Jupiter / Dew

[Solanum]

Had the first really clear evening for quite a while, so got the scope out, had a good view of Venus, and the phase has noticeably changed since I last spent any time looking at it, which is cool.

A little later I span the scope round to have a look at Jupiter and I was just in time to watch Ganymede move clear of the limb, with it's shadow still over the planet, with Io transiting on the other side! I've only seen a moon transit once before so this blew me away! If only I could have photographed it....

The downside is I've just been forced in by dew on the mirror (either that or it suddenly got VERY dusty). BOM says temp is 7.9, apparent temp 5.8, dew point 5.7. There was dew on the tube almost as soon as I took it out. I had the fan running on the mirror, but it hasn't stopped the dew. We have cool evenings here in Mildura, so can anyone give me any good ideas on preventing this happening?

[rmcpb (Rob)]

Many ways to handle dew but the simpilest is the place to start.

Try making a tube extension out of a bit of cardboard - it will have to extend about 15" beyond the front of your focuser for your sized mirror. Hold it in place with some velcro or even gaffer tape for the prototype. This simply makes it harder for the dew to get down the tube. If you think its worth it make one out of some thin plastic sheet from the local hardware.

Now we get into contradictions. We all say to observe objects that are high in the sky for the best views and that is true BUT if dew is a problem then try to save all your really high observations till the end of the night. So the dew cannot fall straight down your OTA onto your mirror.

Keep your fan going all night.

Many people have a 12V hair dryer to blow on the dew to get rid of it - never used one but they appear to work from all reports.

Dew is just one of those things you get used to

[Tannehill]

We were watching that very same satellite event at the MPAS site last night. Very cool.

Dew on your primary mirror, but not your secondary?

Scott Tannehill

[Solanum]

I realised when I put the scope away, I was wrong about the dew, it was on the secondary not the primary, hard to tell in the dark with all thsoe reflections! As the secondary is flat is it more tolerant to warming, i.e. using a hairdrier? Presumably as soon as it cools right down again it'll get dew on it though?

I never understood the hairdrier thing, surely it's going to leave all sorts of air movement, for a while after it's removed and the sides of the tube will warm up and cause more air movement, or doesn't that happen? If not, why not? Also, once everything has settled back down again, doesn't the dew re-form?

Thanks for the tip about the tube extension, I will give it a try.

[Tannehill]

Okay, that (secondary dewing up) is the more familiar pattern. While primary dew problems do occur, it's must less common. The primary - being larger and less "exposed" to the clear night air, will much more slowly radiate itself to a temperature below the dew point. It's also a tad more common (it's said and the science of radiative heat loss seems more consistent also) in truss designed scopes with a more exposed primary (no surrounding mirror box or tube to reduce the radiative heat loss).

Plus warming your primary is really the last thing you want to do, as that will impair image quality by causing distortion of the mirror and creating a boundary layer of warm air to layer along the front of the mirror.

One solution: as suggested, a dew shield will slow dew up by blocking radiative heat loss from the secondary mirror.
Another: as needed warming the secondary up with a dew blaster (or a small hair dryer). 12V products are out there like rmcpb suggested. If you have electricity at you site, I suggest a small 'travel' style hair dryer. Mine - purchased at Big W I think - has a "nose" just the right size to slide into a 2" focuser. I put it on the low setting, stick it into the (empty, obviously) focuser tube, and take a 4-5 min break while it warms the secondary and thereby removes the dew. Yes, it can contribute a bit to tube currents but that'll be better than not seeing at all.
Best solution is a low output dedicated secondary heater designed to warm your secondary mirror to keep it just barely above the dew-forming temperature. Many commercial products are available. This requires either routing power to the secondary (thin wires running along, or designed into, the secondary vanes) or getting one of the 9v options where the 9v battery actually mounts above the secondary so as not to block light to the primary. Hard to believe a 9v can warm anything, but I've heard many folks say they work.

WORST solution - wiping your secondary with a towel! You could scratch it and it will be dewed up again in 3 minutes....

Scott

[iceman (Mike)]

As Scott said, dew shield + hairdryer in the focuser hole will fix that secondary dewing up!

[OneOfOne (Trevor)]

Originally Posted by Solanum

[snip]
A little later I span the scope round to have a look at Jupiter and I was just in time to watch Ganymede move clear of the limb, with it's shadow still over the planet, with Io transiting on the other side! I've only seen a moon transit once before so this blew me away! [snip]

Saturday night we had a Star Squared Party at our place (my wife and I go dancing at "Star Studios" in Bentleigh and had about 20 friends around for a party with the scope out for anyone interested in looking, hence the "squared"). I didn't know what was happening on the night and was just glad of the clear night. The first target for the night was also Jupiter. At first I saw a moon right on the limb and then noticed a black dot in one of the belts. During the night about a dozen people came out in to the cold to have a look and every one of them was really surprised at seeing the transit! Mot reckoned it was "amazing". It was also a first for me too, I have never seen one myself. My first "big thing" was the ocultation of Jupiter a few years ago, only a couple of weeks after I actually bought the scope.