Q's about dew shields - again

[bombassa]

I have been assaulted by the dewmonster and is now considering a dew shield for my small newton but:

How long should this be? I have seen numbers like 1,5x apparature to 2,5x apparature but what should I go for?

Will a dewshield affect the views in any negative way? I would think that the shape of the cone of light reaching the primary mirror could be changed somewhat?

[John]

I use one that extends about 14" above the end of the tube of my 10" newtonian. It does not affect the views at all, other than i) keeping stray light out of the top of the tube and ii) keeping dew off the secondary mirror.