Dew heater wattage

[Ian]

I'm thinking about rigging up some sort of dew heater for DSC for the Megrez and the guide scope. What sort of wattage would you recommend?

Ta.

[Mike]

Not much. Some commercial dew heaters go up to 60 watts which is ridiculous. I'd say no more than around 10 watts. Any more and you'd fry your battery too quick.

[Ian]

that's my concern really. It's only a little scope, and I don't want to overheat it, particularly as I want it to be (at least at the moment) permanently on if there's a risk of dewing. 10 watts would need about 15ohms...

[Mike]

How are you going to make it? A string of resistors in parallel?

[Ian]

i've got some nichrome wire, but the resistance is too low. It'd be good for a 12v toaster, so yup, string of carbon film resistors or if I can do it out of the random pack of power resistors I have, I'll use them. I've got plans for a 555 based pwm power supply but I've not got time to build that before DSC.

[JohnP]

Ian - I would say 10W is enough... 6-8 is probably sufficient... Here is a spreadsheet that I found some time ago on the web that will help you...

http://homepage.ntlworld.com/john.punnett/graphics/oas/Dew_Heater_Designer.xls

John

[MarkS]

Even with my C11, I generally have it ticking along at 12W on all but the very dampest of nights.

But it's worth having plenty of spare capacity in reserve just in case you get hungry and need to quickly make some toast.

Mark

[Mike]

A spare dew heater wrapped around your fingers makes a nice hand warmer too.

[Rocket Pooch]

Mine were made form 330ohm 1/4 watt resisters about 2cm appart from each other, and they can get really hot if you crank up the juice a but.

Don't know if this helps at all really.

Also I do have a reel of copper resistance wire I made a really simple dew heater for the 80mm guide scope just by wrapping the stuff about and putting a switch on the end of it.

[Ian]

that's a useful spreadsheet now I've got my head round it John. Is it your work?

[Mac]

That quite a good little spread sheet.
Saves me working it out on paper.
well i've just calculated with 2,000,000 330ohm 1/2w resistors i can get a nice 872Kw dew heater.
Oh and my 120AH battery will last all of 0.49seconds, if it dosent explode.

[Ian]

use that at dsc and we'll get rubbish pictures. Not due to sky glow, but Mac glow...

You could get the same effect by eating readybrek.

[JohnP]

Ha... Ha...

No Ian it was something I found when surfing around for dew heater info when I made some... TBH - I started of using resistors myself & they worked well but I have now bought a proper dew strip (only cost about £20 for the ZS66) - which I use with my homemade controller. They are just a lot more flexible then resistors & wrap around scope nicely etc...

John

[RobertM]

I read somewhere that you only need the dewband temperature to be a few degrees above dewpoint.  That seems to figure as I've got away with it in the minimum setting so far, the warmth of the dew band can barely be felt.

[Rocket Pooch]

I can make breakfast on mine when its flat out on the 8", much better than a hair dryer, but the tube glows red.