Hi David,
Part of the question is easy to answer. If the light undergoes an even
number of reflections (refractor without diagonal, newtonian, SCT without
diagonal), it is always possible to match what you see in the eyepiece and
what you see on a map only by rotating the map (always looking at the same
face). But if the light undergoes an odd number of reflections (refractor
with a diagonal, SCT with a diagonal), you have moreover to reverse your map
(looking through from the back, and hoping it is a bit transparent). With
Guide, you can simulate looking from the back of the map with "Display" ->
"Inversion menu" and choosing either "Mirror image E/W" or "Mirror image
N/S".
Now you have still to provide Guide with a rotation angle (in the same
dialog; in Guide, the positive sense is clockwise, that is in the opposite
of the position angle used by double stars observers). It is the angle by
which you would have rotated your map (in its own plane).
If one uses a diagonal and that it is oriented so that its "output" is
contained in a vertical plane, then what you see upwards in the sky is
upwards in the eyepiece (assuming you look from behind the scope and not
from its side, because in that case what was up is now left or right). Then,
the easiest thing to do is to choose "Alt/Az (zenith up)" in the same
dialog, leaving "Rotation" at zero.
Now, if you rotates the diagonal clockwise by an angle A then the image
rotates by the same angle clockwise too and you have then to enter that
angle A as the "Rotation" value. But doing that, you ought to rotate your
head in block with the eyepiece by the same angle A and you arrive in a
rather incomfortable position, with the line joining your ears beeing no
more horizontal. Probably you will reset your head in its usual position.
You do this by rotating it about the eyepiece optical axis by an angle B
that is the same as A but int the opposite sense (conterclockwise in this
example). In conclusion, the diagonal rotation rotates the image in one
sense by an angle A and you rotates your head in the other sense (B), so
that you see the image rotate by an angle A + B = 2A relatively to your head
(and eyes). So if you rotates your diagonal by an angle A and you always
hold your head with the "ears line" horizontal, you have to enter 2A as
Guide's rotation angle. The difficult thing will probably be to estimate the
angle A accurately enough.
In the case of a Newtonian the reasoning is about the same, but the
rotation angles are lees easy to "see", because the diagonal mirror is in
the tube.
I hope this will help.
Laurent Zimmermann
http://users.skynet.be/zmn ----- Original Message -----
From: "David Higgins" <higginsdj@...>
To: <guide-user@yahoogroups.com>
Sent: Wednesday, August 13, 2003 6:57 AM
Subject: [guide-user] Inversion Settings
> I'm having a real brain strain at the moment..... Can someone
> confirm what I should have the screen orientation/inversion settings
> set to so that what Guide shows me is oriented exactly the way I
> will see it through my EP. I am in the Southern Hemisphere, using
> an SCT with a standard Mirror Diagonal.