Dew Management

[ZeroID (Brent)]

Imaging last week when I came to close down and pushed the dew sheild back on the Lunt so I could swing the roof back over I discovered quite a heavy dew load on the front of the objective. So I guess it's time to DIY a dew heater system.
I have an old electric blanket that will donate some heater wire but with a refractor where is the best position to mount the heater ? OTA Exterior just below the objective or the dew sheild itself ?
What sort of heat output required for a 102mm scope ?
I'll need to make it able to slide down the OTA at evenings end so I can close up the roof.
I have a power supply or three and can setup to switch between 5v, 12 v for a low\high setting. Is it really necessary to get finer control with pulse width controls etc ?
Suggestions and advice welcomed

[ZeroID (Brent)]

While attempting to separate the heater wire from an electric blanket (ever tried to do that? What a mission !! ) I had a brainwave.
I used to do homebrew and in the cooler months used to put a small heater pad under the brew vessel. I just dug it out, 6 watts in a flexible pad form with power connections all ready to go !
I am just going to make up an insulating sleeve with Velcro or similar and wrap that against the underside of the OTA just below the Dew shield.
I might even have a ready made controller for it ....

[alistairsam]

hi Brent
thought of making my own straps and got nichrome wire and stuff, but in the end just bought the dew not straps from agena astro on ebay for 25. works great.
for the controller, I dont think higher pwm frequencies are required, just a simple 555 variable square wave generator driving a mosfet for 1 to 2hz or so is more than adequate from my testing.
I made a 4 channel one with a single picaxe and 4 mosfets for less than 30.
I the strap around the objective for my st80

cheers

[ZeroID (Brent)]

You're missing out all the fun part Alistair. I'm DIY king.

One of the electric blanket controllers is still working and it's basically a PWM unit with timer. (Just pulled the dud one apart to check the circuit ) so a change of plug will give me an infinitely variable heater controller from about 1 watt to 6 watts. Well within the controllers capability of 250 watts.

I've never had dew problems before but the weird wet warm weather and a refractor rather than a reflector has precipitated the urgent need. The controller is just a small plastic pendant which can hang down beside the pier no problem with timer and temp indicator glowing dull orangey red. Power cord will easily reach the dist panel just behind. Actually quite a convenient setup.

Total cost $0
Fun Priceless

[GeoffW1 (Geoff)]

Originally Posted by alistairsam

hi Brent
thought of making my own straps and got nichrome wire and stuff, but in the end just bought the dew not straps from agena astro on ebay for 25. works great.
for the controller, I dont think higher pwm frequencies are required, just a simple 555 variable square wave generator driving a mosfet for 1 to 2hz or so is more than adequate from my testing.
I made a 4 channel one with a single picaxe and 4 mosfets for less than 30.
I the strap around the objective for my st80

cheers

You are on the geek list now, no risk

[Ajemsa (Andrew)]

Recently I built a dew detector circuit. I got the circuit diagram from here:

http://www.electroschematics.com/608...etector-probe/

I coupled the output to a flashing LED circuit using a 555 timer and added a buzzer. I put the components together on a breadboard and left it running in a corner of my observatory.

Last night I was collimating my newtonian and doing some basic checks on Saturn when the buzzer sounded. It startled me (I suppose that it is its purpose anyway) so I checked the surface of a few things and then quickly closed the roof. Soon after closing the roof the detector stopped. I noticed that my telescope tube was still dry to the touch but my roof was moist.

I have not fine-tuned the settings of this circuit but I was very impressed that it performed in the way it did, i.e. detecting moisture before its a problem. Has anyone else tried something like this?

[ZeroID (Brent)]

If you could get it to turn on the Dew Heaters it might be very useful.

I ran my little 6 watt system the other night. Temporary setup, velcroed it onto the OTA, wrapped it in a towel for insulation... no dew, despite everything else getting wet. Gonna buy me another couple of those things, cheap enough.

[Zubenel (Wes)]

I made this years ago, might even have a spare board or 2 left over for you to populate . I use this circuit to keep the secondary free of dew. There is a lot of resources on the web to calculate the heater size . See if I can dig up the formula.
http://www.clementfocuser.com/images...trol_Print.pdf. Works like a treat. Cheers Zub.

[Zubenel (Wes)]

This might get you on the way to calculating the size and configuration of a resistor bank. http://webhome.idirect.com/~rsnow/resister.html
Cheers

[Ajemsa (Andrew)]

Brent, this dew sensor circuit has an optocoupler (switch) on the output so you can essentially trigger anything you want. I'm still experimenting with it and put a simple alarm for now.