I have succeeded in getting many hours of unguided exposures
of 4 minutes duration each with perfectly rounded stars (using K3CCD
Tools for polar alignment) using a Mach 1, with a 4" Pentax Astrograph
and a side by side mounted 4" Takasahi FC100,.this set up
worked flawlessly for 10 days at an astrofest. Whilst the alignment
was demonstrably good I do not know the precise accuracy.
Can anyone supply an equation to relate duration of exposure
in seconds to error of alignment in arcseconds, CCD Pixel size,
and OTA focal length.
Polar Align Max will report alignment error but seems hard put
to achieve arc second accuracy whereas a more time consuming
exercise with K3CCDTools can achieve considerable accuracy. If I
knew in advance what alignment I could tolerate for a given Camera/
OTA it could tell me how accurate I need to be.
Any help would be greatly appreciated.
Gerald.
it took me some time to absorb what you actually wrote. 
