Expert: Paul Wagner - 8/30/2008

Question
What do the telescope filters do?Does the blue filter attract the blue color or does it block it or what?

It is said when i build a telescope the tube should be 2 inches bigger than the diameter of the primary mirror.So is that the whole tube from the outside or from the inside without the tube's width?

Answer
Hi Hady

Good to hear from you again.  

A blue filter let's only blue light through---but it makes some things clearer.  For example, things that are red usually turn almost black, and more visible, with a blue filter.  

The reason you make the tube bigger is for ventilation and a clear light path.  Measure that from the inside of the tube to the outside of the mirror.  It would be good to have about one inch clearance all the way around the mirror.

Paul Wagner