Misty Middle

[TopHouse]

Due to it being nice and clear tonight i was out with the scope most of the night, tried my new 32mm Hyperion out on Pliedes and a few clusters, oriion nebula etc, was really pleased, review coming later. Anyway, views were fine.

Because of my position re house, patio etc I have to wait till about 1 am to get saturn and there was no way i was finishing up till I'd had a looky, turned out the moon was too close to it anyway so views were Rubbish!

BUT

Since I was first out at about 7 pm, at about 11 I came in for a warm, left the scope outside, had a coffee etc until first the moon then Saturn arrived. Started with the moon on my new 32 and I had a misty patch right in the center of view, at first i was cursing because I thought it was the secondary I was seeing, I then moved onto my 8mm Baader Hyperion, this also had the little foggy patch in the middle. I hadn't noticed any of it pre 'moon' with either eyepiece, however the temperature had dropped quite a bit and i had a fine layer of ice on the OTA when i brought it in, any ideas what it was and what might have caused it, I shone my red torch down the tube at one point but couldn't see any particular dewing of the secondary.

[Space Bat]

Most likely your eyepieces were misting up - your body warmth can do that so you have to be carefull breathing near your eyepieces as well especially after a warm tea....etc

Did you check your eyepiece ?

Chris

[acey]

Keep your eyepieces covered when not in use, either by capping them or putting them in a box, otherwise they'll get misted up. Holding the eyepiece in an ungloved hand can put in enough heat to make them start to de-mist but it will also give you a cold hand. Misting can be on either end of the eyepiece so is not always obvious until you look through it. A partially misted eyepiece will have a misty middle.

A dewshield will stop secondary misting, a cardboard tube on the front end of the finder will have the same effect there. Primaries don't usually mist up but if they do then either some insulation down that end or else better air-flow are the simplest answer.

[TopHouse]

I have shields on the tube, finder and telrad, I'm inclined to go with the eyepieces thing, but I've been out in colder weather, came in once and there was a layer of ice on my primary! Not had this happen before, I'm going to watch for a recurrence. Would it reveal this if i removed the eyepiece and shone my red torch down it do you think?

[Space Bat]

Yes you could see the mist with the torch....

Also the effect of cold isn't the main culprit with what essentially is caused by the dew and the dew point. Which has many factors - air temperature - temperature of surfaces for which dew can form - barometric pressure...etc

If you really want to have the fun of dew - you need to get an SCT

Chris

[TopHouse]

We do get some strange weather here being rather high, there's a long uphill road here and it can be nice and clear, get withing 50 yards of our turnoff and the fog awaits you.