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 REFLECTING TELESCOPE: (AKA Relflector):  A reflecting telescope uses a large parabolic mirror as its objective lens.  This component is convex in shape, and is referred to as the primary mirror .  The diameter of this mirror determines the telescope’s ultimate light collecting capability; the aperture.  The primary mirror is situated inside the truss tube. If we continue to think of the telescope as a “light bucket”, the primary mirror is situated at the bottom of the bucket, waiting to catch and focus the incoming light pouring in.  The focal length of the primary mirror determines how long the truss tube must be.  It must be long enough for the incoming light to hit the parabolic mirror and refocus at a focal point, where the secondary mirror bounces the crisply focused image toward the eyepiece to be magnified.

DiagramReflecting Telescope; inside view of vintage Astroscan
Vintage Astroscan

Reflecting Telescope
Dobsonian Reflector at SBO
Reflecting Telescope; inside view
Peering inside of a Reflecting Telescope
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