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. Due to the fact
the SCT has a magnifying secondary, as you move the mirror further from
the secondary, the focal length increases. That explains why the focal
point moved out when you pulled the mirror further 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 generally given
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

It's conceivable the secondary has some effect, but I believe what i've
observed is that when the mirror is pulled back the focal point shifts
outward. I say pulled back, since CC, in my mind, equates to lowering the
mirror. When I move CC, then I need to pull the eyepiece further out.

Ah, I just found Ken Novak's Cass Notes. Let's see if he says anyting about
focus. It doesn't seem to be of much help. I think I have another book on it.


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

Web Page: home.earthlink.net/~mtnviews