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Originally Posted by netwolf
I thought that a closed tube would be better to combat light pollution.
Regards
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The only light that matters is the light that can reach the eyepiece.
If light is reflected off the primary at some oblique angle that wont make it to the eyepiece who cares? There are some considerations like light comming in from underneath the secondary cage and illuminating the inside and light scatter from dirt on the primary.
Other reasons for having a shroud is keeping dew and dust off the primary, and preventing thermals from body heat entering the light path.