Dew alert!!

[StuartT]

Set up last night, got a nice polar alignment and star alignment. Focused and found my chosen target. Started taking subs and went indoors to watch some TV while my lovely data accumulated.

1 hour later I went back to check everything was still ok and the front of my SCT was completely fogged up with condensation!!

Un-mounted the scope, brought it in the house and blew a cool air fan over the front for 20 mins. Soon it was crystal clear. Set it all up again and resumed taking subs (assuming that the weather had probably now cooled down enough to have dropped all its dew).

1 hour later completely fogged up again!

Now I know why all you guys have dew heaters!

Lesson no. 27 learned 😞

Edited June 5, 2021 by StuartT

[CloudMagnet]

For me dew is more frustrating than cloud cover, at least you can see cloud and forecast it with reasonable accuracy. I've got to the point of buying a dew shield for my Newtonian to see if that helps.

Always good to try to find a low tech solution first to save on complexity.

[StuartT]

Dew shields... ok, now this has always puzzled me. How on earth can a cylinder with an open end protect against a gas? Dew is water condensing out of the air as the temperature drops. Air is a gas and so surely it goes everywhere. It's not directional surely?

[CloudMagnet]

4 minutes ago, StuartT said:

Dew shields... ok, now this has always puzzled me. How on earth can a cylinder with an open end protect against a gas? Dew is water condensing out of the air as the temperature drops. Air is a gas and so surely it goes everywhere. It's not directional surely?

The way I understand it is that dew forms easily on surfaces exposed to the night sky as heat can radiate away to the coldness of space. But if you cover that surface (with the shield) then you stop that mechanic from happening. I'm sceptical, but for the price i'm willing to give it a try. As a side effect, it does also help stop stray light coming in as well.

Sky at night has a good article explaining it as well:

https://www.skyatnightmagazine.com/advice/skills/how-stop-dew-forming-on-telescope/

 

[Clarkey]

It's not that a dew shield stop the moisture from reaching the optical surface but it prevents the optical surface radiating heat too quickly and thus does not reach the dew point temperature. From what I know about SCT's a dew shield or heater is 'mandatory'. One of the reasons I went for a RC.

[StuartT]

ah! I see. thanks. So I guess a dew shield wouldn't be effective if the scope is pointed at the zenith (since the heat can rise straight out)?

I have invested in one of those dew heater bands (though I am now puzzled by how those don't mess up your careful telescope cooling...)

[Ohgodwherediditgo]

I too suffered with the dew last night. I'm not sure if the dew was responsible for not getting focus with my DSLR or the exposure was too short to pick up the stars, PHD was really struggling so i gave up. Lessons were learned though so not a total loss.

[CloudMagnet]

1 minute ago, StuartT said:

ah! I see. thanks. So I guess a dew shield wouldn't be effective if the scope is pointed at the zenith (since the heat can rise straight out)?

I have invested in one of those dew heater bands (though I am now puzzled by how those don't mess up your careful telescope cooling...)

The orientation of the scope shouldn’t matter. What causes the dew to form is that you have the entire night sky visible to the glass at the front of your scope. That means the glass has the whole of the sky to radiate heat to, so this increases the chances of it cooling down enough for dew.

The shield "covers up" a large portion of the sky so vastly limits the available area that heat can be radiated away, helping keep the glass from dropping in temperature as much.

It is a bit strange at first to get your head around, ill admit. Its not convection of heat that the shield stops, but radiation.

[StuartT]

5 minutes ago, Ohgodwherediditgo said:

I too suffered with the dew last night. I'm not sure if the dew was responsible for not getting focus with my DSLR or the exposure was too short to pick up the stars, PHD was really struggling so i gave up. Lessons were learned though so not a total loss.

Well... I am stacking them now, so let's see what we got. It wasn't properly dark where I live until 11ish, so I was up late 😴

1 minute ago, CloudMagnet said:

The orientation of the scope shouldn’t matter. What causes the dew to form is that you have the entire night sky visible to the glass at the front of your scope. That means the glass has the whole of the sky to radiate heat to, so this increases the chances of it cooling down enough for dew.

 

The shield "covers up" a large portion of the sky so vastly limits the available area that heat can be radiated away, helping keep the glass from dropping in temperature as much.

It is a bit strange at first to get your head around, ill admit. Its not convection of heat that the shield stops, but radiation.

Thanks for explaining. I have often wondered about this

[StuartT]

ok, so just as a stop gap for tonight, I have fashioned a makeshift dew shield from cardboard (I don't have any neoprene to hand).

Should I add any aluminium foil on the inside or outside? Or should it work as it is?

[CloudMagnet]

Should work ok, only worry would be that the cardboard gets so wet that it bends inwards, getting in the way of the image. I would avoid foil on the inside for reflections but shouldn't cause any issues on the outside.

[StuartT]

43 minutes ago, CloudMagnet said:

Should work ok, only worry would be that the cardboard gets so wet that it bends inwards, getting in the way of the image. I would avoid foil on the inside for reflections but shouldn't cause any issues on the outside.

ok, I shall keep an eye on it. Thanks. I have ordered a dew heater band thingy which should arrive next week 

[StuartT]

absolutely amazed by how well my cardboard dew shield worked! No dew at all on the scope even after 3.5 hours!

[CloudMagnet]

2 hours ago, StuartT said:

absolutely amazed by how well my cardboard dew shield worked! No dew at all on the scope even after 3.5 hours!

Good to hear! Sometimes the simplest solutions are the best.

I had considered a full dew heater setup but the expense and added complexity of a battery/cabling didn't seem worth it compared to if a simple shield would solve the issue. It's nice to have one less thing to worry about when imaging.