Dew or dew not...there is no try...

[Stonius (Markus)]

So as far as I understand it, dew forms on scopes at night essentially because the telescope tried to reach thermal equilibrium with both the ambient air temperature AND the radiated temperature going out into space (like the inverse of having your scope out in the day and the sun making it hot).

The heat that gets radiated out to space drops the temperature of the scope below that of the surrounding air, and if that point is below the dew point, water condenses out onto the scope. Please correct me if I got that wrong.

So my question is when is the scope at the most thermal equilibrium?

I mean we cool our scopes for as much as an hour to remove tube currents and give us the best views. So is a telescope that is naturally cooled at thermal equilibrium and therefore, has the least tube currents in spite of the fact that the presence of dew would require a thermal gradient between the scope and the local air? Or do we sacrifice a little stability for dew heaters? Or are they simply bringing the scope optics up to parity with the surrounding air (the radiated heat being irrelevant for local thermal currents?).

Cheers
Markus

[billdan (Bill)]

Markus,

All I can contribute is, when the primary mirror starts to dew up it means it's time for bed. A hairdryer poked in the tube only gets you another 5 minutes before it dews up again, so I don't bother now I just shut it all down.

But everything that you described is correct.

Cheers
Bill
Edit: The dew heaters are not really hot enough to give thermal instability, only enough heat to just get over the dew point.

[Stonius (Markus)]

I figure if they gave terrible seeing people wouldn't use them. But I'm wrestling with conflicting concepts here; 1) warm telescope = tube currents = bad. 2) Got dew? = heat telescope! = good!

Obviously, it's a matter of degree (haha, no pun intended). But it makes me wonder what the optimum is. Does a dew heater lead to fewer tube currents or vice versa? I know in practical terms it probably doesn't matter that much but I'm curious as to where the optimum lies.

Markus

[csb (Craig)]

Have you thought about using a dew shield? No power needed.

Shields can extend your viewing time, depending on weather conditions, by a couple of hours. Cheap to buy or easy to make too.

[Stonius (Markus)]

Oh yes, I'm aware of that, just exploring the physics involved as a little thought experiment. :-)

[mental4astro (Alexander)]

Hi Markus,

Have a read through the article I wrote on Newtonian cooling. It also goes over dew control as the two are closely related. I covers the use of fans and heaters, their best practice & incorrect applications:

http://www.iceinspace.com.au/63-771-0-0-1-0.html

I don't always look to control dew formation on my optics. Many times I let things just happen, and just call it quits like Bill does - it also means everything is also getting wet, soaked and uncomfortable. Other times, for increased productivity, yes I implement some dew control measures.

Alex.

[Stonius (Markus)]

Thanks Alex, that article is exactly what I was looking for (nicely written too:-). It seems that heat needs to be done properly if at all. I've never had much cause to control dew as my scope designs have not required much intervention, but I was curious as to the plethora of optic heating accessories and how that plays out.

I especially liked the following quote;

"Heat can be used on scopes, but it needs to be carefully employed. Secondary mirrors benefit from gentle heating as it eliminates potential problems from fan vibration. But heating a secondary is more complicated than just sticking a heater onto it. The secondary mirror holder plays a vital part in this. The wrong holder and it will render any heating efforts futile. Heating of the secondary is very effective as it is small and the heating system can be designed to quite uniformly and gently heat it. Heating of the primary mirror I would suggest is not the best option, for the optical problems un-uniform heating creates is the same as for fan cooling."

Made a lot of sense. Thanks a lot for the heads up about the article. Very informative! :-)

Cheers

Markus

[MortonH]

I thought the title of this tread was:

Dew or dew not...there is no dry.

[Stonius (Markus)]