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Thread: SCT design

  1. #1's Avatar Guest

    Default SCT design

    Just a question I've wondered about recently. While browsing .66X
    reducers for SCTs,
    I keep seeing the quote that Celestron doesn't produce a f/6.6 SCT
    because the design
    would require too large a central obstruction.

    Why would this be so? Why couldn't they produce it with the same f/2
    primary, and
    make the secondary 3.3 magnifying instead of 5X, and keep it the same

    I know there must be a fatal flaw in my logic, but I don't know what it
    is. Any help?

  2. #2
    Chuck Taylor's Avatar
    Chuck Taylor Guest

    Default SCT design

    > Why would this be so? Why couldn't they produce it with the same f/2

    Doing that would mean the light cone would remain fairly
    steep. Therefore the focal point would be inside the tube,
    rather than behind it where you can put a diagonal and eyepiece.

    Clear Skies

    Chuck Taylor
    Do you observe the moon? If so, try

    If you enjoy optics, try

  3. #3
    Roger Hamlett's Avatar
    Roger Hamlett Guest

    Default SCT design

    <> wrote in message
    Think about the geometry for a moment. The light cone is tapering at the
    final focal ratio, forward from the focal point (which typically is
    perhaps 8" or more behind the primary), all the way to the secondary. The
    secondary has to be large enough to 'cover' this cone, and provide any
    extra size needed for the fully illuminated field. If you take F/10, and
    have the primary at about F/2, then the secondary can be about Primary
    Diameter*1.5 from the primary, giving a cone of light that is 25% of the
    diameter of the primary at this point. If you then have the secondary at
    perhaps 33% of the diameter of the primary, you have the required margin
    to give an illuminated field. If the cone then tapers at F/10 from the
    secondary, it will focus at about one mirror diameter behind the primary
    (D*2.5 from the secondary). Now change the ratio of the secondary, so it
    gives F/6.6. The cone will now reach focus D*1.65 from the secondary. This
    puts the focal point just behind the secondary. No good. If you redesign
    to give the same distance to the focus point as the F/10 unit, the
    secondary has to grow to 37.8%, _just to give an illuminated dot in the
    centre field_. Assuming that you need the secondary to give some field
    beyond this, it'll typically need to be about 5 to 8% larger than this,
    taking it up to about 45%...
    The big minus, is the size of secondary needed. The big plus is that you
    get a flatter focal field.
    On all scopes of this sort, there is a relationship between the
    illuminated field, the final focal ratio, the distance that the focal
    plane is behind the primary, and the size of secondary required. You can
    get scopes with very small secondaries, built to have tiny fully
    illuminated fields, or with the design focal point close to the primary,
    again giving smaller secondaries for a given focal ratio.

    Best Wishes

  4. #4
    Llanzlan Klazmon's Avatar
    Llanzlan Klazmon Guest

    Default SCT design wrote in news:1132001348.204341.85850

    The reason for the size of the secondaries in all reflecting scopes is to
    get a well illuminated field of view - generally for scopes that have
    primarily visual use you can get away with only a small fully illuminated
    field and therefore a correspondingly smaller secondary - this is seen with
    a lot of Newts and Mak-Newts where they have a very small central
    obstruction to optimise contrast at the expense of field illumination. For
    a scope used for photography, you want the entire field of the sensor or
    film to be well illuminated which means a larger secondary obstruction.

    For a faster scope (lower f ratio), you generally need a larger central
    obstruction to get the same illumination as you would for a slower scope of
    otherwise the same design. SCTs tend to have fairly large obstructions
    anyway but are even larger than you could get away with because they are
    usually designed as general purpose visual/photographic scopes.


  5. #5
    Phil's Avatar
    Phil Guest

    Default SCT design

    I wounder about that too, Meade used to sell the classic LX200 with a choice
    of ratio's f/10 and f/6 so it is possible

    <> wrote in message

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

    Default SCT design

    On 14 Nov 2005 12:49:08 -0800, wrote:

    The flaw is that a secondary that magnifies 3.3 times has to be 1.5
    times larger than one that magnifies 5 times. You can't keep it the
    same size and still catch all the light from the primary..

  7. #7
    Mike Ruskai's Avatar
    Mike Ruskai Guest

    Default SCT design

    On or about 14 Nov 2005 12:49:08 -0800 did dribble

    Because you'd move the focal plane inside the tube (or close to it).

    No matter how you go about it, the secondary will need to be larger to
    get an equivalently illuminated field as the focal ratio goes down.
    - Mike

    Ignore the Python in me to send e-mail.

  8. #8's Avatar Guest

    Default SCT design

    Phil wrote:

    Several of their early LX200GPS models were available as f/6.3 too, but
    they've been discontinued for awhile (same as the last production of
    LX200 7" Maks has just been completed; I believe the last run was just
    50 or so).

  9. #9
    Stephen Paul's Avatar
    Stephen Paul Guest

    Default SCT design

    Right, but the 6.3 had a larger secondary....

    "Phil" <> wrote in message



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