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Messages posted by: wildcard
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Mike Van Lysel wrote:
Hi Gary – Thanks for the reply.

I certainly understand the importance of the azimuthal reference points provided by the initial 2-star alignment. And I suspected that, as I believe you say, the IE TPAS-term benefits from different azimuthal-samples. However, it wasn’t clear to me that there is any other azimuthal component to the modeling.

To provide a little background; I am working on a telescope (a DOB with the AZ-encoder mounted to the azimuth pivot bolt) that has ALT-issues. For one thing, the ALT-bearings are eccentric. I, just like everybody else who turns to TPAS, am looking for term(s) that are stable over time. I’ll determine these carefully and save them for future use and then resync the model with a few star samples before each observing session. So far, it looks like ECEC is significant and stable for sure; less certain but promising are NPAE and CA.

But I encountered something unexpected in my TPAS runs (I have performed 6 on this scope). The scope has a fundamental raw rms residual of about 20 arcminutes. I started out doing all-sky runs but these produced modest results. I tried various combinations of terms but the best case improvement only brought it down to about 10 minutes. Then one night I did a restricted sampling in one quadrant of the sky and the TPAS model brought the raw 20 minute residual down to 2.5 minutes. I confirmed this behavior the next observing session; a sample that was pretty much pure ALT brought 20 minutes down to 2.5 minutes. Something about the all-sky samples produces worse results than the more limited samples (I have some ideas about what, but maybe that’s for another time).

So that’s the genesis of my posted question. In the perfect world I would just go out and do a lot more TPAS-runs, but in the real world of Wisconsin weather I am trying to maximize the productivity of the chances I get by asking for help here. For my next attempt I was thinking of sampling 4-stars of equal ALT spaced at 90-degrees in AZ, then adding an ALT-ladder of about 15-stars at similar AZ equally spaced in ALT between 80 to 20 degrees (in the northern hemisphere this time of year there’s a very nice ALT-ladder to the NE in the early evening). Is this strategy smart or dumb?

Thanks much,

Mike Van Lysel

Hi Mike,

Thanks for the post.

As mentioned in my previous response, there are additional terms that are always modelled but are hidden from you.
These terms include azimuthal components. However, because the user does not establish any consistent zero reference point in
azimuth, these hidden terms aren't saved in NVRAM. A short sampling run re-establishes them on a subsequent observing session.

With your eccentric bearing, you may well benefit with either or both of ECEC and ECES.

Typically you might find either one of NPAE or CA to be of benefit as well.

It sounds like from your sampling run that you might have something going on in azimuth as well.
Chances are you performed the Daytime Encoder Test but if not I highly recommend you do so in the azimuth axis.
This might then betray something slipping. For example, if your azimuth pivot bolt is secured to the base of the ground
board like on the Obsessions, try and have a look under there and make sure the plate is still rigidly affixed.
Also make sure that the azimuth tangent arm is ideally floating a little. Look out for if it is becoming foaled on the base
of the rocker.

Nothing will trow out a pointing model faster than a misidentified star. Be sure to periodically review the sampled stars
in the REVIEW DATA sub-menu. You can change it to display between raw and fitted residuals. Have a look a the raws
and check for any obvious outliers. Don't delete a correctly sampled star simply because it has a larger error residual.

I recommend you sample the whole sky including with changes with azimuth. Otherwise you are biasing your sample
to include only stars that give you a small RMS.
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