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    Scope position relative to frame 2
    From: Bill B
    Date: 2007 Jan 26, 21:32 -0500

    An off list reply:
    "Bill, I have waited in the hope that this thread would be continued, as
    it hasn't, here are my suggestions. Some lenses have what is known as
    vignetting, that is, a softening or dimming of the image toward the
    outer edges of the field of view, or it may be off-centre spherical
    abberation, which at the low magnification used may not be readily
    apparent, but, enough to cause error. Which leads me to ask why did the
    early makers pay so much attention to telescope alignment, with
    adjustable 'scope mounts, rising pieces, sight tubes with cross-hairs
    > I've not had the time and sun simultaneously to do extensive checks, but
    > initial results of three rounds of IE checks have raised questions in my
    > mind.
    > During Frank's laser IE check I attempted to center the horizon glass/mirror
    > intersection in the scope, and shoot the laser down the center of the scope.
    > Earlier this week I was doing IE checks with the sun with the scope mounted
    > in the same position as the laser tests.  To my surprise the IE had shifted
    > 0!3 towards off the arc from its usual. (On the arc figure reduced by 0!3.)
    > 4SD was 0!1 over target in all three rounds, and n-1 sigma (s) was less that
    > 0!1 in all sets.
    > I then moved the scope to the position closest to the frame (usual position)
    > and did more sun IE checks.  That brought the IE back to the usual figure.
    > I understand why it is important to center the images vertically in the
    > scope (ideally the center of the scope will coincide with the center of the
    > foreshortened surface of the front-silvered index mirror when at the highest
    > angular setting on the arc).
    > I am at a loss as to why a sun IE check should vary depending on scope
    > distance from the frame.  Only two things immediately come to mind.
    > 1. Distortion caused by scope optics when off center
    > .
    > 2. Collimation error caused by an less-than perfect scope mounting system.
    > Two initial questions:
    > 1. What are the symptoms/affects of collimation error?
    > 2. Can anyone suggest why this is happening, other than operator error?
    > Any input would be appreciated.
    > Thanks
    > Bill
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