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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
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