Expert: Paul Wagner - 9/8/2011

Question
Hi I have a 8" reflector telescope. with  2" secondary mirror. If I extend the tub to about 1400 mm so a 35 mm secondary mirror will  work  will that increase the power, and is it worth it? Will it be clearer at higher power or about the sane? sincerely John

Answer
HI John

Nothing you can do with the secondary mirror will affect the power of the telescope--because the power of the scope depends on the FOCAL length of the primary mirror and the eyepieces.  And that won't change.

Think of the primary mirror and its focal length as a long skinny triangle.  The base of that triangle is the mirror itself...and the light from that mirror has to focus at a specific point---the top of the triangle.  

Your secondary mirror has to catch that light BEFORE it comes to a focus and direct it out to the eyepiece....so that your eyepiece can see the image.  If you move the secondary mirror farther away from the primary, the triangle will come to a point before reaching the secondary mirror.

In that case, the scope will never focus, because the focus point will now be inside the telescope tube-where no eyepiece can see it.   

Does that make sense?

Paul Wagner