How long should a dew shield be?

[Mav359]

Hi guys

I'm going to make a dew shield for my 9x50 finder scope tomorrow as i use it for guiding.

How long does the dew shield need to be?

Cheers

[Bizibilder]

There is no rule for this - basically as long as is sensible! Usually about one and a half times the aparture.

The idea is to prevent heat radiating away from the objective lens, if it does the lens cools and will eventually get below the dew point - hence dew will form. That is why they are more effective if he scope is "horizontal" and next to useless if the scope is "vertical" - (if you see what I mean!). Dew will still form but will take longer to do so. The only way to stop it is to heat the objective to a temperature fractionally above the dew point - hence the use of dew heaters etc. All complicated by the fact that the dew point is a moving target.

[RikM]

I have a fairly long dew shield on my finderguider, probably sticks out 4 or 5 inches.

[Grunthos]

...basically as long as is sensible! Usually about one and a half times the aparture...

1.5 x Diameter is a good rule of thumb.

[Mav359]

Cheers guys