ATM Earl of Crawford mount - HELP

[astrojoe@erols.com]

I pulled an old issue of Sky & Telescope ( Sept 89 ) and read an article
concerning a DOB mounting with a tracking arm that allowed the dob
platform to be driven in one axis ( azimuth ) at the sideral rate. This
is accomplished, in the article by Maurice Gavin, simply by attaching a
string to the bottom board via an extension arm off that board and the
other end is attached to the bottom of the OTA tubing just under the
spider ( eyepiece position ). Make the ota tail heavy and the string
will always be taught. Align the mount to Polaris and away you go....

I took this one step further and made an adjustable telescoping pole
instead of using the string. Problem is that it tracks rather poorly. If
the field is low power, you get good results; but a star still drops in
the field after a few minutes. If the power is up to 250X, each time a
star transverses the field, it sinks lower and lower after just 3
passes. It should stay on a field centerline. All I should have to do is
correct by nudging the tube in azimuth every so often to keep up with
the earth's rotation.

Any ideas as to why it is not working right ? One design difference is
that my DOB mount is for a refractor and has it's altitude axis in back
of the azimuth axis by 7". Could this be why it is not tracking on the
proper sideral arc ? 

Joe Castoro