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SkyWatcher 300P Dew Control


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I'd like some advice on dew control for my scope.

I'm going to be buying the AstroZap shroud which I know will go some way to helping with this, but the secondary is still going to be very exposed. I'd like some advice on dew control for the secondary, potentially the eyepiece and the finder scopes (normal & Telrad).

Answers on a post card please. :D

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I had the astrozap shroud on my truss tube dob and it's a great piece of kit, the best money can buy.

To stop the secondary dewing up is a different matter. You can buy dedicated secondary heaters but these cost a lot of money.

You can place a resistor on the back of your secondary secured by a spot of silicon, or like me make a mousemat coat to surround your secondary holder. Inside this coat is some nichrome wire and this heats up the secondary stalk which in turn heats up the mirror.

Either way you connect the wire to your dew controller via a phono plug. Place the wire along the top of your spider vane and drill a little hole through your OTA to your dew controller.

Hope this helps.

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My secondary has a couple of ceramic resistors glued to it, it isn't pretty, but it works.

These are controlled from a four channel controller, which also looks after the eyepiece and finder heaters

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Nice post Craig, I am currently doing the exact same as you, Astrozap dew shield will be bought at the end of the month and I have just got all my power requirements sorted, just need to get the control box and heaters.

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  • 2 weeks later...

After last nights debacle, dew control is more urgent now than other purchases.

Here's what I am considering...

  1. AstroZap shroud for the flex-tube
  2. Heated strip for the EP (would need 1.25" and 2"?)
  3. Heated strip for the finder scope.
  4. Ceramic resistors for the secondary (Martyn's system was in action last night and it worked, he was the last man standing!)

I also plan to buy a Telrad finder scope at some stage and would therefore need to control dew on this too.

Can anybody recommend me a suitable Dew Controller (looks like I would need to control 5 outputs?) and strips.

Martyn - Can you tell me where to get those ceramic resistors and what wiring I would need?

Thanks :)

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Craig - I think you only need 4 outputs - secondary heater, eyepiece, finder, and telrad. Get the 2" ep dew band and you can use it on the 1.25" ones as well. I picked up a second hand Kendrick Digifire in perfect order for only £60.

Or if you do go mad then Hitec Atro do a 6 output controller. Upgrade your field pack too - an 85ah or 110ah leisure battery would do - any local caravan sales store will have them - or websites like Towsure :)

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Yup - I've been looking at the Alair bands too - need to kit the 925 out with dew control - they do look good :)

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The two glued to the secondary are 7 watt 47 ohm wire wound ceramic resistors.


The one in the Telrad is a 3 watt 33 ohm wire wound ceramic resistor.


These are just connected via a two core flex and phono plug


I use four different colours, so I know what Iam controlling at the top end of the scope.

I didn't build the controller, this came with the scope, as did the heated secondary, but Iv'e modified the wiring by using a scart cable, so only one cable goes up the scope tube. At the top, I have made a box with four phono outputs which the heaters plug into.

This controller has individually adjustable outputs, the Kendrick is only adjustable on two channels.


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Guest wisewoman

I was getting a lot of dew in my 200P and made a foam cuff/dew shield out of a £1 camping mat from Poundland and it defintiely helped. I left my dob outside with this improvised dew shield last night whilst I went to Sawley. When I came back there was only a little dewing and I could still see things fairly clearly. Not as high tech but kinder on the bank balance and it will do me for now :D


Edited by wisewoman
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It don't matter how much things cost, what matters is if they work.

From the sound of it your Poundland dew shield is up there with the best.

Iam off to muller a camping mat in a bit, as last night that light from the power station shot straight down the focuser tube, that would have not happened with a camping mat shield.

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