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After having spent the last 2 evenings in the back garden in amazement I have realised that I need to invest in some sort of dew control equipment. I notice that there a 2 main sorts out there, those that run with controllers and those that say a controller is not required. HHEELLLPPPPPP !!!!!!!! My budget is busted but I know that if I just go for the cheapest option it will be wrong so a little advice would be gratefully appreciated.


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Where is the dew forming, on the primary, secondary or the eyepiece? I ask this because a newtonian tube normally acts as a dew shield. A friend of mine uses an elasticated shower cap on the primary end of his newt, you can try this before making a dew heater. If the secondary is dewing up you may need a 12v hair dryer to clear it. If its the eyepiece use 2 eyepieces, keep one in your pocket ready for swap over. Making a dew heater is reasonably easy once you have sourced the materials, in your case making one for the primary (150mm primary) if the tube is say 7" outside diameter. This would be approx 22" circumference. The rule of thumb for a lot of people is 1W per inch of diameter therefore you would be looking at about 7W. The formula is watts W=V squared / resistance ohms. If you are running at 12V then V=144, now I would suggest about 34 ohms/meter nichrome wire. As your circumference is 22" then 22"/39" = 0.56m of nichrome which will give about 19 ohms, therefore 144/19 ohm = 7.6W. 

To make a heater what I would use is Duck tape 2x24"x2" strips, this will be enough to go around and overlap by a few inches. A couple of meters of speaker cable or enough to go from your heater to the battery. Solder the 22" of Nichrome to the ends of the speaker cable, heat shrink the joins if you have any, pin one of duck tape pieces to a length of wood with drawing pins then lay the nichrome wire the length of the tape. The tricky part is sticking the second piece of duck tape to the first. Connect a 12V cigar lighter male to the other end of the speaker cable. I normally finish the heater strip with black velour where you can buy a roll from Wilkinsons for £5. If in the future if you want to buy a dew controller you can replace the cigar connector with a phono connector which is the normal for controllers..

Hope this is of some help.

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I just made my second dew heater, but this one is for my DSLR camera lens.

I have a bunch of small resistors, so this time I made a parallel resistance heater. 20ea - 1K Ohm resistors  give a nice gentle heat from a Velcro band I can wrap around my camera lens. It kept my lens clear all night compared to the night before where I wasted my time and shutter actuation's for naught.

Since I am primarily at home, I used an old power supply to power the ladder of resistors. 9.5 volts with 3.5 amps of output. It puts out a nice gentle heat that can be felt at the band, and induces enough into the lens to apparently keep the glass clear.

If I had a heavier dew problem I would consider one of the more expensive offerings. But I'm getting by fine doing it my way.


So far I haven't melted anything, nor caught my stuff on fire yet. :icon_biggrin:

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Thanks for the comments and advise guys. I have invested a whole £26 on "T'Bay" and have been able to make:-

1 x 4 channel controller

8 x connecting leads of differing lengths

9 x heater straps again of differing lengths

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