William Hamblen wrote:
On Mon, 31 Jul 2006 20:29:56 GMT, "W. Watson"
wrote:


My understanding of the focus knob found on SCTs (all?) is that it is
attached to a long screw thet is on the back of the mirror. It would seem to
me that if one turns the screw CC that it would pull the mirror toward the
observer; thereby moving the focal point of the mirror inward. I tried this
with a 4" Meade SCT, and the focal point moves outward? Perhaps this depends
on where one things the head of a typical screw is in the device. I would
guess it essentially be where the focus knob is located. Can anyone clear
this up?


The mirror is attached to a fitting that runs up and down the threaded
rod. When you turn the knob clockwise the mirror moves to the back of
the telescope and when you turn the knob counterclockwise the mirror
moves to the front of the telescope.

It seems like if you go CC, then the screw would descend--taking the mirror
down, and the focal point would move into the eyepiece further. I observe
the opposite. However, if the screw thread is the reverse action of a
conventional screw, that would explain it. I'm not sure the screw analogy
completely explains this all. It would seem like a descending screw would
force the focus knob down too, which obviously dosn't happen. A drawing of
the innards would help. I have a 4" SCT that might get sacrifices. :-)


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