Beginners question - what is a dew shield ?

[BWBlackett]

I've noticed a few questions about dew shields, including the recent one about portable dew shields, can someone please explain what one is and why I might need one on my soon to arrive Skyliner 200?

Thanks,

Brian.

[Kaptain Klevtsov]

A dew shield is required on a 'scope with a front lens to keep a stationary "blob" of air close to the lens. It helps to prevent condensation on the lens which messes up the optics. Dobs don't need dew shields as the primary mirror is way down the tube.

Captain Chaos

[themos]

A dew shield is required on a 'scope with a front lens to keep a stationary "blob" of air close to the lens.

Captain Chaos

I thought that a better way of putting it is that the shield reduces the amount of cold, cold sky that your optical surface "sees". That makes it stay warmer and reduces the condensation on it. It's the same phenomenon that causes car windows to be misted up in the morning more on the road side than the pavement side. The pavement side sees more house wall and less sky, the road side less house wall and more sky. The phenomenon is strongest on a clear night which is exactly when people tend to use their optics :-)

[Cloudgazer]

Dobs don't need dew shields as the primary mirror is way down the tube.

The secondary mirror is still at the top though.. wouldn't that get dew on it? My dob (a Skyliner 200) went all misty while I was out looking at Jupiter the other morning and I assumed it was dew.. does anything else make your scope mist up?

Scott

[Macavity]

I thought that a better way of putting it is that the shield reduces the amount of cold, cold sky that your optical surface "sees".

I heared a very plausible and interesting(!) explanation. Basically, the scope is looking at a "black body" at the canonical "3 Deg Kelvin" (microwave background and all that!). A bit like a radiant electric fire in reverse... albeit filtered by the atmosphere? Perhaps the dew shield stops warmer, moist air settling on this ever cooling surface. I do find it a BIT surprising that this is "helped" by active dew prevention via warming tapes etc. I'd've thought introducing hot air convection above the "objective" would make things worse. Whatever works, in practice, I guess?

[themos]

I heared a very plausible and interesting(!) explanation. Basically, the scope is looking at a "black body" at the canonical "3 Deg Kelvin" (microwave background and all that!). A bit like a radiant electric fire in reverse... albeit filtered by the atmosphere?

's wha' I said, inni'? At least, that's the way they taught us at Imperial College in 1983.

Perhaps the dew shield stops warmer, moist air settling on this ever cooling surface.

The difference is that the optics see a lot of the inside of the warm (273K) dew shield instead of the cold (3K) sky. You basically cut down on radiative heat loss, the convection effect is second-order.

[Macavity]

's wha' I said, inni'? At least, that's the way they taught us at Imperial College in 1983.

Sorry. Not my intent to try to upstage anyone. Just something that interested me...

[themos]

Oh dear, I came across as prickly when I meant to be humorous. I must remember to use the smileys

[Macavity]

S'alright - I might remember to more closely read previous posts sometimes.