Quote:I just got my first non-refractor telescope which is a Celestron C6. I always thought that "clear aperture" was the objective diameter minus the diameter of any obstruction such as the secondary mirror. However when I look at specifications of the C6 they say clear aperture of 150mm. Shouldn't the clear aperture be 150mm minus the diamter of the secondary?
The reason I'm asking is because I'm trying to compare the light grasp of a C6 to something I am familiar with such as my 120mm refractor.
Regardless of the exact definition of "clear aperture", I calculate that the C6 will have a light grasp about 1.4 times greater than the 120mm refractor. Correct? I used a 50mm secondary diameter for that calculation because I don't know the true size in a C6.
Quote:Hmmm... the last two posts seem like two different definitions of clear aperture.
The description by Mitch sounds technically accurate to me. For an SCT with a 150mm corrector plate and a 56 mm central obstruction it seems like the clear aperture should be listed as 139mm (using the method described in the last line of Mitch's post) instead of 150mm as Celestron does.
Quote:Clear aperture in a catadioptric telescope (SCTs, Maksutovs) is often considered to be the width of the corrector plate from one side to the other. In SCTs, the primary mirror tends to be just a bit larger than the corrector plate's diameter, as the corrector plate deviates the light a bit requiring a bigger primary to catch it all and make it converge towards the secondary mirror. The C6 has a clear aperture of 5.91 inches. It has a secondary obstruction diameter of 2.20 inches, so the total surface area that collects light is 26.6 square inches. This would be equivalent to the light gathering area of a 5.39 inch (137mm) aperture unobstructed telescope like a refractor. However, the resolution of the instrument would still be about that of a 5.91 inch aperture telescope. Clear skies to you.
Rob 18" f/4.3 Starmaster 8" Meade LX200 Classic Celestron 15x70 Skymaster Binoculars