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Steve-G

A quick question about dew & condensation

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So far, I haven't had chance for any prolonged sessions outside, usually only about an hour or so at the most. By this time the condensation build up has pretty much spoilt my fun anyway. I've been giving my scope a good hour or so to cool down before use & have a dew shield on the way which hopefully will help but was wondering if the condensation eventually dissipate over time or is it pretty much permanent once it's started to accumulate?

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Once it appears it's there for the night dew heaters are a small fortune or as cheap as chips the DIY route is cheap but a proper brought controller and band is invaluable. Invest now as once winter comes it's doom ,summer as well red hot one minute cold the next what size scope do you have ?

Pat

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Dew forms when air, saturated with water vapour, comes into contact with anything that is below the "Dew point" ie at a lower temperature.  Cold metal scopes are notorious for doing just that!  The best way to get rid of dew is to warm the scope until it is just above the Dew point and no dew will form.  This sounds like the opposite of the general rule to "cool things down" but it isn't.

Generally you need to cool your scope from "room temperature" (as you store it indoors) to outdoor ambient temperature.  This outdoor temperature may or may not be below the Dew point on any particular night.  If it is above then no dew will form, if below it will.  Often all you need to do is warm the scope a couple of degrees or so.  To do this the most common piece of kit is a dew heater - a band that is electrically powered and which wraps around the objective of your scope and warms it up (ever so slightly!).  This will cause this part of the scope to raise above the dew point and the dew will evaporate.

Unfortunately the dew point is a moving target and depends on the humidity level of the air - warm air can hold more water vapour than cold air - so you can't say at exactly what temperature the problem will occur.

Hope this helps.

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when you leave the scope out to cool, point it down wards so damp air is less likely to collect on the scope aperture, a battery hair dryer can be a help

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nightfisher I'm STILL looking for a battery operated hairdryer for that purpose. Do you have one?

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A quick point on any hair dryer is make sure the back as no dust there's usually a fine mesh at the back that collect dust some times that can blow through and land on optics dried on is a real pan Hoover the dryer first

Pat

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nightfisher I'm STILL looking for a battery operated hairdryer for that purpose. Do you have one?

Something like this is perfect.  http://www.365astronomy.com/365astronomy-12v-hair-dryer-defroster-with-cigarette-lighter-plug-p-2753.html?zenid=3207f27befdabcbbe242b1f0e14f1df9&gclid=CjgKEAjw-6WcBRCsgNjFy-2OuGYSJADf4R2sFzG1xuCl40KY3GQISfJ447sh6WJwCRd8ihvFkNHtOfD_BwE

Terrible for drying your hair, great at keeping dew at bay.

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Thanks for all the interesting comments & advice. A dew heater sounds like the best way to go but can see that working out as rather expensive what with the heater itself, controller & some way of powering the thing. I'll have a rummage around eBay for a battery powered hair dryer in the meantime; which is bound to earn me some funny looks from the wife :D

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This topic has had me rummaging in my garage but to no effect - I am sure I had a box kit that contained a rolled up heating element of the type people used to stick on back windscreens of cars to clear mist I suspect the pro version heating coils are the same principle - has anyone experimented with such a kit? It should work off 12V easily enough.

Ian

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You may be able to use one of those grain filled "heat pads" that you heat in the microwave oven - just wrap it around the scope to keep it warm.

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Prevention is better than cure in my experience. Once dew has formed on an objective or corrector lens, it takes a fair amount of energy to clear it ... by which time you probably heated the optics a bit more than is ideal for maintaining good seeing. A commercial or DIY heating band only needs a few watts (dep. on scope size) to prevent dew from forming in the first place if you switch it on at the start of a session, especially if used in combination with a simple wrap-around foam dew shield.

I've never really seen the need for sophisticated control electronics. All my scopes have foam dew shields and home-made low-wattage heating strips that I always leave switched on throughout a viewing or imaging session. You can make them very cheaply from thin ni-chrome wire in heat-shrink tubing (or sandwiched in stitched strips of fabric), or from a series of resistors soldered end to end and fitted inside a length of rubber tubing. Depending on your local conditions, you may only need say 5 watts for a 8" scope - not too demanding on batteries if you don't have mains power. Rechargeable NiMH battery packs used for radio-controlled electric models are useful and stand up to deep discharge much better than small lead-acid batteries.

Adrian

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Thanks for the advice, Adrian. I may look further into this in the future when I've got more time for some serious observing sessions. Meanwhile, the dew shield has arrived, been strapped on & immediately made a massive difference to keeping the condensation at bay. All I need to buy now is one of those retro-looking cloud guns from FLO  :grin:

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