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Tony,
Thats an important question. At short FL's
(<1000mm) there's always
a guidestar. At longer FL's (>2500mm) you will
need to plan the imaging
session using "the Sky" software with its FOV
indicators and template
for the SBIG cameras and self guide chips. This
will tell you how to orient your
camera and center your object so that a suitable
guidestar will fall on the guiding chip.
The other factor is the limiting magnitude of guidestars which will depend on your aperture.
With my RC (12.5") I
can guide on stars as dim as 13 magnitude. That may not sound that dim but
once you start placing filters in front of the
camera for tricolor work the guidestars can get very dim,
especially with the blue filter. I'd say that if
you use "the sky" you will be able to get a suitable guidestar
99% of the time if you are willing to change the
orientation of the camera and the centering
of the target.
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