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Old 22-10-2010, 12:21 AM
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gary is offline
Join Date: Apr 2005
Location: Mt. Kuring-Gai
Posts: 5,476
Argo Navis on a GEM - A primer

Originally Posted by Alchemy View Post
Back again.
Well the sky was clear enough.... Almost full moon, but bright stars are available. Set the encoder direction easily enough, time for a one star alignment as I had selected German equatorial exact....... Seems mine isn't so exact as although I had things in the finder it wasn't what I hoped. Now given I have pulled bits apart and tinkered with it during the instalation that isn't suprising, so I figured let's go for the rough align ( too bright to use polar scope with the moon).....

So here's where I get stuck, it's me so I apologize, but I can't figure where to point the scope for the fix alt ref. if I have the scope pointed to the south celestial pole, where do I go from there....

I got confused with turn north as I couldn't make any sense as everywhere is north from south.

The diagram of the fork seemed to be pointing to the SCP ?

So if I am standing behind my scope looking at the SCP do I turn it 90deg to the left in dec axis (east) and leave he RA unchanged, should the mount be running. I hope that makes sense. Feel free to talk in non technical terms
Hi Clive,

Thanks for the post.

Right now, you will be just getting familiar with things and the GEM ROUGH ALIGN
setting, which really means the scope is not polar aligned at all, combined with a
FIX ALT REF step and two star alignment will be the most expedient approach for you.

In the long term, which means in a few more observing sessions once you become
familiar with things, I recommend migrating to the GEM EXACT ALIGN setting
but combining it with what we call a "short TPAS run". That probably won't
mean much to you now, but in a future thread we can explain what we mean by that.

Let's talk about the FIX ALT REF step and two-star alignment process on a GEM

By way of background, the optical encoders fitted on your mount are of a type
called an "incremental encoder". When they power on, they have no inherent
zero reference point. As it transpires, you only need to establish the reference
point for the Dec (i.e. ALT) encoder and this is the job of the FIX ALT REF step.

One can actually set the ALT REF point to any convenient value, but typically
on a GEM, one might either use the 90 degree point or the zero degree point.
Let us stick with the zero degree point. If you look at the Dec axis on the mount,
it will have a mechanical setting circle on the Dec axis. The zero degree point
corresponds to the orientation of the OTA that results in the mechanical circle
reading zero degrees. If the mount were polar aligned, and in this instance it need
not be, that would correspond to the OTA pointing to a Declination of zero in the
sky, which is the Celestial Equator.

Part of the first time setup is establishing what are called the encoder direction
sense signs which are the +/- signs in SETUP ALT STEPS and SETUP AZ STEPS.
Let us run through that process now.

The drive motor need not be running to do this process, but feel free to switch
it on if you wish, as it helps if the mount tracks whilst you do a star alignment.

Picture if you will a line that runs across the sky from the SCP, through
the zenith and heads due north. That line is the meridian.
The stars are rising in the East, crossing the meridian and setting in the West.
Start by having your mount roughly polar aligned. Position the OTA in RA so that it is
on the East side of the meridian. Now rotate the OTA so that the mechanical
setting circle shows it is pointing at a Declination of zero degrees. Since the OTA
is roughly polar aligned, it will be roughly pointing to somewhere along the
celestial equator. Power on the Argo Navis. MODE FIX ALT REF will appear.
ALT REF=OK will appear for two seconds. Press EXIT and dial up MODE
ENCODER and press ENTER. Whilst watching the right-hand displayed
value, slowly rotate the OTA to point toward due north, If the numbers increase,
i.e. +0, +1, +2, etc. all is well. If they decrease, go to SETUP ALT STEPS and
reverse the +/- sign and repeat the above procedure by repositioning the
OTA to point to the Dec=0 mechanical mark, performing the FIX ALT REF step
and the doing the check for increasing ALT angles in MODE ENCODER.

Once you have established the Dec (ALT) sign, keep the tube so that it is on the
East side of the meridian but try and identify a bright star on the west of the
meridian. Do not choose any star that is between the SCP and the southern horizon.
Argo Navis has a convenient list of bright alignment stars in MODE ALIGN STAR
but using a combination of MODE CATALOG and MODE ALIGN, one can actually
align on any two objects. For now, say you choose FOMALHAUT. Now dial
up MODE ALIGN STAR, ALIGN FOMALHAUT and center it in the FOV.
Press ENTER. Argo Navis will briefly display what is known as a WARP factor.
Now pick a second star. In so doing, try and pick one that has a different
elevation to the first and that is at least 30 degrees away or more. You are
now free to flip the OTA over the meridian if you need to. Say you pick ALDEBARAN.
Perform the MODE ALIGN STAR procedure and align on it.

Now dial up MODE RA DEC and press ENTER. Point the scope somewhere
roughly along the celestial equator in the western half of the sky and sweep in the
RA axis only from West to East. As you so so, watch the left-hand displayed value.
RA should increase as you sweep from West to East. If not, reverse the SETUP
AZ STEPS sign and repeat this procedure including the FIX ALT REF step
and two star alignment.

Once you have determined the direction senses, performing a real alignment using
a FIX ALT REF step and two star alignment uses the same steps as just described.

When performing the FIX ALT REF step and aligning on the very first star,
one can have the OTA on the East and picking an alignment star in the West
that is not between the SCP and horizon or one can have the OTA on the West
and pick a star in the East that is not between the SCP and the horizon.
However, if you established the SETUP ALT STEPS +/- sign with a procedure
that started with the OTA on the East and on a subsequent observing session
wish to start with the OTA on the West, then you need to reverse the SETUP

The reason for this is because when the OTA is flipped across the meridian,
the apparent direction sense of the Dec (ALT) encoder reverses. This is an
attribute of spatial geometry rather than some caveat of Argo Navis.

As mentioned earlier, after some familiarization, we recommend to begin to
use the GEM EXACT ALIGN setting combined with sampling the positions
of at least four stars in a short TPAS run. This has several advantages.
Firstly, one need not perform the FIX ALT REF step. Secondly, even if
the mount is only within a degree of the pole, the TPAS feature will
compensate for any polar misalignment. Thirdly, you can add additional
mount error modeling terms which can often dramatically improve the pointing
performance of many GEM's, such as a G-11. But that can be the topic
of a future thread.

Please do not hesitate to ask for any clarification on the above, either by
a post here or by emailing or feel free
to give us a call on 02 9457 9049 here in Sydney during normal business hours.

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
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