Which Way Is Out on a SCT Focus Knob?

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?

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

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?

Don't know about the mechanics of the situation, but for the purposes
of star-testing, toward infinity is inside focus. This direction is
indicated on many SCT knobs.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

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

Clockwise...

Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
http://skywatch.brainiac.com/astroland
The Annual SCT User Imaging Contest is Underway!
http://www.rothritter.com/contest/2006/

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

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

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.

--
The night is just the shadow of the Earth.

RMOLLISE wrote:
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


Clockwise...

Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
http://skywatch.brainiac.com/astroland
The Annual SCT User Imaging Contest is Underway!
http://www.rothritter.com/contest/2006/

Why? What are the details of the device and does the direction differ
between mfgers? Further what does out mean? the Mirror moves out or the
focus movies out?


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

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

Web Page: home.earthlink.net/~mtnviews

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

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