RASC-KC 24" dobsonian Kevin's dew zapping system
RASC-KC 24" dobsonian Kevin's Dew Zappers Project

Page last updated: 2002 April 22

Start of the Project: 2002 Feb 01

The RASC Kingston Centre's 24" dobsonian project has been completed. First Light was 2002 April 20 It is past time to start looking at dew systems.
A quick search on the internet reveals that most people use 1.5 to 2.2 watts for 2" eyepiece heaters. I would assume this is for preventative heating and not trying to defrost an eyepiece after the fact. For 8" corrector plates I've seen 3-19 watts listed. My own personal design so far includes 1 watt eyepiece heater (5/4") and 1/2 watt for a secondary mirror on a 7" scope (on the order of 1.1 to 1.5").
For the 24" project we need a 2" eyepiece heater (2 watt) and a secondary mirror heater (3 watt - should be enough)

Initially the eyepiece heaters will be off/on duty cycles (0% and 100%). Later we will be adding a duty cycle controller (0% and 40-100%) to give a finer control on the power used and the amount of heat delivered. Same with the secondary heater. Later the secondary will be getting its own self regulating feedback controller, housed inside the secondary holder itself.
Duty Controller: eyepiece port, telrad port, secondary mirror port, finder port.

2" eyepiece heater

See Dew Zapper Project page for details on a 1 watt eyepiece heater. Build another using 2 watts and a larger circumference (6.5"+). The power cord can be shorter as it will be plugging into a power jack located on the secondary box.
Parts list include RCA male plug, strain relief (duct tape or shrink wrap) up to 2' of power cable (20-24 gauge), velcro and insulating material, resistors (to be listed later). 2 watts at 12vdc = 166 miliamps and 72 ohms. Assuming 1/2 watt resistors we would need 4 to dissipate 2 watts, add another to be safe. Wired in parallel gives 5x72 ohms = 360 ohm resistors. Wired in series gives 14.4 ohm resistors, again hard to find. Try parallel.

Focuser Heater

It was recommended that we look into heating the Crayford focusser as well as it tends to get sticky in the cold. Assume 2 watts.

Primary Mirror Heater

No plans at this time to use any heating elements on the primary mirror.

Secondary Mirror Heater

Our secondaries were built out of 7" blanks and cut into elliptical shapes after the polishing. It is approximately 5" x 7" by 1"? thick. We do not want to place the heating elements on the back of the glass but rather on the secondary mirror holder structure, which is approx 4" x 5.75".
3 watts at 12vdc gives 48 ohms and 250 milliamps. The circumference of the holder is approx 15" or 1.25'. We have a choice of nicrome wire (the kind we have in stock is 10 ohms/foot from an old toaster) or discrete resistors.
  • Nichrome wire: for 50 ohms roughly we would need 5' of wire and it would wrap around 5/1.25=4 times around the holder. It would need to be "packaged so as to have each loop be electrically isolated, then a final wrap of some kind of insulating material.
  • Resistors: Assume standard 1/2 watt resistors, we would need 6 to handle the power dissipation, and add a couple more for a safety factor, so 8 resistors. Wired in series this would require: 50 ohms/8= 6 ohm resistors. Kind of hard to find. How about in paraller? That would take 50 ohms * 8 = 400 ohms. Much easier to find. So let's get x8 400 ohm 1/2 watt resistors. Evenly spaced around the holder would give 15"/8=1.875" apart.
    It would be nice to make this easily removable, so let's pretend we are making a very large eyepiece heater and use the same design, only bigger.
    Parts list include RCA male plug, strain relief (duct tape or shrink wrap) up to 2' of power cable (20-24 gauge), velcro and insulating material, resistors (to be listed later).
    Parts list include RCA male plug, strain relief (duct tape or shrink wrap) up to 2' of power cable (20-24 gauge), velcro and insulating material, resistors (to be listed later).

    Telrad heater

    We have an existing telrad x1 finder with heating capacity. It has a 200 ohm (=60mA=720mW) heater builtin. This is two 100 ohm resistors wired in series, each 1/2 watt.
    Parts list include RCA female jack, up to 5" of power cable (20-24 gauge), two 100 ohm 1/2 watt resistors. If a gunsight style finder is used, look to build a small eyepiece heater design style.

    Finder heater

    We will need a better than x1 finder (telrad or gunsight) for this 24" (610mm) f4.6 fl=2800mm (110" = 9.2') telescope and therefor another dew zapper.
    Parts list include RCA male plug, strain relief (duct tape or shrink wrap) up to 2' of power cable (20-24 gauge), velcro and insulating material, resistors (to be listed later). Depends on the placement of the finder

    Total Power Budget

    To date total: 18 watts.
    Eyepiece heater 2 watts, focusser heater 2 watts, secondary heater 3 watts, telrad heater 1 watt, finder heater 2 watts, primary mirror (top surface operating laminar flow) fans(3) 5 watts, primary mirror (bottom surface cooling fans)(2) 3 watts