Dew prevention

[dark star]

Hi, I have a 14 inch truss dobsonian, which I made myself. Does anyone have any suggestions for dew prevention/ heaters, for the primary mirror, secondary mirror, 80mm finder scope and telrad? I went to a dark site last December and everything dewed up! There are a lot of different dew prevention kits on sale, so I find it confusing. Something to attach to the back of the primary mirror to gently heat it seems a good idea.

I use a shroud, but there are some holes cut in to the mirror box to reduce weight, I don't know if this will make any difference? Also, suggestions for a battery to power everything. I have bought a 12v hair dryer which I  would prefer to run from a battery rather than the car.

David

[alanjgreen]

Hi David,

I have a 20" truss dobsonian so understand your pain.

1. Fit a shroud. (I see you have ).

2. Add a fan to the underside of the scope blowing air up the tube, this will help to stop dew forming on the primary mirror.

3. Stick a secondary heater tape to the back of the secondary mirror. There are several "types" available depending on what sort of secondary mirror holder your scope has. I use a Kendrick secondary heater tape from Altair Astro. Here are some links:

http://www.kendrickastro.com/pdf/2029-M.pdf

https://www.altairastro.com/kendrick-split-newtonian-secondary-heater.html

I use the Kendrick Micro Dew Controller to provide the connection on the side of the UTA (after scope assembly), the cable runs up the inside of the truss tube. I used to use the other socket on the dew controller for eyepiece heater tapes but discovered the secondary only needs very low setting (or you get a wobbly view!!!) whereas 2" eyepieces need a very high setting. In the end, I ran a separate cable up from the power sockets in the rocker box to the eyepiece tapes directly.

https://www.altairastro.com/kendrick-micro-dew-controller-with-2x-12v-rca-heater-ports.html

Of course, this all depends on whether your scope has any internal power source? I use an astrosystems powered ground board on mine.

https://www.astrosystems.biz/ASPFTinst.pdf

https://www.astrosystems.biz/drive.htm

This kit then gives you plenty of power sockets (and switches) to connect up everything and turn on/off as needed.

The supplied "foot" holds the single entry power socket that you use the supplied 12V cable to connect to external Lithium battery.

I use a 24Ah Tracer battery and it lasts for weeks between charges.

https://www.firstlightoptics.com/batteries-powerpacks/tracer-lifepo4-12v-24ah-battery-pack.html

It takes a bit of messing about and changing things before you get a setup that works just right

HTH,

Alan