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