Power levels for Dew Heaters

[Stub Mandrel]

It looks like being an autumn with many clear nights, but I didn't bother last night because the humidity was forecast at about 95%!

I have all the bits to make some dew heaters for the 130P-DS. What is holding me back is I haven't a clue what sort of power hey need, i know it isn't critical, but a rough idea of what others use would help put me in the right ball park. The three I think I need are:

• Power resistor of back of secondary support
• Resistant wire band around the tube near the main mirror
• Resistance wire band around focussr (to keep coma corrector clear - I assume this will need a fair bit as the heat has along way to go?)

What sort of powers/resistances (assume 12V leisure battery power) do people use in these places?

[cjdawson]

I've got 2 heating elements on my Meade LX-90.  One is infront of the corrector plate.  The other is in front of the main objective on my Guide scope.

I tend to run both of these at about 20-30% of maximum power (using a PWM for each).  The result is that in reality they then to draw about 300Ma each, roughly.  At full power the 8" element uses about 2 amp, and the 80mm element is about 1 amp (figures from memory so I could easily be wrong).  The end result is that my 70Ah leisure battery will happily power the scope, heaters, camera, autoguider, Skyfit, charge my iPhone, iPad and Mifi all night long.   I'm even tempted to add my MacBook pro to this setup on the same battery.  That will pull about 5amps MAX, and normally more like 1amp when running the Mac version of Nebulosity.  Running VMWare fusion, and windows 10 is a sure fire way to get the 5amp draw.

[cfpendock]

You only need to get the optics a touch above ambient -I use about 300mW per centimetre of dew band.  This works very well for both my 4" refractor and 11" CPC.  But in both cases I have to use dew shields, otherwise it is not enough.

 This site may be helpful: http://www.blackwaterskies.co.uk/2013/05/making-your-own-nichrome-dew-heater.html

Chris

[Stub Mandrel]

Thanks both, I have a foam mat dew shield already (lined with flock as otherwise it's white!)

Those give me some starting points.

I have two 50-ohm heating elements, cut in half and paralleled one of these will give 12.5 ohms, for 12 watts which is 0.3W/cm if wrapped around the 150mm shell of the 130P-DS. :-)

The secondary heater will need PWM, I will have to experiment, same with the Coma Corrector.

[cjdawson]

I'd suggest that you get PWM for everything, there are links on other posts to LED Dimmers available on E-bay that do the job nicely.  They cost under £2, so should be cheap enough to be able to use them 1-1 with your dew bands and not have to regret not doing it when you find that most of the time you won't need them on at full power. ;-)

[Stub Mandrel]

I already have PWM on my scope controller board - I just need to add the FETs and program it :-)

Just I only have only allowed for two channels, although I can bodge another one in.