Second time out with new scope

[machinist]

So the skies finally cleared again and as the last few clouds were disappearing I was there on the sundeck setting up my scope right after work. Alignment worked first time and I was observing within 10 minutes. First target, again the brightest point in the sky, Jupiter, looks great, then moved around to a few other objects including the ring nebula, great sight looking like a little smoke ring in the sky.

Got hungry after a couple of hours so went in for a bite leaving the scope set up, after eating I enthusiastically dragged my daughter out to look through the scope at Jupiter, she was singularly unimpressed and complained that Jupiter looked like a, "fuzzy aspirin", "What are you talking about", I asked moving her aside to take a look, she was quite right, what had been crystal clear had become a fuzzy mess of a view. The problem was immediately obvious, all the optics had completely dewed up, and worse, the temperature must have dropped a few degrees below zero because the moisture on the precious corrector plate of my scope was freezing up into a white layer of frost! I took the scope inside and the frost instantly turned into water again dripping off all the cold surfaces, ended session for the evening. I surmise that there was still lots of moisture left in the air from recent rains which immediately condensed onto everything in sight when the air got cold, next morning the deck was white with a thick layer of frost.

Any advice on what to do about this rather severe dewing and frosting up problem would be greatly appreciated, I don't remember the problem being so bad in the past, but then I was either using a refractor or an old Newtonian with a fibreglass tube.

Some equipment notes, red dot finder, not great, I dug an old finder scope out of my junk box that has not seen the light of day in 30 years but seems much preferable to red dot thingy. I should be able to fabricate a suitable bracket to attach it to the scope in short order. Backlash in drive gears, can be worked around but is a nuisance, you'd think the manufacturers of all telescope drive mechanisms would by now have heard of backlash eliminators and various other mechanisms for preventing this problem, but as they say, you get what you pay for I suppose, still the scope wasn't exactly cheap either.

[Altair40]

I'm a beginner and all I can think of is a long dew shield. I understand sometimes fans are used but this may be more to do with eliminating small temperature gradients in a reflectors tube.

I'm probably talking rubbish here but no one has replied to your post. I've seen condenation form on my newt but only when I've brought it into a warm room from a cold outside.

Anyway:

A relector probably needs to be at the same temp as the outside air to stop condensation. Maybe a dryish garage or something like that before going out? (warming the scope would cause image problems, I think).

Maybe you had some odd weather conditions with very high humidity. I know it's been 90-95% relative humidity recently and you don't need much to start drops nucleating. A rough paint job maybe?! Supercooled water vapour, anyone?

But more likely a sudden change in temp in very humid air (foggish). I don't know how much of a temp change you need - maybe not much?

Anyone else have these problems?

[Astronut]

HI Bill. SCT's corrector plates dewing up is a common problem. What you need is a dew shield which fits over the end of the 'scope. These are available to buy (Astrozap make good ones) but you can easily make your own from suitable material.

Dew Prevention - Astrozap Flexible Dew Shield

The recommended length is 1.5x the 'scope aperture. They're quite effective at holding off dew formation on the corrector plate but for more efficient dew prevention you can couple a dew shield with an anti-dew heater strip such as this one

Dew Prevention - AstroZap Dew Heater Tapes

You'll also need a heater controller. Be aware that anti-dew heaters can drain your power supply pretty quickly though.

Ian