Condensation, talk to me :-)

[GasGiant]

Hi all

 

So I had my first look through the new dob last night. Eager to get out I was out early while the moon was not too high. Being a full moon as well it was super bright, but hey ho.

Earlier that day I had set it all up and just left it outside.  

The condensation on the scope when I came to it was all over the wood mount, the lenses, the tube was wringing ! and obviously made any viewing totally impractical. 

We cant get away with it. Do you guys look at the humidity/dew point and just not bother if its high, last night was 75% humidity for me. Or what are your fixes ? Heated tubes, do you 'lag' the eyepieces etc 

I didnt run the fan last night, should I have ? 

Im a pretty practical guy, Id like to try some homemade fixes if poss 

 

Carl 

 

[bosun21]

On the nights of heavy dew I just used a standard dew shield and if really bad I used heated bands on my eyepiece and finder. You can make a dew shield from a yoga mat or the like.

Edited September 30, 2023 by bosun21

[John]

7 minutes ago, bosun21 said:

On the nights of heavy dew I just used a standard dew shield and if really bad I used heated bands on my eyepiece and finder. You can make a dew shield from a yoga mat or the like.

Good advice above to keep dew off the mirrors.

Keep your eyepieces a little warmer than the outside temp and they will stay dew free and not mist up from body heat on colder nights. I keep mine in foam lined cases. The foam seems to retain a little heat for some time.

 

[dweller25]

As others have said, use a long dewshield to keep the secondary from dewing over or attach a heater to it and keep the eyepieces warm.

The tube will protect the primary from dewing over for a longtime.

The tube and mount will be fine even if they are covered in dew (or ice in winter), just towel them dry when you put them back into storage.

You could try experimenting with the fan to see if it helps, though I found on some scopes - a Maksutov in particular - it made things worse if you used the fan too much.

Edited October 1, 2023 by dweller25

[Nik271]

10 hours ago, GasGiant said:

 

Earlier that day I had set it all up and just left it outside.  

 

If you leave the scope outside to acclimatise in the evening the danger is that it may become significantly cooler than the ambient air by radiating heat into the sky.

What I do is drape a towel over the scope, it is very effective at blocking the IR. A tarp for covering motocycles/barbeques is anothe cost effective option.

 

 

Edited October 1, 2023 by Nik271

[CraigT82]

As Nik says any surface that can ‘see’ the sky is going to radiate its heat away and become colder than the ambient air temp and eventually below the dew point temp, This is when the dew forms on those surfaces. 
 

When I set up early and then leave it I’ll usually throw a large beach towel or tarp over it all, helps to prevent this radiated heat loss to the sky.

 

[Carbon Brush]

Good advice from the above.
This thread also contains some useful comments.

 

[Jules Tohpipi]

The really significant factor in predicting this is the dew point figure rather than the relative humidity figure (albeit they are linked). But it's not an entirely intuitive concept to understand the mechanics of, so most weather forecast services don't bother quoting it to the public. Nor are most of the usual online references very good at explaining the dew point concept in a correct or understandable manner. The overall upshot being if the humidity levels are medium/high with the dew point relatively close to the actual temperature, then taking out a large metal tube will result in dew forming sooner and particularly later (where it will cool at a faster rate than the ambient temperature, plunging it down to the dew point). 

In these more extreme situations, the only real solution is to get the temperature of the equipment up beyond the dew point - or stop it cooling too much. Personally I don't bother if it's bad as I'm trying to de-clutter the set-up process these days.

Of course, risk of dew is considerably higher when it's been a warm and wet day, then the air temp drops if the skies have cleared. The warm day allows the air to hold more moisture but the same air can't hold all that moisture if the temperature drops - generating lots of dew. As has been the case the last few days in some parts of the UK.   

Edited October 1, 2023 by Jules Tohpipi

[Moonshed]

I don’t own a dob and have no experience of them but with my Celestron I cover the whole set up with a TeleGizmos scope cover and that does a great job at keeping it dew free. Could this not be used to the same effect on a dob? If this idea is stupid please feel free to tell me 🤪