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Old December 16, 2012, 10:44 AM   #5
Senior Member
Join Date: August 27, 2011
Location: western Iowa
Posts: 204
One inch is about 25mm. It's not that the diameter is that much larger but the area (think garden hose). Area of a 25mm tube 506mm, area of 30mm tube is 706mm and transmits about twice as much light. It's also the size of the front glass, bigger glass has better light gathering power. Many tubes have flared front ends to support larger glass.
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