Dew on optics in a dome

[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 :shrug: 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]

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]

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]

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]

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

[avandonk]

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]

Michael
I got one of these

http://www.ebay.com.au/itm/OZ-Auto-Swich-20A-12V-24V-Solar-Regulator-Panel-Charger-Battery-Controller-Light-/370752995316?pt=AU_Solar&hash=item565299bff4&_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-Digital-Air-Humidity-Control-Controller-Temperature-Sensor-Meter-2-Probe-/130909198876?pt=AU_B_I_Electrical_T est_Equipment&hash=item1e7acbda1c&_uhb=1


http://www.ebay.com.au/itm/heated-towel-rail-new-in-box-/190847779427?pt=AU_Building_Materia ls&hash=item2c6f6a0263&_uhb=1

Cheers
Stephen

[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

[h0ughy]

which dehumidifyer do you have Marcus?

[cohiba]

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]

Mike,

You can get some silica gel from here (http://www.silicagel.com.au/categories/Silica-Gel-Dehumidifiers/)
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]

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]

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]

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]

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]

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

[gts055]

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

[Joshua Bunn]

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]

I bought from Darryl a few years ago at

http://www.dampsolutions.com.au/store/dehumidifiers/dehumidifiers-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.

[strongmanmike]

Thanks for all the feedback guys much appreciated :thumbsup:

The AG12 has a built in 12V tube heater moulded directly into the tube just in front of the primary, combined with the rear extraction fans and secondary mirror heater pad, I have no problem with dew during imaging sessions, not even on front corrector element that is exposed to the inside of the tube.

I had thought of leaving the tube heater on all day but it is 12V so batteries will go flat and I am not sure about leaving anything electrical like that on 24/7..? If I had mains power connected I could put its power supply on a timer and it might be the way to go, I could set it to come on for a few hours each day, likely in the mornings..?

So, until I have mains power I have ordered two rechargeable desiccant boxes (http://www.silicagel.com.au/products/Hydrosorbent-Silica-Gel-Dehumidifier-450-grams.html) designed for use in a much larger space than the inside of the AG12, I will place one in the tube after an imaging session and put the cap on the OTA, I will place a bag over the rear end of the OTA where the three fan outlets are and this will then become a fully enclosed space. When the box needs recharging I will just pop it in the oven and put the fresh one in the tube to replace it. I will also get a couple of the items Bob mentions and place them in the observatory.

I'll reassess as time goes by and should I get mains connected I will likely go down the dehumidifier on a timer option :thumbsup:

Thanks again guys

Mike

[Tandum]

I don't get any of this in my foam dome. My problem has always been keeping rain water out. That's what ya get when a technician does building work :)

[DavidTrap]

Mike - what sort of desiccant are you getting.

I had some blue desiccant that could be recharged, but apparently the blue stuff uses cobalt and carcinogenic.

I bought some silica gel in 100g packets on eBay for $3-5 each. I've but a bag in every box of gear I have. They can be recharged in the oven, although I'm yet to do that - apparently you can weigh them and see if they need recharging.

DT

[Nico13]

Hi Mike,
A mate of mine (not on the forum) has used the electric blanket method for years and swears by it and his setup is in a plain tin shed with roll off roof.

[strongmanmike]

Hi Dave, as per the link in my post, this one (http://www.silicagel.com.au/products/Hydrosorbent-Silica-Gel-Dehumidifier-450-grams.html)

Sounds like a good idea Ken but again having something electrical on all the time worries me a tad... :question:

Mike

[Tandum]

I thought people were installing these solar exhaust fan (http://www.ebay.com.au/itm/Solar-Powered-Vent-Extractor-roof-or-wall-mounting-Exhaust-Fan-/221089300886?pt=AU_Solar&hash=item3379f30196&_uhb=1)s to equalise things.

[strongmanmike]

Yes they look good and I imagine this might be good for removing heat from an observatory but I wonder how effective it would be with reducing the internal humidity if the outside humidity is high...?

Mike

[kinetic]

+1 :)

Steve

[bert]

Where does all the water go that was in the wet concrete mix bert? Its released into to the air, in Mikes case into the dome, which was my point of my post. And this lessens over time as the cured concrete dries.

Its true that concrete keeping concrete wet will make it stronger, also water can be helpful in cooling the concrete in Higher temperatures, where the exothermic reaction heats the concrete as it hardens.

I have seen the effect of condensation first hand in building where the concrete is still 'green' ie not fully cured. Even with sisalation and other methods of condensation control, ends up raining inside every morning as the concrete releases moisture. This effect leesens over the next few weeks.

[DannyV]

Hi All,
I'm running a small aircon duct from my room in the house with an internal fan to the dome that I'm building.
It used to be from my shed A/c for summer, but I'm changing it now winter's on the way.
The fan will have a speed control to just keep it ticking over, but i'll fill the little dome with enough room air. The dome will be sealed as much as possible - just leaking a bit here and there. My dome is made from 40mm polystyrene and silver foil.

[Logieberra]

I purchased a Delongi dehumidifier from Harvey N in Fyshwick. I have it set on a timer, 'on' at 8-830 each AM. Surprising how much moisture it collects from the astro shed. Very effective after a night open to the Cbr air. Love it.

[strongmanmike]

Hi Logan

yes I will likely do the same if I get 240V connected...because we are renting and digging some long trenches though garden beds etc isn't realistic or appealing, I'm thinking of simply running an extension lead through a garden hose and leaving it out on the ground 24/7..?

Mike

[marc4darkskies]

That's thinking outside the box ... errrr ... observatory! :thumbsup: Frankly, a dehumidifier is the only real / complete solution to your problem. Otherwise your obs will be permanently dank.:fishing: (always wanted to use that emoticon)

[mswhin63]

Hi Mike, I too live in a rental premises and I can't do anything below ground, even more so in my situation. I am investigating some low power ideas to get all the features including dew capture as well, I am hoping your consideration with the packet moisture collection products work out.

Mainly my solutions will be through solar power. I am not sure you have looked it this already but I hope to make it so that I can move everything along with the electronics as well. Maybe a idea to look at some auctions to get cheap panels. Or maybe the offers that are going around for cheap solar installations. Should be enough for an observatory. Mine will be designed around low power Beaglebone, Arduino and FPGA technology.

[Phil Hart]

We (the ASV) purchased a dehumidifier for our dome once we hit this problem. Once you take the plunge, you need a dessicant dehumidifier as standard models are not effective in cold temps. We got this one:

http://www.dampsolutions.com.au/store/dehumidifiers/dehumidifiers-home-office/8L-low-temp-desiccant-dehumidifier

Seems to be working well but have had to extend the hours of operation on the timer. It's a bit of a battle in Victorian conditions but it must be helping.

Markus.. how well sealed is your dome? We don't get ours down to 50% very easily but the problem is that it's 2 hours away so I don't get to check in on it very often, and when I am there it's clear and we've got the dome open and are imaging.

Mike.. I think you will have to get over your fear of living power on.. there's a lot to gain.

Phil

[marc4darkskies]

Not very well. But even on the wettest nights (dripping down the walls inside the dome after a session), everything is dry by late morning after buttoning up around 3:00am. It's also not very effective below 5 deg so I'm eying some of the newer models. Also during prolonged wet weather, it will humidistat off and on with it set at the driest setting (40 or 50%). I've been thinking of sealing around the door and the pier but that would leave the some gap around the dome circumference and the the slit so it would be only marginally effective at saving a bit of electricity.

[Paul Haese]

Clayton Bay is the dew city of the entire solar system. I gauged 100mm last year in dew. Summer and winter the only time dew does not occur is when the wind comes from the north. Totally bizarre. I use a dew heater on both scopes and that is all. Dew drips off everything. Next day I open up the observatory for a bit to get the heat in again and that is all I do. Nothing is rusting or corroding. I would not bother too much. Canberra is very dew too as Anthony and I have talked about this issue many time. Personally I would not bother with dessicant as you will be re-charging it every couple of days. Your gear is built for dew and will last out the dew better than you might think.

[strongmanmike]

Cheers guys

Thanks Paul, yes, as I said, my dew abatement during imaging is working fine, it is just the coming and going of moisture in the closed observatory over time that I am concerned about, mould on the optics etc, I think tackling it in some way is probably worthwhile. You may eb right about the desiccant, we'll see :question:


Mike

[strongmanmike]

Well....I opened up tonight after last imaging a week ago and the focus was perfect, no adjustment needed, I did a goto the same target NGC 6164 and after a couple of short exposures and slews I was on frame within a couple of pixels, easy...bah, who needs plate solving :lol:

Observatory life for me :thumbsup:

I do have to be very careful moving around inside the dome in the dark though, not much between my shoulders and the counterweights :scared: :lol:

Having no breeze to deal with makes it a little more comfortable too. I set it for 1hr worth of exposures, position the dome to give maximum travel across the opening and come inside, so far seems 1hr is as far as I can stretch it and I go out at the 45min mark just to be sure ...yes this will become annoying I guess buuuut baby steps... :)

Mike

[BC]

I notice a couple of people in this tread have mentioned a lack of dew with a foam dome. Would there be some benefit in lining an obs with foam? Geodesic triangles in an existing dome, of flat sheets in a ROR. Would any form of insulation assist (not sure if Mike's dome has this)?

Bruce

[Logieberra]

Mike, if you go dehumidifier route - consider a DeLonghi (Type CF08E). I grabbed one from the Harvey Norman Bargain Basement in Fyshwick after a glowing report from CDKPhil on this forum. We grabbed the factory refurbed ones and saved quite a bit.

He uses the external water removal via user supplied tube. I use the internal bay only (both are possible, see pics). Many settings, speeds etc. They don't miss a beat.

A few pics here:

[Logieberra]

In the 2nd pic, that yellow thing to the left is an auto-cutoff feature. The foam raises with water level and turns it off before overflow.

In the 4th pic, that disc can be pushed out to provide access to the water tube if you wish to re-route outside the observatory (5th pic).

The only caveat - a small modification is required to run the unit on a timer. Normally, you would have to manually press the 'on/off' button (1st pic). As said before, we use a timer to automate dew removal. A small soldering job fixes this and means that when the timer powers the unit, the unit turns on without user interaction. A simple 5min soldering job, easily reversable and does not damage the unit.

[strongmanmike]

Nice one Phil

The operating temperature for that model is +2C to +38C, many days in Canberra it is below 2C for half the day..? It might need a temperature operated switch incorporated..? I think the desiccant models still work when it's colder..?

Mike

[Logieberra]

During operation it raises the room (or observatory' s) temp slightly. In a small, contained space this makes quite a difference. No issues to date, regardless how cold the night.