maintain the constant temperature?
It keeps the temperature of the RH200 within +-0.1C of set temperature. Friday night it was just below zero C in the observatory and the controller was at 40% power or about 40w and the RH200 was at 17.0C. This of course was distributed over five dew straps. These dew straps would be big enough for a ten or twelve inch scope.
