Quote:
Originally Posted by Amunous
i have a lot of trouble getting the argo aligned most nights.
i will do the 2 star alignment and when i try to do a tour it will find some objects and not others, and im talking about obvious objects like orions nebula or omega centauri.
sometimes it will find planets no problems but its way off on deep sky. other times deep sky is fine and planets are way off.. i dont know what the issue is but it used to work fine... maybe its a little out im not sure but some nights ill spend 1-2 hours just trying to get the argo to work properly.
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Hi Michael,
Thanks for the post.
We categorize the cause of pointing errors into two classes. What we
refer to in the User Manual in the section under SETUP MNT ERRORS as
"Category 1" errors includes setup errors, encoders slipping in
couplings, misidentified alignment stars and similar errors of
essentially a random nature. What we refer to as "Category 2" errors
includes systematic errors within the mount, such as non-orthogonalities
between the primary axes, non-centered encoders or eccentric bearings,
gravitational flexures, etc.
Firstly, for any first-time installation or any installation where there
is a large pointing error residual, we highly recommend that the user
perform what we refer to as the Daytime Encoder Test which is outlined
on pp 116-117 of the most current version of the Argo Navis User's
Manual. As the name implies, the Daytime Encoder test should be
performed during the daytime as it is easier to work with a fixed
terrestrial pointing reference and it is easier to get down and closely
examine the encoder installation whilst the scope is moving.
I cannot over-emphasize the importance of this test as it can help
betray many different types of mechanical problems such as an encoder
slipping.
Beyond the Daytime Encoder Test, uses should always use AUTO ADJUST ON
when performing the FIX ALT REF step.
To set it up, DIAL up MODE SETUP, SETUP ALT REF and enter a value of
+090.000. Then when you perform the FIX ALT REF STEP, DIAL up ALT
REF=+090.000 AUTO ADJUST ON. Perform your two star alignment as normal.
The WARP factor should then be 0.00 (A) where the (A) indicates the ALT
REF point was automatically adjusted. If you see a non-zero WARP factor
when AUTO ADJUST is ON or an (X) instead of an (A), it means something
is amiss, such as a misidentified star or cable not plugged in. Keep in
mind that though a WARP factor of 0.00 is a prerequisite for good
pointing performance, it does not necessarily guarantee good
performance. The reason is that the AUTO ADJUST mechanism bends over
backwards to correct the ALT REF point so as to produce a WARP factor of
zero wherever possible, even if you have misidentified the alignment
stars. AUTO ADJUST ON adjusts the ALT REF point based on your two star
alignment.
AUTO ADJUST ON can make a major difference in the pointing as it simply
uses the manually set ALT REF point as a "hint" and then refines it
based on the two star alignment. In other words, one need not worry if
the ALT REF point is really 89.123 or 92.456 as even a rough hint, such
as 90 degrees, will allow it to provide a refined value.
Generally speaking, Argo Navis does not use the date/time or location as
part of the alignment process when aligning on a couple of stars.
However, if SETUP REFRACTION is ON and say the SETUP DATE/TIME TIMEZONE
is set incorrectly or the lat/long is significantly out, then Argo Navis
will apply refractive corrections as if the users locale were those as
set. This might, for example, result in the system believing the local
horizon is at some much higher elevation in which case it could apply a
correction of a degree.
Finally, all mounts are not perfect and have what we refer to as
"fabrication errors". A worn Teflon pad can result in the scope sitting
higher in one bearing compared to the other and this can result in the
Az and Alt axes no longer being at right angles. These axes being at
right-angles is a pre-requisite for good pointing.
Likewise, if the scope is out of collimation, the Alt and optical axes
will not be at right angles and again this will result in a pointing
error residual. To help accommodate these class of systematic error and
some others, Argo Navis has an advanced feature called TPAS - Telescope
Pointing Analysis System.
See the User Manual in the section on SETUP MNT ERRORS for details. Argo
Navis can then analyze and potentially compensate for several common
systematic errors. One can optionally save a pointing model for use on a
later obsession session. On a subsequent observing session, one can
re-synchronize the model by using TPAS and sampling the position of at
least four stars. The beauty of TPAS is that it can then improve your
whole-sky pointing performance.
Anecdotally, on some Dobs, some of us find performing a short TPAS
run and fitting either the IE error term (see Manual for details) or IE
in conjunction with one or two other terms can make a significant
improvement to the pointing performance. Whereas AUTO ADJUST ON adjusts
the ALT REF point based on just two stars, TPAS and the IE term takes
over the job of AUTO ADJUST ON and adjusts it not just on the bases of
all the stars you have sampled, but also takes into account any other
errors that have been included in the pointing model as well.
TPAS reports the pointing performance in terms of a statistical metric
called the Root Mean Square (RMS) pointing error so it gives you
valuable feedback to let you know how well the scope points both before
and after modeling. TPAS even has a facility where one can look at the
individual "raw" and "fitted" (i.e. those applied after modeling)
pointing error residuals for each sampled star, which can often also
show whether something random appears to be occurring which is a cue to
go back and perform a Daytime Encoder Test.
TPAS, however, is the "heavy guns" of pointing correction and performing
the Daytime Encoder Test, double checking SETUP parameters such as the
DATE, TIME ZONE, TIME. LOCATION and using AUTO ADJUST ON should always
be the first port of call. Many users never require to use TPAS, but it
can be the best prescription for those whose mounts have a systematic
error. In the end, the most important attribute of a pointing system is
its ability to find stuff, and features such as AUTO ADJUST ON and TPAS
are there to help fulfill this most important of attributes.
If you require further assistance, please don't hesitate to email us at
sales@wildcard-innovations.com.au or to telephone us here in Sydney
on 02 9457 9049.
Best Regards
Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Phone +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au
Mark, any of the Stars in the alignment catalogue will do 
