Newt cooling down - caps on or off?

[PeterM]

Hello,

I have a 200mm newtonion scope on a Dobsonian mount.

So far I've just been putting it out in the garden about an hour or so before I want to start observing. I've been concerned about premature dewing so have been leaving the caps on (main tube and focuser) but is that right? It does seem more logical that a tube would cool more effectively with the caps off as I'm really only observing for an hour or two and I've never really had much of a problem with dew on my refractors over that time period.

Would it be better to have the caps off? With the caps off would putting the scope in a horizontal position during cool down help?

Thanks v much,

Pete

[oldfruit]

In my limited experience:) i have taken the caps off but have placed the tube almost horizontal to avoid anything dropping into the tube. After 1 hour i get good views and only after a couple of hours do i get dew if any.

[barkis]

Well, logically. any warmth that may be inside the OTA, would be retained a bit longer if the caps are left in place, so on those grounds it is better to remove them. Leave the scope horizontal as usual though. Once the Dew Point is reached, it will require other methods to keep it at bay, such as dew strips, hair dryer eg.

Ron:).

[John]

In my limited experience:) i have taken the caps off but have placed the tube almost horizontal to avoid anything dropping into the tube. After 1 hour i get good views and only after a couple of hours do i get dew if any.

I follow a similar routine but I do leave the scope slightly "nose up" to help the warm air currents find their way out of the top of the tube that bit quicker. Only slightly inclined though - not so much that the primary mirror is really exposed.

With my refractors I do the opposite - "tail up" so the warm air can escape through the eyepiece drawtube.

[AndyG63]

Hmm ... I always leave my Newtonian pointing down, a little. Less chance of anything falling in (though don't ask me what possibly could!) and my (probably erroneous) belief that and moist warm air, when cooling, will not settle on the mirror, but pour out the tube.

That said - and on deeper thought - it probably stems from my previous life as a TV cameraman, back in the days of plumbicon tubes, when it was deemed safer to lock a camera off pointing at the floor, than risk it getting a burn by pointing at a light.

Now I am confused!

Andy

[NGC 1502]

I think that John's method, post #4, is the best routine with a Newt.

So for me, it's caps off, slightly nose up, because warm air rises.

Regards, Ed.

[PeterM]

Thank you all for the responses. Yes, in retrospect it does seem rather obvious that if you trap warm air in a big tube it's going to cool down more slowly than one that's open