Re: ATM Earl of Crawford mount - HELP

[Everett Cairns <ecairns@mcmail.cis.mcmaster.ca>]

    Certainly if alt and azimuth axes cross at a point the apparatus will
work. Then you can replace the two axes by an equivalent ball joint at the
meeting point of the two axes. Having done this, you see a rigid triangle
from ball to lower rod end to upper rod end which pivots about the line from
ball to lower rod end. If this line points to the celestial pole than
certainly the mount will be an equatorial.However,when alt and azimuth axes
do not meet the situation is definitely more complicated. I would agree that
this will muck up the works. At the moment I do not see a solution.
                                                        Everett

astrojoe@erols.com wrote:

> 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