|
Hi,
A level Dob is not equivalent to an equatorial mount.
As you appreciate, sky co-ordinates are designated in terms of RA
and Dec. Due to the rotation of the earth around its own axis
and also the rotation of the earth around the sun, the sky appears
to move overhead as a function of time, appearing to revolve around the
celestial poles like clock-work. Therefore, historically, RA is generally reported in
terms of time - hours, minutes and seconds whereas Dec is generally reported
in terms of degrees, arc minutes and arc seconds.
As you also appreciate, if one points a scope at some place in the sky and
if the scope does not track, the heavens appear to move across the field
of view. In other words, the celestial co-ordinates of where the scope
is looking are continually changing with time. If the mount is not polar
aligned, then with the exception of pointing the scope at one of the two
two celestial poles, the RA *and* Dec co-ordinates of where the scope is
pointing will change continually with time.
If one polar aligns the mount, that is, aligns its azimuthal axis with the
celestial pole viewable from your hemisphere, then when the mount is
pointing at any arbitrary place in the sky, only the RA co-ordinate will continually
change if the scope is not tracking. Therefore, one can then conveniently
mark the azimuthal setting circle in terms of hours, minutes and seconds.
When the scope is not polar aligned, making the azimuthal axis in such
a way makes no sense.
When one mounts a Dob on a platform as you propose and then polar aligns the
lot, then marking the azimuthal axis in terms of time then makes sense.
You can convert from degrees to hours, minutes and seconds simply by dividing
by 15.
If one does not have an equatorial platform, one needs to perform a co-ordinate
transformation calculation, going from the mount's co-ordinate system to the
sky's cordinate system. One needs to also know your location on earth and the
current time. This transformation is laborious to do by hand but is readily done by
a programmable calculator or computer.
We happen to manufacture a product that does a similar calculation in real-time multiple
times per second so that one then knows where the scope is pointing in terms of RA/Dec
Best Regards
Gary Kopff
Wildcard Innovations Pty. Ltd.
|