Tpas

[Hans Tucker]

Any Argo Navis users give me some feedback as to how well the TPAS function assists in providing accurate polar alignment.

I am not a fan of Drift Alignment so always looking around for tools and techniques that would be better faster method.

[adman]

Hans - if you use PHD for guiding it is a great tool to quickly polar align - 5 to 10 minutes max. Less once you get used to the procedure. It has been described in a thread here somewhere - I will try to dig it up for you...

Adam

Edit: here you go ---> link (http://www.iceinspace.com.au/forum/showthread.php?t=67244&highlight=Phd+drift+alignment)

[marki]

Or just use alignmaster, takes 2 minute to get it close enough for AP.

Mark

[adman]

Alignmaster didn't really work for me. The spot where I setup at home has a restricted view of the sky, and I often couldn't get 2 reasonable stars from the built-in list to align on.

With PHD, it is a tool that I already have - no need for more software, and there is no need to have an accurate time or lat/long like you need to with Alignmaster - but for me the biggest problem was the built-in star list.

Having said all that - I do like the simplicity of Alignmaster. Its a nice idea, just didn't work in my case.

Adam

[gary]

Hi Hans,

Thanks for the post and Happy New Year.

One of the advantages the TPAS approach has over the drift test is that,
unlike the drift test, it can simultaneously take into account many of the
common systematic geometric, gravitational flexure and eccentric bearing
errors that typically occur in the mount/OTA.

The problem with polar misalignment error is that it becomes entangled with
all the other pointing errors within the mount/OTA and sophisticated
analytical techniques are required to untangle it.

As just one example, if the scope's optical axis is not at right-angles to the Dec
axis, then this will impact upon the polar misalignment correction that would
otherwise come from a drift test, guiding based analysis technique or any alignment
technique that only uses a couple of stars.

However, since exposure times are often relatively short, for many users the
results obtained from some of the alternative polar alignment techniques
provide acceptable enough results. However, many of us will agree
that sometimes the contortions of a drift test can prove to be literally a pain in the
neck. :)

The best way to use TPAS is at some point, perhaps during a non-observing
period such as near Full Moon, is to perform a long sampling run, perhaps
sampling the positions of 40 to 80 bright stars scattered across the sky.
Using this rich supply of data, one can then analyze for any systematic errors
within the mount/OTA and for the error terms that are likely to be persistent
from session to session, store them as a pointing model into the Argo Navis
memory for use on a subsequent observing session.

Then when one performs an imaging run, simply sample at least four to six stars
to then re-synchronize the pointing model and to characterize the polar misalignment
terms in both Az and El.

The beauty of the TPAS approach is that once a prior analysis of persistent
systematic errors is performed by way of a long sampling run, on a subsequent
session only a short sampling run is needed to nail any polar misalignment.
The polar misalignment terms have very unique signatures that assist in them
becoming disentangled from the other pointing errors once a model is in place.

These same techniques are used on large professional equatorial mount's
such as the 3.8m Anglo Australian Telescope at Siding Springs. So much
so that over the decades using these techniques they have graphed that the
scope is slowly sinking in its piers by a tiny amount each year.

Anecdotally, I will plonk the mount down and eyeball the polar alignment
typically within a degree of the pole and then let TPAS do the rest.

As I often remind equatorial mount owners, there is no such thing as a "perfect
polar alignment". Such a concept is a common misconception even among some
experienced imagers. Instead, because of the effects of atmospheric refraction,
there is only an optimal point in the sky to which to align the polar axis that
corresponds to each elevation in the sky you happen to be imaging. Due to
Earth rotation, this means the optimal axis to which to align the polar axis of
the scope is always changing.

This case study using a G-11 might be of interest -
http://www.wildcard-innovations.com.au/group_post_5573/
To view the Scalable Vector Graphics (SVG), a browser that supports SVG
such as Internet Explore is recommended. In particular, I draw you attention to
a comparison between the third graph from the top to the final 5th graph.
MA and ME are the polar misalignment terms in Azimuth and Elevation and
you can see how they change when other additional errors are taken into account.

if you require any assistance, please don't hesitate to drop me a line.

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

[jamespierce]

Just curious Gary is the TPAS alignment applicable to a dob which is setup and moved around alot ? Or does the fact that the base etc, and the trusses are never quite identical mean you need to redo it each time you setup ?

[Hans Tucker]

Thanks Gary for your in depth explaination...so effectively Argo Navis TPAS is similar to Software Bisque T-Point. I have the $$$'s for the Argo Navis system...just trying to weigh up the benefits to adding this to an EM200 mount.

Adam/Mark, thanks for the info on PHD & Alignmaster..previously I didn't give these much thought but I will revisit these..the PHD does look the better of the two.

[gary]

TPOINT was originally developed at the Anglo Australian Observatory here in
Sydney for use on the 3.8m Anglo Australian Telescope. It is used at most of
the world's major observatories, such as at the Gemini's and Kecks.

TPAS provides a sub-set of the most common systematic error modeling terms
of TPOINT and if you feed them both identical data, you will get nearly identical
results, as you would expect.

For example, if you take the raw pointing data from the Hale 200" Palomar telescope
and feed it to both TPOINT and TPAS and model the same terms, the results
for each of the terms and the whole sky Root Mean Square (RMS) pointing error residual
are very similar. The following table shows a comparison between the results
provided by TPOINT and TPAS when analyzing a pointing run of 39 observations.
The resultant values are shown here in arc seconds for this particular telescope.
Argo Navis takes 2 seconds to perform this analysis.


TPOINT ANALYSIS ARGO NAVIS
coeff value sigma value sigma

1 CH +45.82" +- 3.357 +45.77" +- 3.347
2 ID -128.41" +- 1.383 -128.38" +- 1.379
3 HCEC -7.28" +- 4.344 -7.27" +- 4.332
4 HCES +18.49" +- 1.606 +18.48" +- 1.601
5 IH -87.55" +- 4.749 -87.50" +- 4.736
6 NP -2.01" +- 2.101 -1.99" +- 2.095
7 MA -3.58" +- 1.020 -3.58" +- 1.017
8 ME +64.99" +- 1.587 +64.94" +- 1.583

Sky RMS = 3.81 Sky RMS = 3.80"
Popn SD = 4.27 Popn SD = 4.26"

The term called CH stands for Collimation in Hour Angle and results from
non-orthogonality between the Dec and optical axes.
HCEC and HCES are eccentric bearing errors terms in Hour Angle.
ID and IH are what are known as Index Error terms that synchronize the model.
NP is a non-perpendicularity between the RA and Dec axes of the scope.
MA and ME are the polar misalignment terms in Azimuth and Elevation
respectively. RMS is the Root Mean Squared (http://en.wikipedia.org/wiki/Root_mean_square) pointing performance, a statistical metric
similar to the concept of an average and very popular in engineering. An RMS
of 3.8". for example, means that approximately 68% of the objects would
then fall within a radius of 3.8" and nearly all of them would fall within a radius
of three times that, or 11.4".

Keep in mind that the optical encoders commonly fitted to amateur telescopes
provide a resolution of about 126 arc seconds per encoder step so one will
not achieve the same sort of pointing performance as scopes such as the
200" or the AAT, but the point is that the same techniques engineers use on
the world's biggest professional telescopes can also be applied to the
more modest telescopes enthusiasts own.

By way of background, as you will be aware, in mathematics, it is commonly
impossible to be able to uniquely solve a set of equations if there are more unknowns
than equations. Using the same analogy, the pointing error terms, MA and
ME are two unknowns, but there are commonly many others that affect the
results of MA and ME, such as NP, CH, HCEC, DCES, etc. etc. Unless
one samples enough stars, that is, has enough data, the unknowns cannot
be solved simultaneously. This is why in a strict mathematical sense,
techniques such as the drift test or any technique that relies on attempting
to refine the polar alignment with just two or three stars is at best a first
order approximation to the solution. In a practical sense, such techniques
are often sufficient for the type of work many amateurs do. However, if further
refinement of the alignment is required, these first order techniques don't
cut the mathematical mustard and fall short in a practical sense as well.

The analytical techniques provided by programs such as TPOINT and TPAS
represent world's best practice for solving problems of this type.

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