ATM 5 hour error-free straight bolt tracking

[doug kniffen <dk@usmo.com>]

I was building another one of those wide field astrophoto camera platforms
(Trott type 4), when it became evident that I'd  wasn't paying enough
attention and had built something that would double tangent error instead
of eliminating it. While analyzing the error it seemed that the design was
unecessarily  complicated. At this point the idea for a different drive
configuration occured. It seemed deceptively simple, a straight bolt fixed
at 90 degrees to one board and a second board that "rode" the bolt while
sliding against the first. 

The are no "magic" ratios involved and only one critical dimension. Motor
rotation isn't important. Even the drive rate doesn't matter.  If the
distance on the second board , between the bolt pivot and the slide contact
point, equals 6 hours of thread it should track to the mechanical limits of
accuracy. After sketching this thing out it appears that one could easily
get five hours of tracking in the basic configuration. A second
configuration sketched might allow up to eleven hours, though it could be
tough to eliminate mechanical slop as the meridian is crossed.

The design has a semi-virtual polar axis unlike the Poncet or d'Autume
platforms and also unlike more conventional drives which use a hinge or
pivot bolt/bar. On this design, there is a reverse mechanical polar axis is
located at (and perpendicular to) the intersection of the drive bolt and
the first board. There is also a fully virtual polar axis about which the
instrument rotation occurs.

Having built, over a number of years, nearly every type of camera platform,
it occured that I had thought of this a long time ago. Has anyone heard of
such a design before? It doesn't seem to be well known if this type of long
exposure drive platform has been built before. Initially it seemed
appropriate to call this a sine drive, then a cosine drive,
then................Since there a large number of configurations which
utilize the same principle, the best descriptive name for this type of
drive platform should be:  The Large Hour Angle Straight Bolt Sidereal
Tracking Drive Platform

Maybe I'm all wet on this, but I'm going to build one since it looks good
on paper. Anyone have comments??

Doug Kniffen