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Old 23-09-2008, 04:33 PM
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gary is offline
Join Date: Apr 2005
Location: Mt. Kuring-Gai
Posts: 5,430
Hi Rob,

Thanks for the post.

Originally Posted by CoombellKid View Post
Thanks for the tips, I went out and tried them late last night. Can I still
have Refraction On when I have Auto Adjust On?
Absolutely and we recommend this.

The following background may also be of interest.

What AUTO ADJUST ON does is looks at the angular distance between the
two stars (as a function of time to compensate for Earth rotation) and
looks at the angular distance the encoders *appeared* to move (based on
the initial 'hint' value you gave for the Alt encoder reference point).
If you had got the initial 'hint' value *exactly* correct, these two
angles would be exactly equal (when AUTO ADJUST is OFF, the WARP value
is in degrees and is in fact the difference in degrees between these two
angles). However, chances are you will not have got the initial hint
value exactly right. This is where AUTO ADJUST kicks in. It moves the
Alt encoder zero reference point so that the two angles are exactly the
same and therefore the WARP factor will be zero.

AUTO ADJUST ON can do a wonderful job in refining the ALT REF point.
However, it is also a case of "garbage in, garbage out". For example, if
the user mis-identifies one or both alignment stars, AUTO ADJUST will
"bend over backwards" to make the distance the encoders moved match the
distance between the two stars, even if this means moving the Alt Ref
point to an angle that is far away from the initial "hint" value.
Similarly if the wrong number of encoder steps are set or there is a
mechanical slippage in an encoder. AUTO ADJUST ON will try and do the
right thing and give you a WARP factor of zero, but it does not mean
your pointing will then be precise because it will have incorrectly
compensated in face of these more serious problems.

What we recommend for users to do is to initially test the system with
ALT REF = 90 degrees AUTO ADJUST OFF. In fact, when we designed the unit
we could have made the decision to have AUTO ADJUST ON at all times.
However, AUTO ADJUST can incorrectly compensate for a large class of
problems and thereby 'mask' the problem. For this reason, we make users
conscious of the FIX ALT REF step and allow them to switch AUTO ADJUST
OFF so they can help diagnose a problem for themselves.

As I recommended, never become too preoccupied with the initial manual
setting of the ALT REF point and you can put aside mechanical
aids such as spirit levels and set-squares. AUTO ADJUST ON is
your friend. However, if something seems amiss, the ability to
switch it OFF for a while can help you diagnose a problem.

Argo Navis allows you to see what value AUTO ADJUST ON moved
the Alt encoder reference point to. After you do the FIX ALT REF (with
AUTO ADJUST ON) and the two star alignment, place the unit in MODE
ENCODER. Push the scope back to the zenith stop position and read the
right hand (Alt encoder) value. It should be in the ball-park of 90
degrees. If it is more than a few degrees of either side of this, you
likely have a mechanical problem somewhere.

With regards refraction compensation, as you are aware, this becomes
important close to the horizon, a part of the sky most Dob owners tend
to shun. Refraction modeling uses your lat/long (that of your nearest major
town is fine) along with your time zone and local time. From the location
and timezone and local time, Argo Navis can then compute where the
local horizon is and then apply the necessary corrections. Currently at your
locale, your time zone setting will be +10:00 hours.

Having Auto Adjust On, seemed to work ok. I tend to find the alignment
error happens when I do an Align Star in the west and then swing and look
at something in the east. But I also found if I did an Mode Align on the
object (with Auto Adjust On) seemed to fix it, is that how it should work?
I have a feeling this could some sort of mount error. I'll perform a TPAS
once I've become comfortable with what I'm doing.
In the face of mount fabrication errors (all mounts have them to some extent)
the pointing error residuals will be smallest in the neighborhood of the two
alignment stars. Therefore one strategy is to choose one of the two alignment
stars to be in the area of the sky in which you plan to initially observe. If you
then move to some new part of the sky and find that the error residuals have
increased, one can align on a new object in that area. You can align on
any type of object, including for the initial alignment objects, but stars and
plans are best since their astrometric co-ordinates don't suffer the same
uncertainties that extended objects, such as nebula and galaxies, do.

Argo Navis maintains a queue of alignment objects, and the default setting,
which we recommend, is two objects deep. For true multi-star alignment, this
is where TPAS comes into play.

I think I logged 100+ objects last night through to 1:30am this morning. I
think that would have to be a record for me. It was fairly clear with
trans 7/10 but a touch windy. Which made setting the Mode Fix Alt Ref
a bit hard. Turning on Auto Adjust made that a lot easier.
Brilliant and great to hear that it was a new personal record observing run!

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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