Dew inside the objective

[StuartT]

Last night was particularly heavy for dew and this morning I am seeing a fine mist of dew on the inside of my frac's objective. How can I clear that?

[Mandy D]

3 minutes ago, StuartT said:

Last night was particularly heavy for dew and this morning I am seeing a fine mist of dew on the inside of my frac's objective. How can I clear that?

I had dew form on the inside of a camera lens when I moved my camera equipment in from -24 °C in Switzerland to a warm room. I just put it back in the camera bag and let it come up to room temperature slowly. The next time I looked, a few hours later, it was clear.

[DaveL59]

If you have one, a desiccant cap will help pull the moisture also. If not you can make one for 1.25-inch focusers using a 35mm film canister if you have any old ones kicking around. Otherwise as Mandy said, it'll clear by itself when the temps climb - worth leaving the caps off tho so air can circulate to aid this.

[StuartT]

Thanks both. 

I did think of leaving the cap off at the focuser end, but I think my scope has some other kind of lens at the bottom, so is not really an open tube. I have been unable to find a cross sectional diagram to verify this. It's an Esprit 150 ED

Edited November 11, 2023 by StuartT

[Franklin]

Put a dew heater band on and leave it.

When it's cold outside with a lot of moisture around, as it was last night, I put the lens caps on before I bring the optics back inside and leave the dew strips running for a while. There's nothing worse than bringing a freezing cold bit of glass from outside into a warm house, it will instantly fog over. By putting the caps on you are trapping some cold air against the glass which will then warm up gradually and hopefully not form any water droplets but evaporate away slowly. I keep a desiccant bag inside the dew-shield, trapped by the aperture cap so it can't get free and touch the objective and a desiccant 2" insert at the eyepiece end. Never had any problems when doing this.

Edited November 11, 2023 by Franklin

[StuartT]

8 minutes ago, Franklin said:

Put a dew heater band on and leave it.

When it's cold outside with a lot of moisture around, as it was last night, I put the lens caps on before I bring the optics back inside and leave the dew strips running for a while. There's nothing worse than bringing a freezing cold bit of glass from outside into a warm house, it will instantly fog over. By putting the caps on you are trapping some cold air against the glass which will then warm up gradually and hopefully not form any water droplets but evaporate away slowly. I keep a desiccant bag inside the dew-shield, trapped by the aperture cap so it can't get free and touch the objective and a desiccant 2" insert at the eyepiece end. Never had any problems when doing this.

Yes, I have the dew heater on now.

Sorry, I should have said my scope lives outside, so it wasn't a question of bringing a cold optic into a warm house.

[Franklin]

2 hours ago, StuartT said:

Yes, I have the dew heater on now.

Good, that should clear it up with time.