HOW TO FIGURE OUT WHAT SIZE SECONDARY TO GET
This is a common question for which the answer is not that simple. In a
reflector the light that is bounced back to the eyepiece viewed from the
side is the shape of a cone. The size of the cone where it strikes the
secondary mirror is dependent on many factors, such as the primary
mirror size and focal length, the tube diameter, and the focuser height.
There is a program on the web called "newt" that does a great job of
incorporating these factors and coming back with an answer. A small
side note to this is if you are building a telescope for primarily looking at
planets you should lean toward a small secondary mirror. This will give
you a little more contrast.
Want our input on deciding on your secondary mirror size?
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