General moisture problem on my Newtonian

[philjdowns]

Hi All, I'm having a problem with moisture / dew on my Newtonian mirrors and was wondering if anyone has a working solution.

Today I got set-up and before dark took 100 Flats, then after dark 100 Darks, went to uncover the scope for my Lights, and it was running with water, within 5 mins of taking the cap off I had condensation forming on my primary, so gave up and covered it all up.

Now I was thinking of using some Baader TurboFilm (Clear) to seal the front of my tube, then bagging the bottom round the primary and adding 1/2lb of desiccant to absorb any moisture in the tube (drying the desiccant as required).

The scope lives outside under a TeleGizmos 365 Scope Cover tied tight round the bottom if my pier and a bag (2lb) of desiccant inside close to the camera. So is will already be at the ambient temperature before I start anyway. 

I'm thinking that as the film has almost no thermal mass, it shouldn't get much (if any) condensation on as it's always the same temperature as the surrounding air. I could add a small heater to keep the air in the tube just above ambient but not sure if that would set up eddy currents within the scope and give me more problems.

Interested to know if anyone else has other thoughts, or can point out why my thoughts are bonkers!   

Phil

[Carbon Brush]

As your scope is stored outside, it is at ambient temperature. On a clear night (I am reliably informed these sometimes occur) your open tube is pointing towards a cold sky. Using an infra red thermometer you would find upwards (sky) being perhaps 30C cooler than sideways (hedges, buildings, etc). This means the mirrors are going to cool below ambient. I know it is counterintuitive for anything to cool below surroundings. I assume you left the scope open between the lights and flats, allowing cooling.

If an object is below the 'dew point' there will be condensation.The amount of moisture that can be held in air reduces with temperature. Often we see this when bringing the cold scope into a warm house. The warm air in the house can hold a lot of moisture, but when the air encounters the cold scope it is chilled and there is condensation.

I would be tempted to fit an anti dew strap around the primary mirror. Just a bit of heat in there will stop the mirror from cooling below ambient and prevent condensation. As you are not raising the mirror & tube air significantly over ambient, you do not generate air currents.
This of course does little to help the secondary, which is why some fit another small heater to the secondary.

Hope this is useful, David.

[DaveS]

I keep my 'scope under a TG 365 cover, but it's not tied tight unless high winds are expected. I normally leave it open at the bottom, but have an electric pet-bed under the cover to provide warmth when not in use.

Dew heaters are a must in our damp climate.

[philjdowns]

Thanks, thought I might get away with it as my primary has a fan on the back blowing ambient air over the back of the mirror, so looks like I need to change tack.

The dew heaters I have found are mostly the strap kind that fits round the outside of the tube, so that's going to be 1/2" from the edge of the mirror. Would it be better to warm the mirror directly?

I'd have thought that providing a small amount of heat to the edge of a large circular mirror will only warm the outside edge, as the mirror will dissipate the heat quickly, leaving the centre 1/2ish still liable to condensation. Would it not be better to apply the heat uniformly over the entire back of the mirror so there is minimal thermal gradient across the mirror?

Warming the secondary will be the next task! 

The front lens of my comma corrector is about level with the inside of the tube, would that need any action, it's not pointing at the sky but is open to the tube? 

Phil    

[philjdowns]

OK, so I've had a dig about and found enough components to make a flat plate heater for my 8" primary without buying anything - Thats a result!

I've done a few tests and have the following data (hope this works, a copy/paste from Excel!):

10W seems the maximum around for an over-the-counter 8" mirror heater, what I have gets me to 12W, though I'm expecting to be using the lower (green) levels as I only have to warm the mirror to 1°(ish) above ambient and then match the heat loss of the mirror to keep it there.
Does anybody have figures that they use - as in W/in² ? Or how many Watts they use to heat a Newtonian primary?

I'm expecting to start the build this weekend if all goes to plan. 
 

Phil

[philjdowns]

Information on my home built Dew heater posted here Forum | Backyard Astro

Hope that works ? 

[Jkulin]

On my RC, I find having fans at the rear of the primary and a dew heater on the secondary stops any condensation.

On my new Newtonian, I have rear fans and will put a dew heater on the secondary, but I will also put a dew strap around the outside of the primary.

When I discussed putting a dew heater on the primary with my mate, he suggested that although fine, that when it eventually fails, then the whole lot would need dissembling to replace.

HTH