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Joined: 22/02/2021
Messages: 3
Location: Durham, NC, USA
I love the TPAS - it works great with my Dob. I've been wondering if there is a way to save the pointing data after running a pointing test, then reloading it during my next session and continuing to add data to the dataset? Or save two datasets and merge them? I've found no indication in the user manual that this is possible; the data are erased when the AN is powered down.

Yes, I realize that some physical conditions might change from one session to the next, especially if the telescope is taken down, moved, adjusted, etc, and that such changes might "invalidate" the prior data. However, if I leave my scope up "overday" (as opposed to overnight), it might be nice to be able continue with data collection.

Of course, one way to accomplish what I'm looking for is simply to leave the AN powered on. If I can plug it in, that is a possibility. I'm hoping for a way to actually save the data one way or another.

Thanks -- BC

-- BC

Home-built 10" Dob with equatorial platform.


Joined: 17/10/2019
Messages: 122
Location: Mount Kuring-Gai NSW Australia
Hi BC,

Thanks for the question.

Since the pointing data is gathered with respect the arbitrary star alignment, it is not possible to merge two sets of data from two different alignments.

One way to appreciate why that is so is to consider a telescope that has arbitrary geometric pointing errors built into it.
Such a telescope never points at exactly where it is thought to be pointing.

When one does a two star alignment using such a hypothetical telescope, one is effectively declaring - "I solemnly swear that the two points I have just aligned on are free of errors."
Well we know of course that can't be true because there is nothing special about those two points compared to any other and therefore they must be polluted with the same geometric
errors as any other arbitrary two points.

To further exacerbate the issue, the geometric nature of many types of pointing errors is such that they are often a function of the telescope's altitude or azimuth, or possibly some harmonic of that.
In other words, many of these types of errors are not constant across the sky.

When one gathers pointing data, it therefore is built upon the scaffolding of the arbitrary two-star alignment. This is why one should never save the index error terms because they
effectively null out the finite errors of the arbitrary alignment, This is also the reason the data from two different alignments would be a challenging thing to merge.

This is a long-winded way of saying that the workaround is to indeed not power off the system and to use the same star alignment.
But I hope the above background might give you some insight as to why that is so rather than leave one thinking it is through want of functionality.

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Phone +61-2-9457-9049
[Email] [WWW]


Joined: 22/02/2021
Messages: 3
Location: Durham, NC, USA
Thanks for the great response, Gary. Long-winded perhaps, but excellent. I had not appreciated the important role of the alignment stars. Of course there must be a starting point for the TPAS function (which I presume is composed of one or more Fourier series -- or related -- expansions of expressions for the relevant telescope mechanics?).

Nevertheless, your response also begs a couple more questions:

1. It seems that one should NEVER re-align the scope in the midst of a pointing test. That is, don't perform a new ALIGN or ALIGN STAR operation, as this would change the "starting point" for the TPAS function, and thus would result in error data that are inconsistent with the data collected prior to the new (and ill-advised) alignment. Similarly, no new FIX ALT REF operation. Correct?

2. If I want to compare new results (the computed error values) with results from a previous pointing test -- to see if results are more-or-less consistent -- then it would be best to begin with the same initial alignment stars, yes? Doing so might also give me a new computed IE that might not be too far from what I saw in the previous test? All else equal, of course, like scope setup, collimation, etc.

I must have done a decent job building and setting up my scope, because my computed errors rarely differ from zero by more than one standard error. Even so, the TPAS results always give me a substantial improvement in RMS and thus better pointing performance. If the objective is prediction (in this case predicting where a star will be found), it's no statistical sin to keep nonsignificant terms in the model, as long as those terms really do help improve the performance.

Thanks again! -- Bob Clemen

-- BC

Home-built 10" Dob with equatorial platform.
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