Heating scopes to prevent dew formation.

[Macavity]

Something (of the many things) I've never really understood. If scope cooling is important, the influence of WARM dwellings / observatories etc. is a big contribution to degradation of seeing: How come HEATED dew prevention schemes seem to be OK. Not a test or anything, but...

[Psychobilly]

AFAIK .. its all about putting in the minimum amount of heat to just stop the dew forming ... I run my deheaters from the satrt of the session rather than turning them on when it satrts to form as i reckon prevention is better than cure.... if you can get it right i don't think it causes to much thermal disturbance...

Allowing the scope to cool IIRC is to reduce the tube currents...

Peter...

[kev 102 42]

What does IIRC mean,Thanks...............kev

[Psychobilly]

If I Remember Correctly... it means "If I Remember Correctly"

Did you get out lastnight Kev?

[kev 102 42]

no it was misty, hope to get out to night..........kev

[Macavity]

Yeah, I suppose that's the thing - You ain't trying to fry an egg!

I was reading an interesting article in AN about such things. Indeed, prevention does seem to be better than cure, as also learning how to "drive" the thing (tape heater). It is something to ponder - I never seem to get on terribly well with passive (commercial or home-brew) Dew Shields alone.

[Dante]

Hi Gents

I have just returned from a holiday on the Lizard (Cornwall). There were a few nights when the sky was simply awesome. I'm a newbie by the way and after getting my scope set up and aligned (miracle that) I had a tour and then decided to go for a planet. Saturn was chosen. Joy oh Joy my Goto went straight to it. I put in a 2X and had another look it was great. Decided then to use my web cam so set about getting that ready. By the time I did - DEW! it was like running water. Everything was dripping, scope, eyepiece, powerpack, PC the lot. Viewing over.

Any guidance on preventing dew on the eqpt.

Cheers

Danny

[JAO]

Running anti dew systems is a bit of a balancing act. You want the optics cool but not so cold that dew forms. Last night was really dewy here in norfolk so I had the Dew heater on my secondary reasonably high. However as the night wore on I realised although the secondary was dew free I needed to turn the heater down as there was a slight degredation of the image. I find on very dewy nights there ia degree of trial and error.

[brianb]

You want the optics cool but not so cold that dew forms.

Indeed. The point is that the objective, being open to the sky, tends to cool below ambient so that dew can easily form on it. You get tube currents when there is a temperature differential, so warming the objective so that the heat from the dew strap exactly balances the loss by radiation gets everything to the same temperature, improving the image rather than worsening it.

All a dew cap does is slow the radiation down, that's why they're only effective for a certain time.

Getting the right balance between heater & radiation is an art form, made a bit less tricky but not entirely solved by "intelligent" controllers with built-in temperature sensors - but in practise you only need to get fairly close to get reasonably steady seeing (given the state of the atmosphere outside the scope) together with dew free optics. But planetary imagers working in very steady air tend to use neither heater nor dew cap, giving a gentle burst with a hairdryer sufficient to just evaporate the dew when it forms. It's an effective technique but wouldn't suit long exposure imagers, and also uses far more battery power than continuous gentle heating by means of a dew strap.

[brianb]

Any guidance on preventing dew on the eqpt.

The scope needs a dew cap and heater straps for objective, eyepiece & probably finder too. PC goes in a cardboard box which also acts as a glare shield. Keep it running, it should generate enough heat to keep itself dry. In British conditions you probably need ~70AH capacity to keep a mount, dew heaters for a scope and a PC running all night. A "leisure" (camping/marine) deep cycle battery is the best way of powering it.

[Dante]

The scope needs a dew cap and heater straps for objective, eyepiece & probably finder too. PC goes in a cardboard box which also acts as a glare shield. Keep it running, it should generate enough heat to keep itself dry. In British conditions you probably need ~70AH capacity to keep a mount, dew heaters for a scope and a PC running all night. A "leisure" (camping/marine) deep cycle battery is the best way of powering it.

Hi

Thanks for the tips. As most of discussion relates to reflectors etc I was wondering if it still apllies to a 70mm refractor. The PC in the box is certainly worth doing still not sure about the rest though.

[Psychobilly]

Still worth doing with a "frac" ... They dew up as well....

Peter

[Dante]

Still worth doing with a "frac" ... They dew up as well....

Peter

Hi again peter

Dewing ? Whilst in Cornwall my kit looked like I had thrown a bucket of water over it, and the skies were simply awesome! It's somethiing that I simply must sort out for the future.

Danny

[Psychobilly]

yep... a dew shield just delays the inevitable....

Peter...

[eye in the sky]

I dont know if i should or not but i use one of them homebrew heaterbelts for my dew problems on my lx90. It does the job at the fraction of the cost of a dew heater.

dave

[brianb]

a dew shield just delays the inevitable....

but the delay depends on the length of the dew shield. Most are far too short. If 3.5 to 4 times as long as the diameter, and made from non conductive material (waterproofed cardboard, craft foam etc) they will be adequate for small refractors, finders etc.

But such a long dew shield is too clumsy for a Mak or SCT, and in any case will not stop eyepieces fogging over.