Dew

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Dew

Postby Graeme1858 » Sat Dec 29, 2018 9:35 pm

It looked like a clear sky this evening so I set up in the back garden. By the time I got assembled, plugged in, slewed to Vega to align and focused on it, the dew on the corrector plate brought an end to the proceedings!

So will a passive dew shield do the job or do I need a heated one? If heated, do I need a controller or will it work without one?
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Re: Dew

Postby Aratus » Sun Dec 30, 2018 12:52 pm

Dew is the enemy of every astronomer! Its all to do with the sky dropping the temperature of the plate lens or mirror below the dew point. The sky sucks heat out of objects exposed to it!

There are things you can do to tackle the problem.

1 Reduce the amount of sky that the lens/mirror.plate is exposed to. A long dew shield will reduce sky exposure from areas you are not actually looking at. It might not stop it altogether, but it does slow down the cooling, and reduces the chances of dew forming.

2 Reduce the amount of time the telescope looks at the sky. If you are not actively using the telescope, cover the front with a lens cap or cloth. Alternatively point it at your house, or even at the ground. Again that will slow down the cooling process. It might even reverse it to some extent.

3 Periodically blow warm air on to the lens/mirror/plate from a hair dryer. That brings the temperature of the exposed surface up a few degrees, and allow you to contine to observe for a little while longer.

I've tried all these over the years with some success, but eventually I was pursuaded to get a dew strip and controller.
What you need is a 12v source. A leisure battery, power pack or mains adapter. The dew strip is an electrically resistive material which heats up when the 12v is applied. Normally it is wrapped around the telescope at the point where the plate/lens is. (or just behind it). The heat works its way to the glass and raises its temperature.

It will work without a controller, but you only need a fraction of the heat generated to do the job. There is also the danger that the glass will overheat and set up convection currents. That is almost as bad as dew forming!

You can get a purpose built controller, but I've discovered that a simple PWM controller will do the job just as well at a fraction of the price. Put in '12v voltage controller PWM' into a search and you will find lots of them. You will need to pop it into some kind of box. I would go for a 10A version to be on the safe side.

You can make the strips yourself too, but I have always gone for the professionally made ones. It is a lot of hassle making your own

My own experience is that I regret not getting the dew strips earlier. Turn it on, and then forget about the dew problem.

Here are some examples of the equipement that you can get.
Dew Shields
https://www.firstlightoptics.com/dew-prevention/astrozap-flexible-dew-shield.html

Dew Strips
https://www.firstlightoptics.com/dew-prevention/astrozap-dew-heater-tapes.html

Dew Strip Controller (Multi-channel ones are available)
https://www.firstlightoptics.com/dew-prevention/hitecastro-single-channel-dual-port-dew-controller.html

A PMW controller unit for a self-made version
https://www.ebay.co.uk/itm/PWM-DC-12V-40V-10A-Motor-Speed-Controller-Switch-Voltage-Regulator-Dimmer-Knob/183584696600?epid=1494853802&hash=item2abe802918:g:uvcAAOSwVH9cGa-A:rk:47:pf:0
I use an 11" reflector (Celestron CPC 1100) and a 3" refractor, (Sky-Watcher ST80) mounted on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI 120MM, ZWO ASI1600MC and Canon 1300D for imaging.
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Re: Dew

Postby Gfamily2 » Sun Dec 30, 2018 1:41 pm

Graeme1858 wrote:It looked like a clear sky this evening so I set up in the back garden. By the time I got assembled, plugged in, slewed to Vega to align and focused on it, the dew on the corrector plate brought an end to the proceedings!

So will a passive dew shield do the job or do I need a heated one? If heated, do I need a controller or will it work without one?

I understand that a passive shield will help delay dew forming, but only to a limited degree, and less so if the scope is pointing upwards.

An alternative to dew bands is to get a 12V hairdryer (online or from outdoor/ camper van suppliers or from The Range if you have one nearby) and use it to disperse the dew from time to time.
Scopes: Meade 8" SCT, Skywatcher 127mm Mak, Raffle winner of SW ST80
For imaging: Pentax K5, Asda webcam, Star Adventurer (new toy)
For companionship: Mid Cheshire Astronomical Group.
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Re: Dew

Postby Graeme1858 » Sun Dec 30, 2018 11:37 pm

Thanks for that. Seems the way forward is to get a hair drier (there's a Range up the road and there's a 12V bench supply in my man cave!) and save up for one of these:

https://www.firstlightoptics.com/dew-pr ... eld-8.html

The engineer in me is attracted to the diy PMW heat controller. I was toying with the idea of using one to control a motor for a remote focuser with an Arduino counting the focus knob revolutions.

Cheers

Graeme
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Celestron CGX 9.25 SCT
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Location: Kent, UK, 51°28’N 0°38’E


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