Termal equilibration vs Dew Prevention

I know that many recommend that you put your scope outside at least one hour before you use it to allow it to come to thermal equilibrium with the outside temperature, but, when you do, you run the risk of dew formation because everything is the same temp. I live in New England and this past week we had 3 nites where the transparency was above average, but, even witha flexible Astro-Zap dew shield on my scope, it seemed to get covered with dew in almost no time, as humidty was over 85%.
What is the answer to the dew problem?

Thanks.....Mike

Mike Alexander said:

I know that many recommend that you put your scope outside at least one hour before you use it to allow it to come to thermal equilibrium with the outside temperature, but, when you do, you run the risk of dew formation because everything is the same temp. I live in New England and this past week we had 3 nites where the transparency was above average, but, even witha flexible Astro-Zap dew shield on my scope, it seemed to get covered with dew in almost no time, as humidty was over 85%.
What is the answer to the dew problem?

Thanks.....Mike

Mike,

You didn’t mention the type of telescope you have or if your dew shield has a heater. However, assuming that you have an SCT: On high humidity nights you need to start running heat as soon as you put the scope out, with the heat at a high setting. Once dew forms, most dew heaters can’t get rid of it. If you are at home with access to AC power you can use a hair dryer to remove the dew. While running heat might slightly degrade the scope’s performance, at least you are observing – versus not observing at all!

BTW, I live in NY and with this year’s rain (and flood) the ground is soaked. The shroud of my Obsession Dob is generally soaked before it is even dark enough to observe. However my Kendrick dew heaters keep the secondary, Telrad, eyepieces, etc, dew free.

George N

It helps if you can start with the optics a little warmer
than outside temperature. Taking scope from 72F air
conditioned house to humid 85F outdoors will cause instant
dew. See if you can get scope a few degrees warmer than
outside temp -- put it in garage, for example, before
moving it outside.

A heater strap is also essential. My own strategy is to
put it on lowest setting as soon a scope is outside with
optics still covered for hour or two. Once I start
observing I check often for any fog on the optics and
then increase the heat if needed.