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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.

Randall

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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