RE: [APML] Best Scope and mount for $2000 was Scope Question

["Jon Kolb" <jkolb@datawest.net>]

Rob, assuming you're in the northern hemisphere,

> The problem is, with an alt-azimuth mounting, the object 
> stays centered, 
> but since you have two axis moving, the field rotates around 
> the center of 
> the frame.

To see what Jerry's talking about here, go outside after dark and look
for Orion in the south.  He's more or less standing straight up right
after dark.  Then go back out a few hours later, when he's just about to
set.  Without cocking your head, you'll notice that now he's just about
flat on his face, not straight up anymore.  What happened here?

Well, when you first looked at him, you used your neck to rock your head
back a bit to look up at him - that moved the elevation of your eyes.
When you went out later, you turned your head to the west to see him
over the horizon - that moved the azimuth of your eyes.  Your eyes moved
only in azimuth and elevation, but Orion face-planted.  What happened is
that the field of view of your eyes rotated - that is, you saw the field
rotation Jerry mentioned previously.  The go-to scopes essentially move
the same way as your neck, so a camera tracking in alt-az on Orion
during that time would have recorded the stars sweeping in great arcs as
the field rotated - interesting, but not the photograph you wanted.  

That's why we need to use a mount that moves in an inertial coordinate
frame, not a topocentric frame (earth-centered, not driveway-centered).

Sincerely,
Jon Kolb
Adventures in Astrophotography
http://home.datawest.net/jkolb/
jkolb@datawest.net




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