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woodblock

Heated observatories

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I haven't got an observatory but I'm thinking about building one. I keep my telescope in a shed and I have to take it out and set it up outside somewhere. The shed is not heated. It gets quite cold in there but never drops below freezing inside. Last night's observing was cut short because of dew which settled on everything after about 30 minutes. I know we are supposed to let the temperature of the scope settle down before doing any serious stuff. But if the scope is already cold when you start to use it then when you take it outside or open the roof isn't the temperature of the scope  going to fall quickly to a point where dew forms?

So I wondered if people heat their observatories.

Cheers

Steve

 

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I think the usual solution is dew heater bands on the appropriate optics, but the obs itself at ambient temperature

Everything ends up soaked at the end of a session, so a dehumidifier (or very good ventilation) reduces the risk of mould and rust when you close up

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25 minutes ago, Yawning Angel said:

I think the usual solution is dew heater bands on the appropriate optics, but the obs itself at ambient temperature

Exactly this.  In the short term, gently wafting a hair dryer over dew-covered optics can help.  But ultimately dew heaters are probably the best solution.  I've just put them on my OTAs and now I can be using the scopes for hours non-stop without dewing up.

James

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Thanks everyone,

I just can't quite get around the idea that you want the temperature of the OTA to even out but at the same time you put heaters in it. That seems to be the opposite of what you want.

Anyway, I get the idea. I think I can get away with a heater on the secondary and the finder. The main mirror doesn't dew up. The problem is mainly with the secondary.

Cheers

Steve

 

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The principle is that you keep the optical surface just above the dew point temperature, so it doesn't fog. The air volume inside the tube isn't heated, and so is close to ambient temperature, reducing air currents in the tube

I'm not a reflector user, but you'll likely want to heat the secondary, the finder and maybe the eye piece

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3 hours ago, woodblock said:

I just can't quite get around the idea that you want the temperature of the OTA to even out but at the same time you put heaters in it. That seems to be the opposite of what you want.

In a way it is the opposite of what you want.  The problem is that the optics get so cold that they cool the air close to them.  The colder it gets the lower the capacity of air to hold water vapour so it starts to condense out on the surface of the optics and the way we avoid that is to try to raise the temperature of the optics to the point where they're just warm enough for that not to happen.  As Alex says, you still want as much as possible to be at a uniform temperature to prevent convection currents in the air causing distortion of the image, but a dewed-up telescope isn't very useful either :)

Managing temperature in telescopes, particularly large closed tubes can be awkward.  For example, some people have taken to wrapping large SCTs in "bubble wrap" insulation because under a clear sky the side of the tube facing the sky loses heat faster than the side facing the ground, which means the air near the top of the tube cools faster than the air near the bottom, in turn causing air currents.  By adding the insulation they hope to even out the cooling to prevent that happening.

James

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My obsy is heated to 7c above ambient by the dehumidifier which runs 24/7 set at 60%.   It's only switched off when I open the roof.

Here you can see when I entered the obsy and had the light on while I switched the dehumidifier off and opened up.   The temperature drops from 7c to -0.5c over three hours and the humidity rises to 89%.  The outside temp was 2c at the start and -1c when I finished and humidity rose from 84% to 96%.

Without dew bands I wouldn't have lasted more than an hour but even at midnight the optics were clear when everything around me was soaking.  Heating the optics (and the diagonal if I'm visual) by a degree or two the whole time the obsy is open is pretty much essential for me.

 

THL.png

17 Jan to 25 Jan

Edited by LongJohn54
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Excellent stuff LongJohn. Looks like I'd better invest in some dew bands.

 

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I will be installing a bathroom extractor fan in mine this weekend, it has been very damp this winter and I have developed some mold in the obsy. I am also going to get a UVC bulb to run for a couple of hours once per month.

Personally I think that warm and damp is much worse than cold and damp. I also dont think that there is much point in a de humidifier in a role off roof as you are unlikely to have a sufficiently good seal.

So i would just look at ways to keep the air moving.

Adam

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45 minutes ago, Adam J said:

I will be installing a bathroom extractor fan in mine this weekend,

I've fitted a USB fan in mine moves plenty of air, my roof is well sealed as it's the original Keter shed bolt down, bit of gap round the doors.

Dave

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The whole business of telescope temperature may be not quite as it is usually described. There is no need that I can see for a closed-tube instrument to be at ambient temperature. It's just that when it is at ambient temperature there should be no tube currents. However, tube currents can also be prevented by insulating the tube, as James mentioned, so that its internal temperature is uniform and constant. That's the bit which matters. I often wonder if we might not be able to make a slightly heated and temperature controlled 'jacket' to put round our closed-tube 'scopes to ensure that the main tube is at a uniform and constant temperature slightly above ambient to prevent dewing.

For imagers there is also the mechanical aspect of instrument temperature to think about. As the fabric of the scope cools it oontracts, so altering focus. A visual observer just gives it a tweak but an imager using long subs may find it drifting during an exposure. For this reason I run the dew heaters on my fast refractor in summer, not to prevent dew (there isn't any here in summer) but to slow down cool-down and, therefore, focus drift.

While a dose of hair drier will clear a lens the effect on image stability it utterly catastrophic and the dew is back before the turbulence has settled! A far better remedy, in the absence of dew bands, is to drape a warm dry towel over the optics and give it five minutes. But dew bands just work.

Olly

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While what @ollypenrice says is true for the telescope,  don't forget a heated Obs will loose heat from its structure and this will cause convection currents which will degrade the local seeing.

Regards Andrew 

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1 minute ago, andrew s said:

While what @ollypenrice says is true for the telescope,  don't forget a heated Obs will loose heat from its structure and this will cause convection currents which will degrade the local seeing.

Regards Andrew 

Indeed. You can't heat an observatory while observing from it.

Olly

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It is best not to seal anything.  You need airflow under the obsy floor so allow 150mm above the concrete base before running the floor.  Concrete doesn't fully dry in a lifetime according to my civil engineer friend.  I used a roll off roof design and the overlap had a 25mm gap to allow airflow at wall height too.  In this way everything airs off very well, no rust, mould or condensation.  Dew bands are a must if you have a lot of exposed glass like my 9.25 SCT.

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