[ATM] Split ring polar alignment

["Scott Ewart" <scott.e@quicksurfer.com>]

Hi all,
        Work is progressing on my 12.5" split ring.  After getting the tube
bent for the split ring, I wondered how accurately it had turned out and how
accurate it really needed to be. I measured the overall diameter as far
around as possible and it's within about .020".  With a CNC-cut template of
the radius (16.25"), I measured the radius over spans of over a foot at a
time.  That also comes out to under .020" difference.
    Assuming this is accurate, I checked to see how much the drive rate
would change between fastest and slowest, and it turns out that the
difference is negligible.  Under one arc-second per minute, way less than
the difference between solar and sidereal rate.
    It only occurred to me recently that not only does the drive rate change
with split ring radius, but so does polar alignment.  Since it rides on two
rollers, the effect on alignment is sort of an average depending on what
sections of tube happen to be on the rollers at any given time.  I figure,
worst case, polar alignment may vary by over three minutes (+/- 1.5).  Does
this sound about right?  How accurate does alignment need to be for long
exposure photography with a two-meter focal length?  I know I can chase a
guide star easily enough, but there's still going to be some field rotation.
With 35mm film, will this be noticeable?  How good (we're talking numbers)
is the typical drift-alignment?    Will I forever be chasing to pole?
    Thanks for your help,

Scott



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