Argo Navis TPAS Success (SDM Dob Setup)

[jamespierce]

I used TPAS on my Argo/Servocat setup for the first time last night, previous to this I've never bothered. The pointing has always been ok, good some nights, ok others - objects would nearly always fall somewhere inside a 1 degree field and either re-aligning from a near by object or using the Servocat local sync function always tightened things up to search in a region. Last night though I was out a bit earlier, already setup so I figured why not give it a try - the process I followed was thus.

1) Standard 2 star alignment process
2) Turn on sampling mode
3) Sample 4 stars and compute the basic model using only the IE element.

At this point things were better, objects were consistently falling inside a 0.5 degree FOV.

4) Sample about 9 more stars right around the sky using a mag +2 stars tour. The servo cat would drive within about 0.25 of a degree, I'd center, sample that star and move on.
5) Recompute the model with all 5 elements (not sure if I should have used them all with only 13 stars ?)

The whole process took me about 15min and it was my first time. I would say next time it's a ~10min job.

For the rest of the night I slewed all over the sky enjoying a highlights package for the last night of our 6 night observing trip, 50% of objects were dead centre, 40% within 0.1degree of being centred and the remaining 10% were within 0.2 degree. I really noticed an increase in accuracy around the zenith, typically a trouble spot for a dob + argo.

Unless I'm out for a very quick session, or only looking at a very small region of sky I can't see a reason not to do a short TPAS run - it's easy to spend far more than 10min over the course of an evening tweaking the alignment or just chasing down objects which were not found first time. You can do the whole process early in the evening before it's dark enough to view anything properly anyway. It was especially nice to work through a number of very faint galaxies in Fornax with each one dead center, no wondering if you'd found it or not !

[mbaddah]

Thank you James for that report. I look forward to trying this on my SDM once completed :)

[Rodstar]

Hey James, good to hear you found the TPAS made such a difference. I used it for a while, and then became lazy....a lot depends on how much time you have. I agree with you that if you are going to be chasing down lots of objects over a long night, and in particular searching out some faint fuzzies, having the objects in the centre of the FOV consistently makes a huge difference, and more than justifies the time spent doing a simple TPAS run.

Cheers,

[wavelandscott]

I've not used the TPAS function but your post has made me consider reading the manual and do one myself. Although, "out of the box" my AN does well and puts stuff in the FOV most of the time.

[jamespierce]

I did it again last night, with only a 3min 6 star run, even that tightened it up significantly. Again I did it an hour before it was really dark enough to observe anything anyway.

[erick]

Well no more excuses - I have to give it a go. And I just had the scope set up in the same place for 10 nights and am only deciding to do it now! :doh:

[gary]

Thanks for the post and great to hear you are using TPAS on your Argo Navis. :thumbsup:

Rather than adding all the terms, you should experiment and find which combination
of terms results in the smallest RMS and PSD values and where the terms,
with the exception of IE, which you should always use, are only included in
the model if they have a value at least twice the magnitude of its own standard
deviation.

Think of a value representing a "signal" and the corresponding standard deviation
as "noise". Terms whose signals are swamped by noise will not contributed to
the model in a meaningful and in fact may make things worse.

As an analogy, if one surveyed the ages of pupils in a class and found the
average age was 16.2 years +/- 1.3 years old, then that would more likely be a
statistical meaningful result compared to one where the result had been
16.2 years +/- 25.3 years old.

RMS, which stands for Root Mean Square, is similar to the concept of
an average and the smaller the value the better. PSD, or Population
Standard Deviation, is a metric similar to RMS but it can also tell something
about a model's predictive capability at some point into the future. If
you need to choose between a model that provides a smaller RMS or one
that provides a smaller PSD, the one with the smaller PSD is usually the
better to pick.

To perform an 'on-the-fly' TPAS run, perform your two star alignment as
normal. Be sure that the SETUP MNT ERRORS ACQUIRE DATA,
SAMPLE MODE menu is ON. Now, rather than re-aligning on new stars, whilst in
GUIDE MODE, press ENTER, dial up SAMPLE MNT ERROR, center the object
and press ENTER to sample the star position.

When you get to a stage in the observing run whereby, on a Dob, you have
sampled at least 4 stars, use the SETUP MNT ERRORS functionality and fit
a model using the IE term. Once you have at least 5 or more stars, try
adding another term that is relevant to the mount, perhaps NPAE, CA,
ECEC or ECES.

It is worth repeating the rule as to when to include a term -

A term that is relevant will result in the fitted RMS and PSD value
become smaller than the raw pointing performance case and such a term
must have a value that is at least twice the magnitude of its own
standard deviation. Argo Navis reports this to you when you fit the
model and the standard deviation is the number that appears after the
+/- sign.

There are several advantages to using TPAS above and beyond the regular two star
alignment with AUTO ADJUST ON. Firstly, when one
performs a two-star alignment and AUTO ADJUST ON is in effect, then the
ALT REF point is always determined from just the two alignment stars.
With TPAS, the IE term now does the job of AUTO ADJUST ON and it uses
*all* the sampled stars in refining the position. On many Dobs, often
the addition of one extra term in addition to IE can have a pronounced
effect on the whole-sky pointing performance which then circumvents the
necessity to doing any more realignments or even sampling.

In practice, the system is relatively quick and easy to use. Over time,
one might begin to recognize that the mount has, say, a persistent NPAE
term, which one might then decide to save into non-volatile memory
(NVRAM). The beauty of this approach is that once the model is
re-synchronized by sampling 4 stars and fitted IE and the fixed term,
one can enjoy improved whole sky pointing performance.

Anecdotally, this approach is popular with myself and several regular
observing colleagues when using large aperture Dobs.

TPAS is a very powerful feature and the techniques that it uses are technically
sophisticated. This one feature represents many years of research, testing
and work and the software that embodies it represents a significant percentage
of the entire code base.

Thanks again for the post.

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

[jamespierce]

It's cool Gary, very cool ! I'm looking forward to spending some more time playing around with it when I get some more time.