Alignment issue with AN and Gem mount. Need some direction

[[1ponders]]

I'm trying to solve an alignment issue with my argo navis and my G11 and I'm not getting anywhere. Options and advice sought.

When I do my initial alignment from the east side of the mount I can generally get close to objects that I'm looking for. Some nights though the accuracy can be quite a few arcmin out. I've tried using the Align option to reduce the error but that makes things much worse.

Typically for example say I align star on the east side of the mount and I select Alderbaran. Warp = +0.00. I then select a bright star, Rigel. I move to Rige (with meridian flip tonight), and center it in the eyepiece. I then select Mode Align and select Enter when Rigel comes up on the screen. It gives me a warp of +0.00. I then select Canopus from the Catalog and when I get there the RA reading is right but the Dec reading is 88. AN is reading Canopus nearly 90 deg out. I've checked the encoder signs and it all looks ok.

Any suggestions as to what is going on??

[gbeal]

None whatsoever, but the elusive Robbie will know for sure, e mail him. He has had the AN on my old G11 for eons, one of the first if I recall.
Gary

[[1ponders]]

Thanks Gary, I'll drop him a line

[gary]

Hi Paul,

Thanks for the post and hope you have been keeping well.

If you normally align with the OTA on one side of the mount but then decide to
align with the OTA on the 'other side' having performed a meridian flip, then
you need to reverse the SETUP ALT STEPS encoder direction sense sign
(i.e. the +/- sign in SETUP ALT STEPS).

When the OTA flips over the meridian, the apparent direction sense of the
Alt encoder reverses. This is an attribute of three dimensional space rather
than some caveat of Argo Navis.

For example, imagine a scenario where the Dec cross axis is level so that the
OTA is on one side and the counter weights on the other like a weight lifters dumb bell.
If the OTA is pointing toward the horizon, as you raise it in elevation (i.e.
in Dec), the encoder shaft might then rotate, say, clockwise with respect some
external frame of reference

However, if you then flip the OTA to the other side and repeat the experiment, again
raising the OTA in elevation (i.e. in Dec) then the encoder shaft will now be
rotating counter-clockwise.

So you can perform the initial alignment with the OTA on either side of the mount
but you need to set the SETUP ALT STEPS encoder direction sense sign
to the correct sense for that particular side. Once you align, you are then free
to flip the mount and you don't need to change the sign, unless you decide
to align again but on the opposite side.

When aligning, it is best to avoid an apparent ambiguity that can result if one uses
a star between the horizon and celestial pole. One can do so, but one then
needs to be conscious of what the direction sense sign would be. A good rule
to avoid potential ambiguity is if the OTA is on the East to align on a star in
the West and vice versa.

An interesting attribute of a GEM is that you can point the OTA to
most locations in the sky using two distinct mechanical orientations,
i.e. with the OTA either on one side, or the other, after a meridian flip. On Dobs,
the backboard of the rocker prevents the OTA passing through the zenith,
otherwise one could do the same trick. Fork mounts owners tend not to let
the ends of the OTA pass through the forks, otherwise they could do the
same sort of pointing contortionist gymnastics of a GEM. If Dobs and
Forks could, then they too would need the Alt encoder sense sign reversed when
the OTA flipped. :thumbsup:

Please do not hesitate to email me if you require further assistance.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au