Dew Heaters - what is the theory on best placement?

[rogerg (Roger)]

Hi all,

Wondering if anyone knows what is the best place (in theory) to have a dew heater strap:

1) around the dew/sun sheild at the lens end, but not around the lens its self, such that heat currents are created in the area to avoid due but the lens its self is not warmed.

2) around the actual lens/OTA around the end element or filter such that the piece of glass closest to the sky receives the most direct heat.

You'd think it'd be easy to test, but I've found it's not... hard to have exact repeatable situations and easily move my dew straps.

Thanks,
Roger.

[leon]

Roger it probably has different effects on different scopes, but I my Tak I had it at the end of the dew shield, AND IT WORKED PERFECTLY, HOWEVER SOME WHO ALSO HAD THE SAME TAK, had it directly one the objective lens.

Leon

[ballaratdragons (Ken)]

Roger, I tried several places on my 2 refractors and I get the best results wrapping it around the outside of the lens cell.

It warms the cell & glass.

ED80 and 120mm

[avandonk]

There is no correct answer as it depends on the conditions.

When a telecope is pointed at the sky it 'sees' a sky which can be much colder than ambient temperature. It is NOT 3 Degrees Kelvin it is more like 10 to 60 C below your ambient temperature. The dew shield will limit the amount of sky your telecope 'sees'.

The important thing is the front of your telescope radiates heat into this cold sky and can cool below the ambient temperature to the point where it gets below the dew point.

All the heater strap needs to do is to radiate heat to the front of your telescope from the dew shield to stop this cooling. It does not need to heat the optics directly unless you live in Ballarat. Sorry Ken!

About half way up your dew shield and a few degrees above ambient should do it. The better controllers will give you this versatility.

I use a thermostat on my optics for this reason. Too much heat and you get distortion due to air currents etc.

Bert

[RB (Andrew)]

Agreed, I place mine about halfway up the shield and let my dew controller regulate the strap according to ambient temps.
That's why I love using the DewBuster controller with it's Air Temp sensor.

[ballaratdragons (Ken)]

Originally Posted by avandonk

It does not need to heat the optics directly unless you live in Ballarat. Sorry Ken!

Bert

That's OK Bert.

Putting mine around the cell saves me from chipping the ice off the optics later

[rogerg (Roger)]

Thank you very much everyone for your replies .. especially Bert for the theoretical analysis

I'll stick to the dew shield on he refractors with more confidence now, and for my camera lens I think I'll buy a second sun shade and permanently wire on some nichrome.

Roger.