How long should a dew shield be?

[dlp]

I made a dew shield for my 200PDS a while ago from a camping mat, but it's a pain if there is even the slightest breeze. I didn't cut down the width of the mat, so the shield is 56cm/1' 10" long. How far do you think I can cut it down while still retaining its effectiveness?

David

[Stephen]

I may be making this up, but isn't the effective length the diameter of the aperture?

(Edit : Maybe 1.5x)

[Demonperformer]

I heard 1.5x, and this is what I made for my scopes ... have never had any problems of dewing on front lens ... and last night everything else was dripping by the time I had finished.

[Merlin66]

You have a newtonian?

Dew shields don't have a great effect on reflectors (SCT and refractors - yes)

At the best they give you some protection to the secondary...if pain persists I'd cut it back to 200mm or so infront of the tube...

[yoda]

You have a newtonian?

Dew shields don't have a great effect on reflectors.

Is this because on a newtonian the front is compleaty open and the tube acts as a dew shiled in it's own right, or do thay dew up?.

I have only had my scope for a week and been able to uese it on six of thoes nights and I have'nt had the mirror mist up once even at -4.

[lukebl]

.....Dew shields don't have a great effect on reflectors (SCT and refractors - yes)...

I beg to differ! I have had real problems with the secondary mirror of my Newtonian dewing up, so I have constructed a home-made cardboard dewshield. For my 250mm Newt, it's about 200mm long. Seems to work pretty well. It was longer, but I had problems with it sagging and obscuring part of the light path.

[jam1e1]

I have 250mm newt and on few occasions dew has caused issues. Plan to make a shield, what's best material? Will use weed Matt as a cover, it black and doesn't rot, also lightweight material. Use this material for light shield very effective

[brianb]

An overhang of 4x the diameter is required to give good protection against dew. That is often too much for stability ... a dewcap extension of about 1.5x the diameter is useful on nights with a light to moderate wind, but you will need active dew prevention measures (a dew heater strap, controller & power supply) to supplement it on still nights with heavy dewing. If you're still getting vibration issues with a short dewcap, you need a mount upgrade.

[dlp]

Thanks guys, and it doesn't look like the length of mine was far out from 'correct'. Going to make another 300m long with the remaining foam for windy nights. Will test on a very damp night too to see if it's still effective.

David