What the heck

[Tiny]

(Last night)

We look out and the clouds have cleared to give a nice starry night. And it is nice and dark apart from the beacon shining from our lunilight neighbour. The seeing is about 7 to 8 which means I can work Mars with a 6" reflector at x150 but not at x190. The wife has her 8" SCT and with both scopes with filters we can just about see markings on Mars as in light bottom and darkish bands.

Moving on M42 is nice and wispy on the 6" with a 37mm (x20) eyepiece then back on with an 8mm to give x90 and on to Saturn.

Three moons here and maybe a ring shadow plus the, ever more shutting, rings (which have reduced the wow factor). Still Saturn you can dwell on for a long time and that is what we are doing when...

Suddenly Saturn blurs in the view and everything - tubes, mount and all is almost instantly wet. The red dot finder and the eyepiece is misted up and the seeing is down to about 3-4. It has been reasonable (as in coat and slippers) weather but now there is a definite chill in the air though there is little or no wind and no mist. After waiting a bit for things to clear we gave it best.

In many years of stargazing this sort of instant wet misting of equipment hasn't happened. So what the heck did happen folks!

[James]

Sounds like you're kit dropped in temperature down to the dewpoint or beyond. Just as soon as your kit goes below the dewpoint lots of dew will start condensing on it.

We're you using a dewshield with the SCT? Dewshields help to slow the loss of a heat a little so that the corrector lens will stay slightly above the ambient temperature and therefore not get dew on it.

James

[Tiny]

Yes, I was wondering about dewpoint but this was so quick.

Switching on the lights to pack up... The reflector was coated on all the outside, as was the mount. And we knew the same was true of the eyepieces and the reddot, which, one misted had got more and more coated. The SCT, complete with dewshield also misted on front glass and all mount/tripod surfaces.

The SCT front glass took an hour to dry out indoors and we also left the reflector open for that period to get damp out. The reflector (metal) tube dried in the hour but the rest (metal/plastic of tube, mount and tripod) was still damp and only got covered this morning (11am). Half an hour after we were in it had clouded over so little was lost in observing time. But the change was so quick and the condition so extreme. We are used to frost, damp nights etc but this was a whole new ball game.

[The Warthog]

A front line, one of those theoretical things they draw on weather maps, probably passed you. You went from cooler drier air to warmer, moist air in a matter of a few seconds. I have had this happen to me on the highway twice. Each time the car windows misted up completely, and I needed my wipers and defroster to clear the glass, and noticed that everyone was having the same problem. This is a little alarming at 120 km/hr.

[Vega]

I must admit, I noticed the dew was pretty bad last night during my obs session. My table had a river on top of it Misting up eyepieces was my annoyance of the evening.

Matt

[michaelmorris]

I had exactly the same thing happen at around 11:30pm. However, the combination of a large dewshield and dew heaters meant that everything stayed useable.