Dew!

[Andrew*]

wow. Mr Science. Sounds like worth believing to me!!

Basically it's not going to be too hot. Imagine using a hairdryer, which can make things as hot as 70*C from a foot away. It's not going to have that much impact by any means.

It might even be to cool to have any effect.

Just give it a bash and see.

If it makes a dent, don't come back to me!

Andrew

[FLO]

Good post CC

(I will need to print it and slowly digest!)

[Andrew]

Thanks CC - I've only just seen this! Very interesting

[The Warthog]

I was wondering about tube currents being induced by whatever heater you use, but I suppose that's prefereable to having a fogged-up secondary, and not being able to see at all, which has ended my night a couple of times.

[Kaptain Klevtsov]

The best plan of all would be to have the optics a fraction of a degree warmer than the air.

If the optics are colder than the dew point of the air then they will dew up. If the optics are warmer than the dew point then there will be no dew. If the air is at a temperature below the dew point*, then its foggy so game over anyway.

Getting the optics significantly warmer than the surrounding air will cause nasty air currents to spoil the view, but that's not the plan. Because the optics are solid, they are able to transfer heat out to space by radiation, whereas the air, being a gas, doesn't have that luxury. Gases only radiate heat very very slightly and so cool by radiation very slowly. The heater is intended to replace enough heat into the optics to counter the effects of heat lost by radiation and maintain the surface temperatures at close to the air temperature.

Only if the surfaces are cooler than the dew point of the air will dew form of the optics so if the optics can be warmed to even a half degree above ambient air temperature the dew will be prevented. If the humidity is low, the dew point will be less than the air temperature and no dew will form even if the optics are colder than the air temperature, provided that they are above the dew point temperature. Having the optics very slightly above the ambient air temperature will guarantee dew free observing.

* the air cannot be at a temperature below the dew point because dew would form, the humidity would therefore drop and the dew point of the air would change to match the air temperature. You cannot cool air below it's dew point otherwise you get a mixture of air at the dew point and tiny drops of water - fog.

HTH

Captain Chaos

[daz]

CC

How do you then calculate the dew point?

[Andrew]

re: tube currents from heating the secondary

I've recently seen tube currents resulting from heat from my hand rest on the top of the tube. I see no sign of tube currents from my secondary heater. Hopefully that doesn't mean it's generating practically no heat!

[Kaptain Klevtsov]

CC

How do you then calculate the dew point?

The dew point isn't easy to calculate, but it can be looked up from tables (Google Psychrometric Chart) and the like.

What happens is that the air contains a certain amount of "disolved" water, the ratio of this to the air is given as a ratio known as the absolute humidity. At a given temperature, air can hold a predictable amount of water, and the ratio of the absolute humidity to this value is given as a percentage known as the relative humidity. It is the relative humidity that is quoted on weather predictions.

Warmer air can support more water than colder air, so for a given water content, the relative humidity increases as the air cools. At the point of 100% humidity (as the air cools) the air is fully loaded, so to speak, and any further cooling results in condensation of water from the air. The air temperature drops and the relative humidity stays at 100%, but the absolute humidity goes down as water is condensed out of the air.

The condensation that we know and love (NOT!) occurs when the air comes into contact with a surface that is colder than the air, to the point of being below the dew point. The air is cooled where it is in contact with the optics, tube etc. and whenever the air is cooled below the dew point it deposits water by condensation.

So the point is that the air floating past is not at 100% relative humidity, unless it's foggy, so keeping the optics at the same temperature as the air will keep the dew away. You don't need to have anything glowing red hot. This is also why the fans on Newts work, as the heat is put back into the mirror by convection from the air fast enough to compensate for the heat lost from the mirror by radiation to the sky.

HTH

Captain Chaos