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John, The figures you posted show very good alignment, and you have reached a position of diminishing returns, where the time and effort
needed to improve them is not worth the very slight improvement in tracking. Firstly, if you have checked your cone error, and it is good,
stick to 2 star alignment as many of us do. When you talk about 15-20
minute subs, this is meaningless because the possible length of subs
depends on several factors which are different for different people.
Unguided subs seem to be limited on a consistent basis with the HEQ5
to 2 to 2.5 mins using a 1000mm focal length scope. Short refractors
will obviously get a lot longer than that, and unreduced SCTs will get
a lot less.
Guided subs are limited by Focal length, guidescope flexure,
atmospheric conditions, light pollution, quality and sensitivity of the
autoguider, and how well it is set up, or manual guider's concentration, experience of the operator, and the list goes on. Unless you operate
your scope from a nice dark site your subs would be washed out well
before 15-20 mins anyway.
Peter, locking on a single star does not work well for the same reason that you have to triangulate to find your position on earth. The
computer would obviously know where the target star is, but it could not locate itself, because it would not know from what direction it was looking at that star.
raymo
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