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Fahim you only need a dew shield to be about 1.5x your mirror diameter in front of your focuser to work well (in my case that is 300mm so it only sticks out 200mm in front of the tube). With it set up to wrap around the tube end it has a greater diameter than the actual tube it does not vignette the image at all (the tube diameter is 232mm and the inside of the shield is 240mm).
Don't go overboard with the length of the tube and you should be OK.
Cheers
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