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    Re: A guide to AH use
    From: Bill B
    Date: 2014 May 17, 16:20 -0400

    On 5/17/2014 1:21 AM, Greg Rudzinski wrote:
    > Focus at infinity. Try using the Moon at 1st or 3rd quarter. Once
    > focused then mark the position where the ocular meets the tube or tape
    > in place to prevent accidental shifting.
    Greg has nailed it, as have you. You are not focusing on the mirror, you
    are focusing on the object reflected. Can a celestial object get smaller
    when you place the AH on the ground? Absolutely. The celestial object is
    now several feet further away from the AH. Can you discern the decrease
    of several feet added to a minimum of 4 lights years for a star? Not likely.
    "But with the scope taken off, I looked at low objects standing way
    back, and I had to adjust my focus for a higher object that I stand
    closer to get. Shouldn't I be able to leave focus alone if all are at
    infinity? It may turn out to be the tilt of my head changing the
    progressive glasses focus. What do you think?"
    I'm not at all clear on your definition of low vs. high objects. In
    context, moon vs. sun vs. planets vs. distant stars? And when you speak
    of adjusting focus with the scope off, are you talking about your naked
    eye. Or your eye with your progressive bifocal glasses. Or have you
    mixed scope focus into the mix?
    In any case it is complex, as the human machine is amazing put not
    perfect. As Frank posted perhaps 7 years ago as the eye adapts to
    darkness (enlarged pupil) point sources become progressively more
    multi-spiked. Almost like a loss of focus. Plus you have the whole rods
    and cones things in play.
    When you get a lens involved, similar to a prism different wavelengths
    are not refracted equally. A giant red star will not focus on exactly
    the same plane as a blue star. If you have a standard camera lens, you
    may note there is a mark for focusing with infrared film. I'm not
    certain how the human optical system deals with it, but the glass in a
    scope, camera lens or eyeglasses will all exhibit this problem to some
    minor degree.
    Add in progressive bifocals (if applicable) and there are too many fuzzy
    variables without controls to play with.
    When using my sextant scope, I do note a slight change in focus between
    daylight and late twilight/night observations. Why this happens I don't
    know, but is seems to be real for me. For quick reference I have affixed
    red and white pointers of pinstriping tape to the focus ring. The white
    is for daylight, the red for evening.
    Reading your posts, you seem to be one us (oops "them" ) "chasing
    tenths." Even if the NA and sextant were accurate to 10 decimal places,
    scopes were perfect, we still have to deal with the human eye, which
    Neil deGrasse Tyson mentioned on "Cosmos" is less evolved than fishes' eyes.
    That's my story, but I'm not sticking to it. I've been proved wrong too
    often :-

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