Is there a point where there is no point fighting the dew?

[Stonius (Markus)]

After my recent experiences getting dewed out, I ended up making a dew heater from an Arduino that measured the secondary temperature and ambient temp and humidity, then calculates the dew-point and heats the mirror only a degree or two above that level.


I was very happy that it worked like a champ.


At about 2am I noted that the temperature had fallen to 4 degrees and the humidity had hit 100%, which concurred with the fact that everything else was sopping wet while the secondary was miraculously clear.


Unfortunately, my finder had long ago misted over, the replace ment red-dot finder had misted over in minutes, and even the 16" primary mirror was starting to go as well. Even the 'astronomical hairdryer' only gave brief reprieves, so I called it a night.



On the way home I was thinking what could be done. I could add another dew-heater for the finder. Maybe another for the filter rack and eyepiece, but what do people use for a large primary mirror? Do thy even make dew straps that big? Would it deform the mirror shape?



At that point I was thinking it was getting a bit ridiculous. Maybe there are just times you have to accept that dew has won the night.


How far do you go to combat dew, or is there just a point where you admit defeat? Do you factor the likelihood of heavy dew into your decision to go out?


markus

[AstroViking (Steve)]

I think there are some nights when you have to admit defeat and pack up.

[Stonius (Markus)]

Originally Posted by LaraB2

When you are given no other option, yes very much so.

I thought I had found my perfect astronomy spot because the seeing is usually at least 30-40% better than Melbourne. If course that doesn't help much if you can't see a thing for the dew Hmmm. Maybe the search continues.

[glend (Glen)]

When I had my 16" Dob, it's primary also managed to hang on longer than anything else, mainly due to the mass. Pre- heating it worked ok at home but out at the Dark Sites, with no power assist, there was no point struggling beyond midnight, when everyone else was counting meteors and drinking Port to ward off the chill. So yeah, you got to know when to say enough.

[Stonius (Markus)]

Originally Posted by glend

When I had my 16" Dob, it's primary also managed to hang on longer than anything else, mainly due to the mass. Pre- heating it worked ok at home but out at the Dark Sites, with no power assist, there was no point struggling beyond midnight, when everyone else was counting meteors and drinking Port to ward off the chill. So yeah, you got to know when to say enough.

I did wonder if a hand warmer thrown on the ground under the mirror might do the trick

[Dave882 (David)]

It’s worth noting that when the dew is THAT heavy, with that much moisture in the air the seeing is going to be quite poor for any high mag viewing or long fl imaging.

[Stonius (Markus)]

Originally Posted by Dave882

It’s worth noting that when the dew is THAT heavy, with that much moisture in the air the seeing is going to be quite poor for any high mag viewing or long fl imaging.

It was around 1.2" from memory, which is not that great for that spot.

What is the relationship between seeing and dew though?

[glend (Glen)]

Originally Posted by Stonius

It was around 1.2" from memory, which is not that great for that spot.

What is the relationship between seeing and dew though?

I believe the condensation layer, which hangs just above the ground, acts as a filter of sorts, defracting incoming photons. The moisture layer maybe fairly thin, but your stuck on the ground. If you could get onto a high peak, things might be much better. For example, if you have ever seen fog settled into valleys, where it tends go hang on. Of course, this is more typical in calm windless night.
As far as the chemical hand warmers are concerned, I have used them in my pockets but they are more contact warming devices rather than radiant heaters. Heating in my observatory does work well, but you risk rising convection currents. I had thought that an observatory tent might be a workable solution, but there are not any made for large Dobs as far as I know, and you would need an internal heat source in the tent to keep the optics above dew point.

[astronut (John)]

Markus, Do you have a fan on the back of the mirror?
If you do, you should be running it all night.
It will keep the dew at bay.

[Stonius (Markus)]

Originally Posted by astronut

Markus, Do you have a fan on the back of the mirror?
If you do, you should be running it all night.
It will keep the dew at bay.

There is no space below the mirror, unfortunately, being an ultra-light. They're pretty compact down there, but I have recently added a fan to blow across the mirror to experiment with removing boundary layers. But I didn't have it on that night. I think among everything else I was trying to figure out, I just kind of forgot. Or maybe I was concerned about power, even though it only takes 0.3A. not sure why I didn't. I'll do that next time.

I wonder why it is that moving air (which is exactly the same temperature as the still air) doesn't deposit dew?

[Stefan Buda]

Originally Posted by Stonius

I wonder why it is that moving air (which is exactly the same temperature as the still air) doesn't deposit dew?

Don't forget that the temperature of objects "looking" at the sky can drop below ambient, so blowing ambient temperature air at them can prevent this from happening. Also the fan consumes power and some of that is converted to heat. Therefore the air blown by the fan is a bit warmer than ambient.

[N1 (Mirko)]

Out of all of my observing and/or imaging sessions over the years, only one (1) was cut short by dew. The mistake on that night was to take just one optic and no observing plan, or a plan for dew management for that matter. That was years ago.

Dew is frequent and heavy around here, so I usually have spare finders (or even OTAs) waiting in the car or in the house. A quick swap, and the observing continues. If needed, I just switch to some other item from the list of things I'd like to do, which is always much longer than could be achieved with 1 optic during 1 night anyway.

Originally Posted by Stonius

I wonder why it is that moving air (which is exactly the same temperature as the still air) doesn't deposit dew?

Alex (mental4astro) has elaborated quite a bit around that topic. His posts both here and on CN (maroubra boy) are worth finding.