Dew prevention tips

[Steamer Phill]

Finally got a clear night ahead still waiting for pay day to get a dew sheild and was wondering if there are any tips to prolong the onset of dew? I was going to wait until the street lights go out then polar align the mount without scope then bring scope out when I'm ready to start alignment? 

[brrttpaul]

I use them handwarmers wrapped on an elastic band

 

[cotterless45]

You can make a dewshield from a yoga/excercise/camping mat. I bought mine at the £shop along with some self adhesive Velcro strips. Cut it to a length 1.5 times your aperture.

otherwise, observing on grass is better against dew as it cools down slower than a hard surface.

keep the scope horizontal if not in use, better than pointing straight up as there won't be a temperature difference between top and bottom.

a gentle hairdryer will remove dew and give you a half hour of dew free observing.

if the dew is that bad , it'll affect your eps. Keep a couple warm in a pocket.

otherwise get a dew strip and plug it as soon as you set up. Unless you are using a 14" RC , a dew controller is not necessary. 

Its a pain in Autumn and spring when temperatures drop below the dew point,

Nick.

 

 

[Riemann]

On 19/04/2016 at 21:37, Steamer Phill said:

I was going to wait until the street lights go out then polar align the mount without scope then bring scope out when I'm ready to start alignment? 

I see from your profile that you have an SCT.   If you keep your tube in the house you need to let it cool down for quite a while (probably an hour or two for a C8) to get the best performance out of it, otherwise you will get air currents in the tube and you will have your own personal patch of bad seeing inside your tube!    If you keep your tube in the garage or a shed then it is less of an issue.... or if you keep it in the house you could transfer it to a an unheated garage or shed a couple of  hours before using.

Otherwise follow the advice already given.   A dew shield and dew heater are a must in the long run - a dew shield often won't do the job on its own.

[REG78]

Was thinkn about doin DIY job myself frm yoga matt would I need to cut it th full length of my scope so tht it was all covered with th sponge 

[acey]

Dew happens when a surface is directly exposed to a clear sky in conditions of still air. So two ways to avoid/delay it are: 1) minimise exposure to the sky, 2) make air move across the surface. A third way is to heat the surface, which is what dew heaters do. A hair dryer does that too, but also moves air, so doesn't need to be on a warm setting - a fan without heating does just as well.

Points 1 and 2 tell us why the surfaces that dew most are horizontal sky-facing ones, e.g. the roof and bonnet of a car (the other factor is emissivity, which is why metal and glass are especially prone to dewing).

The most widely used dew-delaying tactic is number 1: minimise direct exposure to the sky. This is what a dewshield does. You can't shield your optics from the sky completely, otherwise you'd never see anything, but if you keep optics capped when not in use, or the scope aimed away from the clear sky between viewings (e.g. tilted horizontally) then this helps. Because a dewshield serves only as a screen between optics and sky (blocking infrared radiation flowing from scope to space) it can be made of anything you like, as long as it doesn't sag into the light path. Stiff cardboard, camping mat foam, reinforced yak hide... take your pick. You could even buy one if you really don't trust your ability to cut up a cornflake box...

Eyepieces will dew up if left with their glass exposed to the sky, e.g. uncapped in an eyepiece rack. Cover them up (e.g. put them in a box) and they'll soon de-mist. If an eyepiece mists while it's in the focuser, not exposed directly to the sky, it's most likely because you accidentally breathed on it. Condensation also occurs if you move a cold scope into a warm house. This is not dewing. The way to avoid it is not to move a cold scope into a warm house. Or just live with it. Since you've finished observing it shouldn't be a problem - leave optics uncapped and they'll gradually clear. Cap them net morning. Or keep your scope in an unheated shed/garage etc where the problem won't arise.

Dewing (the real kind as opposed to condensation from breath etc) only happens on still nights. If it's breezy you shouldn't need to worry about anti-dewing measures. If it's very breezy then you'll have other things to worry about instead.

[PeterCPC]

You might want to look at the Astrozap heated dew shields. I am very pleased with mine - no dew problems.

Peter