I made my own heaters

[Fraunhoffer]

I know this has been covered before - but Im so pleased with these.

Im still getting used to my return to astronomy with a s/h scope and after a few frosty and dewy nights decided to make my own dew heaters. Absolutely stoked with them after surviving an all nighter at -3 last weekend.

I followed the basic instructions for using 330 ohm resistors at www.dewbuster.com. 

Parts:
bag of 100 330 ohm resistors
reel of duct tape 5 cm wide
reel of self adhesive neoprene 1.5mm thick and 5 cm wide.
reel of velcro loops and hooks 5 cm wide
speaker cable 2m
12 car accessory plug (or phono plug if using those)
12v 10a supply
(all the above were sourced from amazon and ebay where needed - although I had some already)

wooden board 1m-1.5m long and 199-150cm wide and 4 screws
template for forming and spacing the resistor wires

I peeled the insulation from some spare (2 core and earth) lighting flex for the stiff copper wires and spaced them 2cm apart on a wooden board held in place by screws. Twisting the screws allowed me to get the wires nicely taut and straight. The wires are about 1cm off the surface of the board to allow finders to wrap the resistor wires and slide the duct tape underneath later on.

A piece of wooden construction toy (vintage bilofix) made a great template for pre-forming the resistor wires to 2cm. The first 2 resistors without preforming were a mess. The hole spacing in this toy is 3cm and I used this as my resistor spacing. As I'm not intending to use a controller, (on a tight budget here) the wider spacing allows me to drop the overall wattage per band to nearly half of the dewbuster design. The dewbuster article spacing is 5/8 inches (1.6cm) and recommends using a controller as the band wattage is 'almost twice the required amount of heat'. 

After soldering the resistors and measuring the OTA tube circumferences with a tape measure, I connected the speaker flex wires with a small spacer at appropriate lengths in the resistor ladder. I was able to get the main tube, the finder and a couple of camera lens lengths out of one long ladder.

The rest of the construction followed the dewbuster article for adding duct tape and neoprene. Finally I added the 12 accessory plugs, checked there were no short circuits and plugged into my 10A 12 supply. Each band felt just vaguely warm to the touch and as it was a clear night - these were all pressed into service.

The scope and bands were all attached whilst still light and I eagerly awaited the observing session. It turned into an all night-er and even though the temperature dropped to -3 C and everything was coated in frost, the optics stayed wonderfully clear. So pleased.

I just need to make straps up with the spare velcro to tidy up the wires.

[Carbon Brush]

Like it!

Should an individual resistor suffer a lead break due to repeated flexing, then then the rest will still work.
A benefit of choosing high value resistors and a parallel connection. 

[Fraunhoffer]

I wanted to follow this up with  a Telrad heater. 
I have been successfully using a dew shield made of modelling foam a few times (from the template here: http://www.homebuiltastronomy.com/downbino/HomeBuiltAstronomy-TelradDewShield.html) but on occasions it would start to dew up around the edge of the Rigel window. After musing and sketching various designs using the resistors again I was not really happy with what I come up with. Whilst rummaging through my bits-and-pieces box I came across a discarded automotive ballast resistor. This is used when you upgrade your interior lights from filament to LED so the ECU  doesn't think all the bulbs are blown. Its a 60 ohm resistor in an allow heat sink. Would make a handy 2W (and a bit) heater. It sits nicely on the front shelf of the Telrad secured with a small piece of double sided tape and doesn't obscure the view or fall off sideways.
Happy to report it worked a dream at -2 last night, the front half of the Telrad was covered in frost and the windows on the rear of the unit - quite clear.