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Originally Posted by Starkler
Hi Gary, a few more questions if I may.
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Hi Geoff,
You are more than welcome.
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Originally Posted by Starkler
The CA term seems to be a rather large number at 107.5. My scope is a 10" f5 dobsonian and I collimate my secondary with a laser, and my primary with a barlowed laser, so i would think all should be pretty much spot on.
Does this large CA number suggest a problem in this area?
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A couple of points.
Firstly, keep in mind that CA and NPAE have similar 'signatures'
and unless you sample a reasonable number of stars they can be
difficult to differentiate. Recommend you sample a larger number of stars
chosen with a wide spread of Altitudes. In particular make sure you also
include a sample of stars nearer the pole of the scope (i.e. the zenith) where
CA and NPAE can be better differentiated.
If you look carefully, your CA and NPAE terms are acting in opposite
directions with respect to each other. It may turn out that based
on the small sample size, the two terms are "borrowing" from each other
when in fact you may actually have predominantly one or the other.
Secondly, don't confuse optical collimation with pointing "Collimation
Error". Consider the following primary axes on your scope -
the Az axis, the Alt axis, the Optical axis and the nominal Pointing axis.
You can have all of the optical components within your OTA "collimated"
(i.e. all the optical components are in-line and parallel to each other).
However, you can still have the Optical axis out of collimation with respect
to the Pointing axis. In other words, where the telescope "looks" might
be different to where it "points". Optically, the image will be fine.
As far as pointing performance, however, such a scope will be deemed
to have a pointing "Collimation Error". This error can arise from a variety
of sources, including optics that are out of collimation, optics that are
not parallel to the Pointing axis or a Pointing axis that is not perpendcular
to the mount's Alt axis.
Your mount may have pointing Collimation Error, it may have
"Non Perpendicular Axis Error" whereby the Az axis and the Alt axis
are not perpendicular, or it may turn out it has a bit of both.
On a subsequent run, it will be interesting know your RAW and
FITTed RMS as well as the values for the terms. If any error turns out
particularly large, you may even consider correcting it mechanically
at some point in the future.
You can appreciate that given a mount can have all these geometric errors
as well as eccentric bearings, flexure etc., then any computerized pointing
system that can compensate for them is clearly no longer just a plain
"Digital Setting Circle", as nothing is particularly circular any more.
For this reason, Digital Telescope Computer is a more apt description.
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Originally Posted by Starkler
Does the quality of the initial 2 star alignment affect the quality of results of a subsequent TPAS run (assuming correct object identification)?
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No, it doesn't. The two are independent. When you peform a two star
alignment the system uses only those two stars plus computes the Altitude
Encoder Reference point from them if you have AUTO ADJUST ON. As soon
as you apply a TPAS model, it uses all the sampled stars plus peforms
effectively a "super Auto ADJUST" courtesy of the IE term. If you remove the
TPAS model by setting all the terms to zero, the system automatically reverts
to your original two-star alignment.
I like to sample a few stars, define, compute and put "in use" a model,
then sample a few more stars and repeat the process. Doing this will
not affect the overall result, i.e. one can simply sample a large number of
stars and then do a COMPUTE at the end. However, by refining a model
as you sample, you can take advantage of the better pointing performance,
thus minimizing the chance of mis-identifying a star from the pointing
performance you get from just the plain two-star alignment.
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Originally Posted by Starkler
Thirdly, would a calculated models usefulness be more consistant and useful if my dob scopes base board was always oriented to the same direction each observing session? I'm thinking of the possibility of an off centre or non square azimuth pivot type error here.
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Once you have determined errors such as NPAE, you should save them
for use on a subsequent session by selecting the USE NOW & SAVE option.
On a subsequent session, you should define a model that keeps NPAE
at a fixed value (as well as any ECEC or ECES term if significant) and then
set IE and possibly CH to COMPUTE. You would then sample, say, 6 to
10 stars and then COMPUTE a model to determine those terms that need
to be freshly computed. When you do this, the system is also able to
internally compute an index error in Azimuth term that is analogous to IH for
polar aligned scopes. This then circumvents you having to align the mount in
a particular way on each session.
Since one cannot determine the zero point of the Az encoder from session
to session, there are some class of errors that are dependent on an absolute
Az position that cannot be determined. However, often these errors can be
'mopped-up' by existing terms and an Az pivot bolt that is mounted so
that the Az axis is not at right angles to the Alt angles is handled by NPAE.
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Originally Posted by Starkler
footnote: When you buy an Argo Navis, you not only get the most powerful DSC on the market, you also get superlative after-sales service
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Thank you. I appreciate that. You are welcome.
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
Good luck.
