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  1. #1
    Tenifer's Avatar
    Tenifer Guest

    Default On-axis, off-axis, and neutral axis light




    On-axis light rays are the parallel rays entering the objective lens.

    Off-axis light rays are the rays that don't reach the focal plane
    or eyepieces but reflect off the wall.

    What do you call other rays from the scope? You can see
    the other rays by this simple method. Simply look at the
    eyepiece about one foot away. You can see the central
    obstruction in the center of the eyepiece. Now how does
    the light or image of the obstruction passes thru or
    register in the eyepiece? This is not on-axis
    light nor off-axis light. What is it called and its optical
    path or lines? I've been figuring this out for ten years and
    haven't arrived at a solution. Anyone knows?

    Teni


  2. #2
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default On-axis, off-axis, and neutral axis light

    On Sat, 03 Nov 2007 06:07:55 -0700, Tenifer <tensorsurfer@yahoo.com>
    wrote:


    I still don't understand your confusion on this. The obstruction you are
    seeing is simply a shadow in the on-axis rays coming into the
    telescope's aperture. There are no rays associated with "seeing" the
    central obstruction, only an absence of on-axis rays.

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  3. #3
    John Savard's Avatar
    John Savard Guest

    Default On-axis, off-axis, and neutral axis light

    On Sat, 03 Nov 2007 06:07:55 -0700, Tenifer <tensorsurfer@yahoo.com>
    wrote, in part:


    No, the term off-axis also applies to light rays that do reach the focal
    plane, and do form part of the image you see. Otherwise, off-axis
    aberrations, such as coma and astigmatism would not be discussed.


    You are thinking of things like the "conjugate plane" here.

    Basically, there's no law that says that you have to use a telescope as
    a telescope. You could try to look at a bug two inches from the
    objective lens with it. But it would be out of focus.

    If you took the eyepiece out of the telescope, and stood far enough
    back, though, you could see the bug.

    Basically, a refracting telescope has a lens in the front; it's made out
    of glass. And a reflecting telescope has a mirror in the back. The glass
    lets light coming from all directions go through it, and the mirror
    reflects light from anywhere. Neither of them restrict the light they
    let past them to the light that will form an image that is in focus.

    So if I put my hand in front of your telescope when you're observing,
    you will see a blur blocking your view, and if I were to shine a
    flashlight in, you would be bothered by the glare. The telescope can't
    stop the light, even if the light isn't coming from the right place to
    come to a focus.

    John Savard
    http://www.quadibloc.com/index.html

  4. #4
    Tenifer's Avatar
    Tenifer Guest

    Default On-axis, off-axis, and neutral axis light

    On Nov 4, 2:24 am, jsav...@excxn.aNOSPAMb.cdn.invalid (John Savard)
    wrote:

    That's right, this is closer to what I'm intending to find out. I'd
    like to know the ray trace of this scenerio of the hand put in
    front of the telescope or flashlight shining inside. It appears
    they don't appear at the focus that the eyepiece can use to
    present image to the eyes at the eye relief distance.

    Teni


  5. #5
    Tenifer's Avatar
    Tenifer Guest

    Default On-axis, off-axis, and neutral axis light

    On Nov 3, 10:29 pm, Chris L Peterson <c...@alumni.caltech.edu> wrote:

    So how come the shadow can produce contrast lost in the
    middle of the eyepiece. Is it because black color is added
    to the pure image causing contrast lost. This is unique in
    that we don't usually encounter scenerio where we can
    see an image yet there is shadow casting on the image.
    It only appear in SCTs or other obstructed scopes and
    this presents a unique opportunity to study what happens
    when another image is superimposed on an existing image.
    Would the retinal image sensor desensitizes due to the
    inpinging black image...

    Teni




  6. #6
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default On-axis, off-axis, and neutral axis light

    On Sat, 03 Nov 2007 15:45:59 -0700, Tenifer <tensorsurfer@yahoo.com>
    wrote:


    No, you can't add black that way. What's happening is that you are
    getting less light from the object, but you still have ambient light,
    both through the scope (scatter) and from around you. Think about the
    limiting case, where the obstruction becomes larger, or your pupil gets
    smaller, until you just match the obstruction pupil to your eye's pupil.
    Pretty clearly, you aren't going to be seeing black. The residual light
    is what is lowering contrast when you also have some light from the
    object in its annulus entering the eye as well.

    _________________________________________________

    Chris L Peterson
    Cloudbait Observatory
    http://www.cloudbait.com

  7. #7
    Tenifer's Avatar
    Tenifer Guest

    Default On-axis, off-axis, and neutral axis light

    On Nov 4, 7:42 am, Chris L Peterson <c...@alumni.caltech.edu> wrote:

    Let's take an actual example I used. I tried a 25mm
    eyepiece in the 8" SCT producing exit pupil of
    200mm (aperture)/80x= 2.5mm. At daytime my eyes
    has 2mm pupil size, Now the obstructure area of the
    EP exit pupil is 2.5mm/3= 0.833mm. This means
    there is enough light to form image at the retina because
    2.0mm-0.833mm = 1.167mm of the light is passing thru
    the eye 2mm pupil. Therefore you are not supposed to see the
    obstruction at the eyepiece. The reason is because as you
    know our cornea is like objective lens... the parallel rays
    from the eyepiece converge back to a point at the retina.
    So the obstruction effect is supposed to be cancelled
    at the retina. Therefore you are not supposed to see
    the obstruction using this actual 25mm eyepiece and
    8" SCT with the data calculated above. Yet one can
    still notice the central obstruction in a very faded sense
    at the center of the eyepiece. I think the cause is black
    color being superimposed on the image producing contrast
    lost. A similar effect when hand is put in front of the objective
    or flashlight shines inside.

    teni




  8. #8
    Tenifer's Avatar
    Tenifer Guest

    Default On-axis, off-axis, and neutral axis light

    On Nov 3, 10:29 pm, Chris L Peterson <c...@alumni.caltech.edu> wrote:

    I can't understand why you can't understand this principle of
    image formation even though you own an observatory. Or
    maybe you just take charge of its accounts. Unless your
    pupil size matches the obstruction portion of the exit pupil.
    You are not supposed to see the obstruction because the
    obstruction is distributed in every airy discs (or pixels)
    of the image. Get it so far. Maybe your are imagining our
    eyes cornea has sensor outside that detects each image
    as is.

    The reason why one can still see the obstruction is not
    because of the shadow in the on-axis rays coming into
    the telescope's aperture. Instead what happens can
    be illustrated this way. Hang a coin and slowly lower it
    to the center of a refractor. No matter what the exit
    pupil and eyepiece you see. You can see the obstruction
    as a very faded center image. The cause of it is because
    black is added to the image resulting in lower contrast.
    Get it now?

    Teni


  9. #9
    wsnell01@hotmail.com's Avatar
    wsnell01@hotmail.com Guest

    Default On-axis, off-axis, and neutral axis light

    On Nov 4, 5:00 am, Tenifer <tensorsur...@yahoo.com> wrote:

    <edited>


    The secondary mirror is simply another object in the field of view,
    but too close to be brought into focus. The secondary reduces
    contrast through diffraction effects that increase the size of, and
    light distribution within, the diffraction disk. It also blocks some
    of the incoming light, reducing the brightness of the image.

    Of course, we don't worry all that much about the effects of having a
    secondary mirror in the optical path, because the only practical
    alternative would be to use a refractor, a design which tends to
    become more impractical, compared to a reflector, as the telescope's
    aperture increases. We just try to have the secondary be no larger
    than necessary.







  10. #10
    Steve Paul's Avatar
    Steve Paul Guest

    Default On-axis, off-axis, and neutral axis light


    "Tenifer" <tensorsurfer@yahoo.com> wrote:

    ...snip...


    The shadow of the obstruction becomes visible as you remove the off axis
    light that is illuminating the area under the obstruction.

    -SP


 

 
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