ATM Planetary Newtonian

[Ed Rosch <earosch@home.com>]

Hi,

I've been thinking about building an 8" Newtonian that is optimized for
planetary performance.  A couple of questions:

I would like to make it an f/8 but would like to know how much
improvement would be gained at longer focal lengths?

Would there be any advantage to using a three vane spider?  I know that
it will give six diffraction spikes on stellar objects, but would the
contrast gain on planets be noticeable?

I've heard that there are issues if one is attempting to use a low
profile focuser with both Pentax XL and TeleVue eyepieces?  Is this
true, and could someone explain the issues?

Thanks for the assistance,

Ed Rosch
earosch@home.com