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  1. #1
    Paulie's Avatar
    Paulie Guest

    Default when eye pupil smaller than exit pupil.. and CO question



    Hi,

    I can't find a definite answer to this in the archives so allow me
    to ask this question that has consumed my time the past 8 hours.

    I know that when the eye pupil is as large as the obstruction
    portion of the exit pupil.. one can't see anything because
    there is no rays. How about if the eye pupil is halfway between
    the exit pupil maximum size and the obstructed portion?? How
    come you can notice the shadow of the obstruction in the
    eyepiece when viewing at daytime where the eye pupil
    gets smaller and can even be smaller than the exit pupil (but
    not reaching the obstructed portion but just halfway).

    Someone (Steve) wrote the following and I think it is wrong.

    "The on axis rays behind the obstruction (there are none)
    carry no energy to the focal plane, where the rays of the
    annulus around the obstruction carry both on axis and off axis
    energy to the focal plane.

    By decreasing the eye pupil to a size smaller than the exit
    pupil of the system you remove off axis (and on axis) light
    from both the area on the focal plane that is occupied by the
    shadow, and the area of the focal plane that is occupied
    by the annulus around the obstruction. The shadowed area
    then becomes darker, faster, since it contributes no on
    axis rays to the area under itself."

    -----

    Well. the reason why I think the above explanation is wrong
    is because rays entering the objective lens end up in all the
    points of the focal plane. Imagine a region a few
    degrees from the center of the focal plane. From
    its point of view, the obstruction blocks the center
    part of the off axis slanted parallel rays entering the
    objective lens.

    Therefore the person explanation quoted above is not
    the reason why we can notice the shadow of the central
    obstruction in the eyepiece.The sole reason being that the
    obstruction is like a defocussed image much like what happens
    if you put your fingers in front of the objective lens
    forming a defocussed image that makes you notice
    the shadow of the obstruction forming at the eyepiece
    when viewing bright images.

    What do you think of Steve theory?? Do you agree with
    it or not and why? Thanks.

    Paulie




  2. #2
    MitchAlsup's Avatar
    MitchAlsup Guest

    Default when eye pupil smaller than exit pupil.. and CO question

    It is my understanding that when you eye pupil is smaller than the
    exit pupil of the optical system, then the optical system is operating
    as if it were of a smaller aperture.

    In the case of an optical system with central obstruction, the central
    obstruction would be a larger percentage of the light that does enter
    the eye, and thus, the image is darker overall, and teh diffraction
    effects of the CO would be as if the CO were this larger percentage of
    the aperture.

    In the case where the eye pupil is off axis with respect to the CO
    part of the exit pupil, then the optical system is operating as if it
    were unobstructed! but with only the objective size as allowed through
    by the eye pupil.

    Thus a 20" scope with a 4" secondary (20%) operating at F/5 (fl =
    100") using a 50mm eye piece creates an exit pupil 10mm in diameter,
    the inner 2mm are not illuminated due to the CO. If you place an eye
    with a 4mm entrance pupil off axis so that it admits only that part of
    the exit pupil that is not centrally obstructed, the scope is now
    operating as if it were unobstructed with an objective of 8" (and with
    the coma of an F/5 optical system.) That same 4mm eye pupil placed on
    axis with the scope would see an 8" objective with a 50% central
    obstruction (and with the coma of an 8" F/12 optical system.)

    Now, if a ground glass screen is placed at the image plane inside the
    scope (or inside the eyepiece) then the image tramsitted outwards has
    all the optial aberations of the 20" F/5 objective, no mater where the
    eye pupil is placed.

  3. #3
    dkelvey@hotmail.com's Avatar
    dkelvey@hotmail.com Guest

    Default when eye pupil smaller than exit pupil.. and CO question

    On Jan 17, 12:16 am, Paulie <jones.pau...@yahoo.com> wrote:

    Hi
    First one has to understand what the exit pupil actually is.
    It is the location ( usually outside of the eyepiece for
    modern eyepieces ) where the obstruction is in focus.
    Try holding a piece of paper behind the telescope while
    point at a daylit sky ( no sun please unless you hace a fire
    extinguisher handy ).
    You should see the outside edge is usually the aperature
    of the telescope. If there is a center obstruction, it will be
    close to being in focus as well.
    Imagine that if your pupil was smaller than the central obsruction
    at the exit pupil focal location.
    There would be no light getting to your eye.
    Now, if you moved your head a little to the right or left.
    The entire image would go from dark to light.
    Now for another change, lets say your head was a little
    farther from where your pupil was right at the point that
    the exit pupils image was. You would now see the defocused
    image of the secondary move across the field of view.
    Also, notice the direction of travel of the secondary obsruction.
    If it moves the same way as your head, that means you eye
    is farther from the optimum location. If it moves the same
    way as your head, that means you are closer to the eye
    piece.
    Try this looking at the sky but no where near the sun.
    Dwight

  4. #4
    Paulie's Avatar
    Paulie Guest

    Default when eye pupil smaller than exit pupil.. and CO question

    On Jan 18, 6:44 am, "dkel...@hotmail.com" <dkel...@hotmail.com> wrote:

    Hi, Searching at the archive.. I saw the following you
    wrote that is quite ambiguous. You wrote:

    "Exit pupil talks about how far from the eyepiece to move
    ones eye such that image of the primary's edge and the
    secondary's shadow are in sharpest focus at the pupil of
    ones eye. As long as the size of the primary's image is
    smaller than the pupil, there is not loss of light at
    the center of the field of view. When the image, of the primary,
    at the pupil gets such that it is larger than the pupil,
    you will see some darkening towards the center of the image,
    cause by the secondary shadow".

    What is your context of the meaning that the image of the
    primary being smaller or bigger than the pupil (what pupil
    is it the exit pupil or eye pupil?) Pls. describe by means of
    the exit and eye pupil.

    Also do you agree with Steve that it has something to do
    with the on-axis and off-axis rays forming the image at
    the focal plane. But we know that central obstruction
    effect ends up in all the airy discs and the brightness loss
    should be uniform. Steve believes that somehow
    only the shadow of the obstruction gets darker with
    respect with the annulus or sides which doesn't
    conform with telescope image formation principle.
    Maybe he doesn't undersand how the image
    formation at the focal plane works.

    Paulie


  5. #5
    dkelvey@hotmail.com's Avatar
    dkelvey@hotmail.com Guest

    Default when eye pupil smaller than exit pupil.. and CO question

    On Jan 17, 3:49 pm, Paulie <jones.pau...@yahoo.com> wrote:

    Hi
    I'll state it again. the exit pupil of a telescope is where the
    image of the primaries edge forms. In the case of a Newton
    telescope, this is simply the focused image plane caused
    by the eyepiece lens. Again, it can be see where this
    is by using a piece of paper and placing moving it towards
    and away from the eyepiece to find the focused image.
    When this image plane is right on your eyes pupil, the
    most light from the distant star is entering your eye.
    If you move away from the telescope, the image of the
    primary will defocus such that the light going through
    the telescope will not all make it through your pupil.
    The size of the exit pupil image is a function of the size
    of your primary and the focal length of the eyepiece.
    When the exit pupil image is larger than your eyes
    pupil, you will be losing some of the light from the
    telescope. This happens when, your pupil is small
    from ambient light, the eyepiece is too short a focal
    length, the primary is too large or the primaries focal
    length is too short.


    I'm not sure I understand Steve's statement. The image
    of the primary formed at the exit pupil location contains all
    the light from the primary that goes into creating the image
    of the object you see through the telescope. This includes
    the missing light from the secondary in the center of this
    image.
    The secondary does have an effect on the spacial frequency
    that one sees as a result of the wavelike effect of light.
    I'm not sure what the statement "only the shadow of the
    obstruction gets darker with respect with the annulus or
    sides which doesn't conform with telescope image formation
    principle" means.
    Brightness loss is only uniform at the eye when the eye's
    pupil is right at the focal plane of the exit pupil. Any other
    distance
    from the eyepiece will form a non-uniform intensity loss across
    the field of view.
    This can be shown by the fact that when the eye is right
    at this focal plane( exit pupil ), one can move left to right and
    see a uniform increase and decrease of light across the field
    of view.
    At any other distance you will see the shadow move across
    the field of view ( although not in sharp focus at the image
    formed in the eye ).
    All this really comes from understanding what exit pupil is.
    In a Newton, the secondary does focus at a slightly different
    distance from the eyepiece than the primary but it is so small
    a difference that it has little effect in reality.
    I try to make it clear but it is really hard to do without taking
    one to a telescope and showing them what it is.
    Dwight


 

 

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