If you have ever tried to use software such as Desktop Universe, The Sky, or Starry Night Pro, etc., to determine the best orientation for a self-guiding SBIG CCD camera, you know that it works extremely well.  Except when you try to correlate what the software tells you with the actual camera hanging off the scope.  Here is a handy reference for figuring it all out.

This assumes the default position for your camera is with the cables hanging straight down toward the dovetail attaching the scope to the mount.  This is a position angle of 270° in the software, and it places the guide chip directly away from the dovetail, or due east of the main CCD chip.

Above:  Default position of the CCD camera

The software will give a position angle of the CCD camera.  The charts below correlate this position angle relative to the dovetail plate on the scope when looking at the back end of the scope.  The degrees indicate position angle in the software.  The black bar at the bottom of the circle is the dovetail plate.  Arrows indicate the direction the cables should point for each position angle.  Note that there are two charts depending on which side of the sky (east or west) your scope is pointed at.

An Example

To achieve the proper framing and guide star selection for an image of NGC253, the software indicates a position angle of 110° is required.  If this object is in eastern half of the sky, the camera should be oriented with the cables pointing slightly counterclockwise from the 90° position which is exactly opposite the dovetail.  This would point the cables toward the position of the finderscope on many telescopes, or at about the 11 o'clock position.  (The dovetail is at the 6 o'clock position.  Note that the dovetail is at the bottom of the diagrams, but that the orientation of the scope rotates with respect to the ground depending on where it is pointed in the sky.  Remember the orientation is relative to the dovetail, not to horizontal as defined by the ground.)