Dew on optics in a dome

[strongmanmike (Michael)]

Ok, had the scope in the dome for only a few days now and already I have noticed the optics fogged up when I have a look down the tube the next day the mount and camera were soaked too and this was 2pm. It is high humidity here in Canberra at the moment with fog until midday but I wasn't expecting this.

My dew abatement works fine during the imaging session but what do others do for keeping moisture at bay during the day inside a closed unattended observatory?

Mike

[h0ughy (David)]

Originally Posted by strongmanmike

Ok, had the scope in the dome for only a few days now and already I have noticed the optics fogged up when I have a look down the tube the next day the mount and camera were soaked too and this was 2pm. It is high humidity here in Canberra at the moment with fog until midday but I wasn't expecting this.

My dew abatement works fine during the imaging session but what do others do for keeping moisture at bay during the day inside a closed unattended observatory?

Mike

at the moment i am having the same problem. my novel approach at the moment is to leave both Pc's on to keep some warmth in there, but i was looking at a dehumidifyer. as you know mine is up on a timber deck, but yours in on a slab, so there is only one thing that is consistent - moist air

[strongmanmike (Michael)]

Originally Posted by h0ughy

at the moment i am having the same problem. my novel approach at the moment is to leave both Pc's on to keep some warmth in there, but i was looking at a dehumidifyer. as you know mine is up on a timber deck, but yours in on a slab, so there is only one thing that is consistent - moist air

I was considering getting lots of Silica gel bags and putting some in a plastic container with holes in it and lowering it down to just above the main mirror on some string after a night's work and leave it there inside the capped OTA when not in use. I could then remove it each session and change out the bags periodically..? Would that work? Where does one get Silica gel bags from anyway?

Then there are those moisture traps you can get from Wollies designed to go in damp areas of your house, could put a couple of those in the Obs too..?

I have no permanent mains power either

Mike

[h0ughy (David)]

i haven't explored the 12V fan theory yet with a solar charger to keep the air circulating?

[avandonk]

There is only one rule. You need to keep the temperature of your optics above the air dew point temperature.

Circulating air works fine if the air temperature is higher than the dew point temperature. When there is fog the ambient air is below the dew point.
Dessicant will do the job if there is enough in a sealed environment. It has to be regularly regenerated and is a pain to manage.

Supercooling is your main enemy at all times.

In Mikes case the observatory dome and to a lesser extent the walls radiate heat to a sky that can be far colder than ambient air temperature. This cools the observatory structure to below ambient air temperature. The contents in turn radiate their heat to the now cooler observatory dome and walls.

Condensation occurs when your optics temperature falls to or below the dew point temperature.


I heat my whole optical train to a set temperature that is thermostatically controlled. In summer 20C and in winter 16C. I never get condensation on the heated bits. This also means that focus does not change with ambient temperature. This is fine for a sealed optic.

In Mikes case I would put a dew heater strap around the tube near the primary mirror so the warm tube radiates heat to the surface of the mirror. This straps temperature should be controlled to be a few degrees above ambient air temperature. Same for the secondary and camera. The power needed is only tens of watts if you insulate the dew heater straps and the area being heated. I am going to get that space blanket aluminised mylar sheet that NASA uses on its spacecraft to replace the towel I am using. It reflects back 90% of the heat you would lose by radiation.


Heat will always go from a hot body to a cold one. To stop it or reverse it you have to apply energy in the form of heat or work.

Bert

[bert (Brett)]

Your concrete slab will release a lot of moisture as it cures. This can take upto a month.

[avandonk]

Originally Posted by bert

Your concrete slab will release a lot of moisture as it cures. This can take upto a month.

Concrete absorbs water as it cures. In fact in dry conditions it is kept wet by ponding or spraying. The setting is an exothermic reaction so it can drive off much needed water for the chemical reactions to occur.

Bert

[coldlegs (Stephen)]

Michael
I got one of these

http://www.ebay.com.au/itm/OZ-Auto-S...299bff4&_uhb=1

and a small 12v battery plus a 50W solar panel. Wired the "light" output to a 5w resistor that sits under the scope which has a motorcycle cover over it. During the night it supplies a small quantity of heat which seems to keep the humidity out.
As I'm about to get the obs wired for 240V I've decided to beef the system up so I can leave the camera attached. Looking at getting a humidity controller and wiring it to a 65w towel heater to maintain the temperature above the dew point.

http://www.ebay.com.au/itm/AC-220V-D...acbda1c&_uhb=1


http://www.ebay.com.au/itm/heated-to...f6a0263&_uhb=1

Cheers
Stephen

[marc4darkskies (Marcus)]

Yep, dome is no protection against dew. I leave my dew heater on after a session and put the scope "to bed" the next day (turn off the dew heater and cover up). I also turn on a small de-humidifier after a session to dry things off. This can take a while if the inside of the dome is dripping wet. In fact I run the de-humidifier all the time (except when I'm observing ) to keep humidity at 50%. In dry weather this is not expensive. I never want to have a mould problem.

Cheers, Marcus

[h0ughy (David)]

Originally Posted by marc4darkskies

Yep, dome is no protection against dew. I leave my dew heater on after a session and put the scope "to bed" the next day (turn off the dew heater and cover up). I also turn on a small de-humidifier after a session to dry things off. This can take a while if the inside of the dome is dripping wet. In fact I run the de-humidifier all the time (except when I'm observing ) to keep humidity at 50%. In dry weather this is not expensive. I never want to have a mould problem.

Cheers, Marcus

which dehumidifyer do you have Marcus?

[cohiba (Robert)]

Im not sure how big the Dome is I have a Sirrus 2.3mtr and had exaclty the same problem all the gear soaking wet. I bought from Bunnings a couple of Plastic Container things (Sorry dont know the name, you use them in caravans etc) that you fill with Silca Gell type crystal comes as a kit never had a problem since. I replace the crystals about every 3 months whats amazing is the amount of water collected in the containers. Give it ago they are only cheap before spending a fortune on alternative solutions

Bob

[Joshua Bunn (Joshua)]

Mike,

You can get some silica gel from here
I turn on a dehumidifier after an observing sesion, works well. I also leave the mirror heaters on, on the secondary and primary mirrors and turn them of the next day.

Josh

[allan gould]

Originally Posted by marc4darkskies

Yep, dome is no protection against dew. I leave my dew heater on after a session and put the scope "to bed" the next day (turn off the dew heater and cover up). I also turn on a small de-humidifier after a session to dry things off. This can take a while if the inside of the dome is dripping wet. In fact I run the de-humidifier all the time (except when I'm observing ) to keep humidity at 50%. In dry weather this is not expensive. I never want to have a mould problem.

Cheers, Marcus

I do exactly the same routine now and have no problems. Don t bother with silica gel unless you bag the scope tightly as moisture will still ingress and you will expend more power and effort removing the water from the silica gel than you would have if you left your dew heaters on all night.
Allan

[gts055 (Mark)]

I am wondering if it would be safe to use a single bed electric blanket draped over the telescope. Apparently they draw around 15w on low to 70w on the high setting. Is this a crazy idea? Mark

[Joshua Bunn (Joshua)]

Originally Posted by gts055

I am wondering if it would be safe to use a single bed electric blanket draped over the telescope. Apparently they draw around 15w on low to 70w on the high setting. Is this a crazy idea? Mark

Personally, i wouldn't do that as it traps the moisture under the blanket (and on your gear). We really need to keep the moisture away from the gear.

[gts055 (Mark)]

Hmm, I thought if the tube and optics (lX200 14") were kept above ambient, then condensation would not occur on the telescope. I would leave the diagonal open and vented with a gauze cover. Foil backed bubble insulation could be put over the electric blanket to retain heat ? Mark

[Joshua Bunn (Joshua)]

Originally Posted by gts055

Hmm, I thought if the tube and optics (lX200 14") were kept above ambient, then condensation would not occur on the telescope. I would leave the diagonal open and vented with a gauze cover. Foil backed bubble insulation could be put over the electric blanket to retain heat ? Mark

Are you talking about leaving the blanket on the scope full time?

[gts055 (Mark)]

Yes, I would leave it on continuously, to maintain the telescope above dewpoint. What do you think? Mark

[Joshua Bunn (Joshua)]

Originally Posted by gts055

Yes, I would leave it on continuously, to maintain the telescope above dewpoint. What do you think? Mark

Yeh, somehing like that would work. An electric blanket would probably be a little overkill i would have thought and need some regulation as well. The bubble wrap is a good idea.
Josh

[marc4darkskies (Marcus)]

Originally Posted by h0ughy

which dehumidifyer do you have Marcus?

I bought from Darryl a few years ago at

http://www.dampsolutions.com.au/stor...rs-home-office

I'd spend $300 to $400 at least. Make sure it has continuous draining option - you can extract several litres of water from the air in a day or two so unless you continuously drain you'll have to empty the tank often. You also want a humidistat.