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  1. #1
    W. Watson's Avatar
    W. Watson Guest

    Default 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>




  2. #2
    Brian Tung's Avatar
    Brian Tung Guest

    Default Which Way Is Out on a SCT Focus Knob?

    W. Watson wrote:

    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 <brian@isi.edu>
    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

  3. #3
    RMOLLISE's Avatar
    RMOLLISE Guest

    Default Which Way Is Out on a SCT Focus Knob?


    W. Watson wrote:

    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/>


  4. #4
    EdZ's Avatar
    EdZ Guest

    Default Which Way Is Out on a SCT Focus Knob?


    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


  5. #5
    EdZ's Avatar
    EdZ Guest

    Default Which Way Is Out on a SCT Focus Knob?


    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


  6. #6
    William Hamblen's Avatar
    William Hamblen Guest

    Default Which Way Is Out on a SCT Focus Knob?

    On Mon, 31 Jul 2006 20:29:56 GMT, "W. Watson"
    <wolf_tracks@invalid.com> wrote:


    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.

  7. #7
    W. Watson's Avatar
    W. Watson Guest

    Default Which Way Is Out on a SCT Focus Knob?

    RMOLLISE wrote:
    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>

  8. #8
    W. Watson's Avatar
    W. Watson Guest

    Default Which Way Is Out on a SCT Focus Knob?

    EdZ wrote:
    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>

  9. #9
    W. Watson's Avatar
    W. Watson Guest

    Default Which Way Is Out on a SCT Focus Knob?

    EdZ wrote:
    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>

  10. #10
    W. Watson's Avatar
    W. Watson Guest

    Default Which Way Is Out on a SCT Focus Knob?

    William Hamblen wrote:
    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>

 

 
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