Re: ATM Earl of Crawford mount - HELP

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

 A second thought which may lead to a solution.
    Find the crossover of the tube axis and the azumuth axis. The mount
works if this is a fixed point relative to your fixed rod attachment point
rooted in the earth. This line defines your polar axis.However, since your
alt axis is not on the azimuth axis, as alt changes this point moves. To
restore to the workable configuration, you could continually TRANSLATE
without rotation, the whole tube to keep this point fixed on the azimuth
axis. In practice this may not be possible.Next best would be to move only
the key points-the rod to tube connection and the tube to real alt axis
crossover. Again not allowable-but it does suggest another solution--the
ground point should follow the same path as the original tube to azimuth
axis crossover. AS this is a pivot point where rotation is irrelevant, the
motion is equivalent to translating the third point in the triangle in the
opposite direction as translating the other two points.The triangle is of
course the two ends of the link rod and the alt-azimuth axes crossover
point. I am 99% sure that I have not overlooked the obvious, but if someone
else can verify and of course design the necessary linkage.
                                                                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