Dew control for a truss tube RC (Primary)?

[DrRobin]

I have a 10" truss tube RC and unfortuanetly my location can get a bit humid and damp.  It seems to be the primary that gets the dew.  I have a small heater on the secondary (not often switched on) and another on the camera, but there is nothing on the primary.  My scope is fitted with a canvas light shroud.  The whole lot is mounted in an obs with several fans and is usually somewhere near ambient temperature.

If I spot it happening during an imaging session then a bit of light heating of the primary with a hairdryer sorts it out for another hour, but I can have dew on the primary for up to 20 minutes before I spot it and this spoils the images, even though it is still relatively clear.

I have seen very few dew heaters for a primary and wisdom seems to suggest that they are avoided, the scope has primary cooling fans.

Does anyone know if you use a heater tape or similar to prevent primary dew?

Ta

[David Smith]

Do you run your cooling fan continuously? I was of the opinion that air flow was the best cure for this. Assume the fan is behind the mirror pulling air in over the primary? I've seen some setups that have the fan drawing air across the front face of the primary but the best solution seems to vary by report / setup. Will be interesting to see what other think.

 

[DrRobin]

Hi David,

The fans on the back of the scope run all of the time the camera is on.  I have an extractor fan in the obs to remove air through a duct as well as a desk fan to circulate the air in the obs, these are switchable, but mostly on when I am imaging, even when it is really cold.  There is a small PC in the obs, but it is very low power/heat dissipation.

I am not sure if the fans in the back of the scope blow or suck, I think they suck in air and across the back of the primary. 

The more I read the more I think it might be best to have a primary heater, either a tape round the primary or perhaps a heater element next to the fans.  I would make it switchable of course.

The real problem is my location, the mist forms on the river down the hill (mile and a bit away) and slowly creeps up the hill.  Probably for about 1 - 2 hours before it obscures the stars my primary mists up, so a heater gains me that extra hour or so.  Usually by Midnight it is a waste of time, too much mist.

Have you got a 10" truss tube or is it a solid tube?

Robin

[x6gas]

Hmmm.   Presumably it's a bad thing to have movement of columns of warm air wafting about in front of your primary...

I have similar issues with humidity - I'm a few hundred metres from a river too.  I use a dew band on the corrector of my 11" SCT and it does the job up to a point (and when it's that humid the seeing is so poor it's hardly worth the effort anyway).  On the SCT it's very inefficient in fact because the dew band is on the outside of the OTA but the corrector is insulated from the OTA with a cork gasket.  Anyway, so long as you can get reasonable thermal contact between the dew band and the mirror you should be OK, I'd have thought.

[Jkulin]

With my 10" Truss, I was told to use a dew heater attached to the back of the secondary and to have all three fans on the primary sucking air out of the OTA, not blowing into it, easy to switch if yours is doing it the wrong way, just reverse the polarity.

To check if blowing hold a piece of paper in front of the fans at the back, if it sucks the paper towards the primary then it is going the wrong way, it should be blowing it away from the rear of the primary.