Hot AND cold ?????

[Rooster]

Silly question been nagging at me...

Most accept that your 'scope should be cooled - right ?

Then you add a dew heater and warm it up again !

What gives ?

[nicnac]

the ones for the EPs will equalize the temp to stop fogging due to your eye temp

and i dont bother with the ones for the OTA, my dew shield is good enough, so i couldnt answer that one for you

[brantuk]

The thing is it's not a heater like a radiator. The cooling settles the air currents in the tube and the dew band keeps the mirror just at or above the ambient temperature to prevent dew forming on it's surface (as I understand it). Half or one degree is usually enough to stop the dew. Sometimes body heat just standing next to the tube can affect the air inside it

[Demonperformer]

Rooster, you don't say what type of scope you have.

If it is a refractor, or an sct/mak, a simple dew shield may be enough (I got a piece of black card from Hobbycraft and simply use a rolled piece over the front of the scope).

If it is a newt of some sort, I have no experience of these so cannot help.

[Bizibilder]

A quick explanation of Dew and Dew point -

The air, a mixture of gases, also contains water vapour (ie water in the form of a gas). At any particular temperature the amount of water vapour in the air, which is variable, is called the "humidity" often expressed as a percentage (%). Zero percent has no water vapour at all in the air and 100% is air that is said to be saturated with water vapour (this saturation level varies with temperature. It does NOT mean the air is 100% water vapour!! just that the air can "hold" no more water vapour at that particular temperature). Again, at any particular temperature, there is only a certain amount of water vapour that the air can "hold".

The warmer the air the more water vapour it can hold.

So take some nice warm afternoon air that contains a reasonable amount of water vapour. Allow that air to cool after sunset and it will eventually no longer be able to hold that amount of water vapour and that water vapour (gas) will condense (become liquid) into dew. This temperature is called the dew point. Just to complicate things the dew point is a different temperature for each different level of humidity of the air! -- Humid air, which contains more water vapour, has a higher dew point temperature than air that is less humid.

Telescopes (amongst other things) complicate matters by often being slightly cooler than the ambient temperature (by radiating heat up into the sky). So the air "next to" the telescope may well be below its dew point and liquid water (dew) will start to condense out of the air all over your telescope!.

Warming the telescope slightly (only maybe 1° or so) is enough to keep its temperature above the dew point and thus prevent condensation and hence no dew on the telescope!!

Not very scientific but I hope the above helps!

[mark7331]

Bizbilder thanks for that - it's a really clear explaination.

As far as dew shields for newts I use a camping mat (£5 ish) which protects the secondary mirror at the top of the OTA.

[glowjet]

Excellent explanation of the eternal dew problem, thanks.

[Rooster]

All good stuff ! So why let the scope cool AT ALL then ? Soooo confusing ! lol

(BTW I have a 102mm refractor)

Not that this problem affects me at the mo ! (been ill for months... now I back on my feet(ish) theres nowt but rain :-( - Just a curious sort, and the problem was nagging at me !

[brantuk]

Refractors - being effectively sealed at both ends - take a bit longer to cool down than open ended scopes because heat transfers through the tube and glass.

All materials suffer expansion and contraction due to temperature. This will cause minor distortions on optical surfaces like mirrors and glass. Once at a steady temperature the scope won't change shape any more.

Applies especially when moving a scope from inside a house to outside

[anthony]

Nice explanation Bizibilder!

[Talitha]

Rooster, have you ever seen heat waves rising from a rooftop, concrete sidewalk or asphalt pavement? These heat waves are thermal disturbances (please everyone, no thermal underwear jokes... they're so lame ) rising into the air, dispersing the heat from it's source until the object reaches an ambient temperature. We've all seen birds circling in the sky, gliding for quite a long time without needing to flap their wings. They're actually being held aloft by these heat currents rising from the Earth's surface. The proper term for it is 'Riding the Thermals'.

Now imagine that you've just brought your scope outside from the house into the cool night air. The warmer optics will cause thermal disturbances until they're cooled off enough.. you won't see heat waves, but the view won't be sharp as long as these thermal disturbances are present. This is why it's necessary to let a scope 'cool down' to the ambient temperature before you begin viewing.

The dew problem's already been explained, so i won't go into that part of your question. I hope this helped.