EdZ wrote:
The focal point of an SCT mirror is not constant. If you move the
mirror forward, it would not move the focal point out.

If you turn the focus screw to pull the mirror out, the focal length
increases. If you turn the screw to push the mirror forward the focal
length decreases. Also, the focal length of an SCT is given generally
for the length when it focuses at the back end of the tube. If you
have a diagonal in your f/10 SCT, it is not operating at f/10, it's
longer.

edz

Why isn't ia constant? Think of it as a cone and no diagnoal. If the the
mirror is moved upward without moving the eyepiece, then the eyepiece needs
to be moved upward to regain focus. If you complain that your head would
block the light hitting the mirror, do the same with a diangonal (as in a
Newtonian). You might need to increase the size of the diagonal, but you'd
have to move the eyepiece out towards you. A 6" SCT f/10 has a constant
focal length, right?. Same for a Newtonian. How would it change?


Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet
--
"Summertime,
And the livin' is easy
Fish are jumpin'
And the cotton is high ..." -- G. Gershwin

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