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ORTHOGONALITY
This feature is available for all GTOCP5 control boxes and only those GTOCP4 control boxes that have this feature
enabled by Astro-Physics.
Version 5 of the keypad program includes the Ortho Model routine to correct for orthogonal error. This section provides an
explanation of orthogonal error and outlines common causes.
A telescope is said to be orthogonal when its optical axis is exactly 90 degrees from the declination axis as shown in the
photo. In Alt-Az fork mounts, orthogonality is not an issue. Pointing is
straightforward, the scope never has to flip sides and the mount does
not need to be polar-aligned. However, in German equatorial mounts, a
non-orthogonal telescope will cause errors in any routine that will reduce
the pointing accuracy of the system by exactly twice the orthogonal
error.
Telescope orthogonality requires that two conditions be met:
1. The RA and Dec axes of the mount are at precise right angles to
each other. Astro-Physics mounts are very accurately machined and
fulfil this condition.
2. The optical axis of the telescope must be parallel to the RA (polar)
axis. Some of the factors which could affect this condition include:
a) Improperly machined rings.
b) Wedge in the mounting plate or brackets.
c) Incorrect position and tilt of a diagonal mirror or diagonal assembly that is not machined square.
d) Set screws in focusers, diagonals or adapters which tilt the optical axis.
e) Die-cast tube assemblies that lack the precision squareness of a CNC machined part. The tube points in one direc-
tion while the optics can point several degrees off in another direction.
f) Diagonal displacement or tilt in a Newtonian.
g) Mirror shift in a catadioptric. The optics are not tied down to a reference plane, but can move around and point in
different directions depending on focus position.
h) Collimation adjustment. There is enough freedom in the tilt of a secondary mirror to allow the image to be moved
completely out of the eyepiece field. Therefore, collimating the scope may disturb or change orthogonality.
Whatever the reason, the Ortho Model routine will calculate the orthogonal error and compensate for it as you slew from
one object to another. Refer to page 36 for more information.
90°