Dew on Refractors and Telrad in Victoria

[PeterHA (Peter)]

All,
This is a question specific to owners of 4-5" Refractors and owers of Telrads in Victoria/ Melbourne.
Over the past years, have you seen a requirement for a dew heaters for the Telrad and for your 4-5" refractro optics?
I am considering to equip myself with dew heaters for Telrad and 4" Apo ahead of the autumn and winter season.
Thanks for feedback.

[AG Hybrid (Adrian)]

A dew heater on a telrad? Fancy stuff. I put Velcro on both sides of my telrad and made a 5" long arch that extends from each side of the telrad and over the glass element. Made from EVA75 I got from Clark Rubber. Keeps my Telrad completely dry even on the worst dew nights in winter.

In Sydney 3 days ago I was outside with my scope. Singlet and shorts and my telrad glass element had dew on it without the shield. I suspect you're telrad will dew up in Melb pretty often indeed.

[Bart]

Doesn't matter where you are from, if the conditions are right, you will need dew prevention. I built my own with a PWM box to control the voltage.

Try this link

http://www.dewbuster.com/heaters-330ohm-resistors.html

or

http://www.deepskywatch.com/Articles...ew-heater.html

[PeterHA (Peter)]

Hi Bart,
Thanks for the links.
Well I have build the controller and Telrad, now contemplating the strap for the scope, depending on feedback I get.
The controller and Telrad fix are described in the DYI section of the forum just now.

[MattT]

Peter make a dew shield 3x the aperture in length and that should keep the dew off the objective, at least it does on my 150mm frac. Dew shield is 450mm long.
Matt

[PeterHA (Peter)]

Hi Matt,
Thanks for the suggestion, an extended dew shield is a solution I have considered but from a mobility, rigidity and aestetic point I decided against it.
I have in any case a 12V power source on the scope for the Nexus, the Telrad needs a heater as well, so I use a dew controller. If most have experienced dew on refractor lenses in Vic I will just get (probably) make a 6 W bad for my scope, from the first replys I guess dew will need to be expected on the lens.

[jamespierce (James)]

A dew shield is actually a bit of a misnomer because dew doesn't fall from the sky as commonly thought ! Dew forms as a result of the right combination of humidity and air temperature. All a shield does is stop the glass from having so much heat sucked from it by 2.7 kelvin night sky.

You could have a dew shield a meter long and still have dew forming on your optics... Obviously some scopes are more prone than others (SCT with a huge exposed glass corrector plate cooling below ambient etc)... But even my 16" dob which has a effective 1.5m long dew shield for the primary will dew up on a poor night without thermal management.