Primary Mirror dew fix for Newt (whilst imaging).

[Emuhead (Andrew)]

Hi all,

Wondering what the tried and tested approach to prevent dew forming on the primary mirror of a steel tube Newt might be. The fix needs to be good to use whilst imaging.

One option:
Looking at Primary Mirror fan attached, set to exhaust mode to draw air down the tube and out the back. Do you need a cover on the back also to ensure the airflow is as intended?

My concern with a fan is that it might cause vibration which defeats the purpose as we're all about those details. Have also read that the cover itself can also create added vibrations so not sure if it's better with or without that cover.

Another option:
Shell out for a Primary Mirror dew heater pad (from Kendrick for example).

Concern here is cost, sticking something to the back of the primary.

Hairdryer is out too as my neighbours like sleep, and so do i.

What do other imagers use with success?

Thanks,
Andrew

[John K]

Hi Andrew,

Would another option be to simply put a strap on the tube surrounding the surface of primary mirror?

This is what my CDK scope has and seems to work well on most night although I have a carbon fibre felt lined tube.

John K.

[Emuhead (Andrew)]

Good suggestion, I have tried that and it didn't work that well for me, but having said that I'll try it again and this time turn the heater up from 50% to 100%, see if that helps.

Any other tried and tested methods?

[carlstronomy (Carl)]

The best way to heat a mirror or corrector plate is always direct contact. This is why Kendrick make the heater pads to stick onto the back of the mirror. Heating the tube will not work as the primary mirror does not make contact with the tube wall, doing this would possibly create more of a heat haze in the tube.

I have heard of people wrapping a strip heater around the actual mirror, unsure how effective it would be. The proper pad would definitely be the best if you have major condensation issues, nothing wrecks viewing and imaging more than dew.

[Startrek (Martin)]

I had Kendrick dew heaters for years ( still do ) on my visual and imaging Newts and after a year or two struggling with dew even using my dew control , I don’t even bother anymore as dewy air still affects your visual observing and also to a greater extent results in poor quality images.
If the humidity is up around 85% or higher and the wind is less than 10km/hr for most of the night, I know it’s going to be a waist of time, a quick check at 7pm outside and I know that dew has already started to form and is going to get worse. This is the case at both my sites 250km apart.

[Emuhead (Andrew)]

Ok that's good to know, thanks Martin.

I've already bought the secondary dew heaters so likely going to install them & have to find out this nugget of wisdom myself & I'm sure end up with the same conclusion. That being said, I've got to persist now as I'm already invested.

Guess I'll just try a few things for the Primary, cheapest being attaching a fan in exhaust mode & see how that goes. Would be curious to know how best to do that, such as attach the fan directly to the primary mirror holder vs create a panel and attach it to that, which should also promote better airflow as itll enclose the back, but do any of these create more vibration than another. Failing that, Kendrick warmer, failing that, just don't shoot on dewy nights.