Dew Control via power resistors

[malc-c]

In my search for DIY dew heaters I stumbled across this thread http://stargazerslounge.com/diy-astronomer/89564-diy-dew-heater-80-complete-working.html

It looks very simply, neat and professional, but it seems that the thread died a natural death leaving a few open questions.

The two power resistors effectively use the secondary mirror as a heatsink. Now I know the object of dew control is to maintain the optics at a few degrees above the dew point, but would heating the mirror directly in such a way cause any real problems.

[Reggie]

My initial thoughts are that it *won't* heat the mirror evenly, all the heat is disappated through the back of the power resistor,so not sure how much that would affect your view/images.

[RogerTheDodger]

Hmm, sort of makes nonsense of getting your 'scope (including mirrors) down to ambient. I'll stick with my diy dew tape heater which seems to perform ok.

[Macavity]

I was thinking somewhat along those lines. The Alair Astro (marketed?) pukka, flat elliptical heaters seem to require sticking onto the back of the mirror and are rated at TWO Watts. If the mirror is cemented in, and you only have access to the back of the mirror holder, maybe a few more Watts, might not go amiss...

[pete_l]

If you tried this on a closed tube 'scope wouldn't you just create air currents inside the tube?

I did once consider a scheme for wrapping heating wire around the outside of a metal tubed Newt. The idea being to keep it at a constant temperature to avoid expansion and focus shift. I abandoned the idea without trying it after I started thinking about convection currents.

[Reggie]

There should really be a major caveat mentioned when approaching ambient and that is RH, if the RH + ambient temperature = dewpoint then ambient is not a good place to be I'm guessing the minimal amount of heat to keep the dew away is actually what people should be heading for but everyone seems to be building heater units that just knock out some arbitrary amount of heat.

[malc-c]

Thanks for your comments guys,

I've ordered some 52R (20W) resistors from RS and was going to have a go at this, but have the reservations mentioned, ie it's not dissipating the heat evenly and concerned about warm air currents. However what alternatives are there for Newtonian reflectors ?

[jimmyjamjoejoe]

Many units have have a slide/dimmer switch to adjust the amount of heat you apply. It doesn't need to be complete overkill.

So long as you're sensible about how much heat you're applying (ie if the resistors are too hot to touch, you're doing it wrong lol), then thermal currents won't be too much of an issue. Tube currents aren't as bad as people make out, the main probelm with heat and newtonians is the "boundary layer" which forms over the primary and acts as a lense.

When faced with the choice of a dewed up, unusable mirror, or some barely noticable tube currents, i know what i'd choose.

[malc-c]

Many units have have a slide/dimmer switch to adjust the amount of heat you apply. It doesn't need to be complete overkill.

So long as you're sensible about how much heat you're applying (ie if the resistors are too hot to touch, you're doing it wrong lol), then thermal currents won't be too much of an issue. Tube currents aren't as bad as people make out, the main probelm with heat and newtonians is the "boundary layer" which forms over the primary and acts as a lense.

When faced with the choice of a dewed up, unusable mirror, or some barely noticable tube currents, i know what i'd choose.

Thanks for that.... I've done some experiment with some 330R 20w power resistors across 13.8v and I couldn't notice any real increase in temperature, even after 20 mins or so... looking at the original thread the chap used 47 ohm resistors, and the nearest I could get is 52 ohm.

I will be running them from a controller (more of that to follow), so that I can control the heat levels, and as the idea is just to warm the mirror to that magic point which is just above the dew point to prevent it from forming.

With regards to that boundary layer, what would be the ideal way to prevent that happening, bearing in mind that the 200P is based in an wooden ROR observatory, so cooling down time is less of an issue. Would placing a heater tape around the tube near the primary, or placing heaters / fans under the primary be a solution ?

[jimmyjamjoejoe]

If it is based in an observatory, i doubt you need to worry about it tbh, so long as temps in there are ~near~ ambient, it won't take long for the mirror to equalize.

Most people go with fans behind the primary, but only because they store their scopes somewhere warm and need to shed the heat quickly.

You shouldn't need to heat the primary. Dew on the primary is pretty rare, happening after hours and hours out on the dewiest of nights.