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not a problem with separate guide scopes in which the subject can always be framed as desired
and a reasonably bright guide star can be selected quite far afield from that by adjusting the aim
of the guide scope. Furthermore, acquiring the only suitable guide star may result in the off-axis
guider being oriented at some very inconvenient angle for monitoring the guide star, like
pointing down toward the ground (Figure 3). The author has spent many hours guiding
astrophotographs while in these uncomfortable positions.
Figure 3. Off-axis guider and DSLR attached to the author’s Compustar 14. This is a
classic example of an uncomfortable position of a standard illuminated cross hair guiding
reticle eyepiece in an off-axis guider. Imagine positioning yourself beneath the eyepiece to
guide a long exposure astrophotograph.
There are a few strategies to mitigate some of these problems. The rotation of the off-axis
guider‟s field of view relative to the camera‟s field of view can be changed by loosening the set
screws on the camera‟s T-ring, rotating the off-axis guider as desired, and then re-tightening the
set screws. This can permit better framing of the subject with certain guide stars. High-end off-
axis guiders have provisions for easy rotation of the off-axis prism and some even permit
adjusting the position of the prism farther in or farther out from the center of the camera‟s field
of view. These strategies provide some flexibility in the framing of the subject with a particular
guide star. The most effective strategy is to actually determine one‟s choice of guide star prior to
the imaging session. This can be done fairly accurately if one spends the time determining the
field of view of the camera through the imaging telescope, the field of view of the guiding
prism/eyepiece through the off-axis guider, and their relative separation on the sky (for details on
how to do this, see the author‟s article in Sky and Telescope, February 1993). One can then make
an overlay showing the fields of view of the camera and off-axis guider/eyepiece at the same
scale as the sky depicted in star atlases, photographic atlases, or planetarium software. The
overlay can be used to determine the orientation of the camera frame‟s long and short axes
relative to the cardinal directions on the sky, the position of the off-axis guider stem relative to
the camera frame, and the actual guide star to be used. This saves a tremendous amount of time
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