Project LX90 equatorial wedge

"ChrisH" wrote in message
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[snip]
I looked at your pic and it leaves me a little confused as to the
function of the device. I take it the parts shown constitue a
levelling plate only? Such a thing is useful for a permanent pier
where a scope is to be removed/replaced, and so it is necessary to
provide a level datum surface. A 3-point contact is based on sound
engineering priciples, however, such adjustments could be more simply
carried out using threaded rods and locknuts only - the plate can then
remain on the Pier. I do not see what advantage this device offers
when used in combination with a tripod?

Sorry, comes from being too close to something. I'm trying to address about
4 issues at the same time. The first, and ultimately most important, is
cost. If I can't keep the manufacturing costs really low, I'll end up just
making a very simple one-off wedge for my own use, then I don't need to
consider what anyone else would like. I'm beginning to regret having asked
the question, now.

The second issue was to provide an adjustable equatorial wedge (I'd settle
for a fixed one, myself), the third was to allow quick mounting, and finally
the fourth was to allow easy interchange between alt/az and equatorial. By
using the idea of interfacing plates, I can address all these issues at the
same time. The basic interfacing plates serve for rapid mounting in alt/az
mode, and a rigid assembly using the same design of plates then allows the
introduction of an equatorial wedge between the original pair, which can
also be done rapidly. Reusing the plates for both purposes allows for
reductions in manufacturing costs.

If a levelling plate were all that I hoped to achieve, then you're quite
right about simply using threaded rods. And if a simple and quick alt/az
mount for a pier were enough, reducing the spheres to rounded ends on the
rods, and matching receptacles for them (to provide the correct orientation)
would also serve. However, as I trust is now clear, I'm aiming higher.

For a fixed pier, the advantages are obvious (I hope). Alt/az when you want
(once the adjustable plate has been calibrated for your location - not an
easy task but one you only have to do once), and a quick change to
equatorial (once the wedge has been calibrated - again once only) and back.

Now for the tripod. First time is not a problem as the same calibration
would need to be done as for a pier. Using the tripod a second time won't be
so easy, because the tripod (with it's interface plate) needs to put back in
much the same position as when the plates were first set up. However, this
should prove to easier with the telescope mounted alt/az. Adjusting the
tripod legs should allow one to bring the tripod into the correct position
and orientation by sighting through the telescope on a couple of landmarks.
Sensible choice of leg position in the first instance (one leg to the north,
the other two lined up on E-W seems sensible and landmarks to the north and
either east or west). Once this position is achieved, the previously
calibrated equatorial wedge will inevitably set the telescope back to the
correct position and orientation for this mode.

I don't expect the tripod to allow for really accurate set up, but I
understand that quite good results can be obtained if the thing is pointing
in roughly the right direction as field rotation is very significantly
reduced - which seems to be the only real advantage to using equatorial for
long exposures. I am assuming through all this that the telescope
orientation with respect to its plate is not an issue - there is a facility
to park the telescope, so once it is positioned the telescope electronics
will remember which way it's pointing (what a waste of time if this isn't
so).

When you go on to describe the modifications required for making the
thing into an adjustable equatorial wedge I get totally lost. I know
the addtional parts have not been detailed, but the equatorial wedge
needs to be both rigid and also provide very precise orthagonal (E-W /
N-S) movement in order to polar align the scope.

The wedge would consist of one each of the interface plates welded together
(for example) at roughly the right angle for equatorial mounting (I'm not
looking to cater for large changes in latitude). The plate with the
adjustment on it will then allow the wedge to be fine-tuned to the location
for which it is intended. Hopefully, the wedge will be cheap enough so that
anyone who consistently uses more than one site will be able to purchase a
separate wedge to adjust for each site. Only one set of plates would be
needed for the telescope and tripod.

Tweaking in Altitude
and Azimuth by a few arcseconds places particular demands on the
mechanical design and I don't see how this is achieved with a 3-point
suspension on a rigid inclined plate. The method of adjustment you
propose does not lend itself well to (for example) drift-aligning a
telescope in azimuth.

My main concern has been getting the interface between components right so
the telescope can be quickly mounted in exactly the same position as
previously thus obviating any need to make further adjustments. As with
adjusting the tripod, the original positioning of the telescope on the
interface plate can ease the task of initial setup. One support sphere to
the north with the other two lined up E-W should help. Adjusting the
northern one then affects elevation, and adjusting the other two by equal
amounts in opposite directions will alter the angle with respect to the
great circle passing through the zenith. Introducing finer controls on
particular axes would almost inevitably bring with it a huge increase in
cost (unless you know different).

For someone using a pier, these procedures are only required once. For
someone using a tripod, the adjustments to the plates and wedge are still
only required once but adjusting the tripod is required each time it is
moved. However, adjusting the tripod in the simpler alt/az mode will bring
with it the automatic benefit of setting the plates in the right orientation
so that the wedge can be simply introduced for equatorial use.

Those are just my thoughts, I don't want to
sound negative so perhaps a final design or a working prototype might
answer the questions I have. Nice to see someone actively thinking
about making things rather than buying them!

Not a problem. Your comments are appreciated. I hope my reply has answered
most, if not all, of your questions. If there is anything else that needs
clarifying, please ask. Incidentally, the reason for using spheres on the
adjustable rods is to simplify whatever ends up being used to clamp them to
the plates as then they always present the same profile to the clamps at the
same distance from the plate so only one clamp need be designed.
Grim