Quote:
Originally Posted by goober
I had a first play with TPAS tonight with my alt-az mount, sampling a dozen stars. The best combination of terms was IE (always use this, I assumed) and NPAE (-16.2 +/- 4.2). It gave me a fitted RMS of 4.5' and PSD of 5.9'. For 5000 step encoders, I think that's near the limit of resolution. Adding other terms, especially CA, blew out the results.
After the sample, I tried the model out with my 154x/32' eyepiece, and everything hit bang on, well within the field of view. Couldn't fault it.
Of course, then I powered off the Argo and lost everything. Just to compare, I did a two star align and went around the objects again without TPAS, and the hit rate was just as good as with the TPAS model. Go figure  |
Hi Doug,
Thanks for the post.
Great to hear you are out with the Tele Vue and Argo Navis.
It is always interesting to note what your RAW RMS point performance was
by way of comparison. If you have sampling data still in memory, If you then DIAL
up SETUP MNT ERR, REVIEW DATA and spin the DIAL clockwise until it reads
END OF DATA, the bottom line will show the RAW RMS, designated with
a capital Greek Delta symbol (shaped like a triangle). If you then press ENTER
and spin the DIAL one click, you will see DELTAS=FITTED. Press ENTER again
and the display will show the FITTED RMS for that data, designated by the
lower case Greek delta symbol (shaped like a stylized d). Using the REVIEW
DATA menu and the ability to change between RAW and FITTED display mode,
it is sometimes interesting to scroll back and look at the error residuals of
the individual sampled stars.
The bottom line is that the results TPAS reports are "as it really is" in a statistical
sense based on the data that you have collected by sampling.
Quote:
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The other strange thing was I went through Gary's FIX ALT REF test to get an accurate fix on my docked scope. At rest my scope points slightly down, and I had set it an angle of -12.67 degrees (read it somewhere). Went through the alignment, and put the scope back at docking position, go into MODE ENCODERS. My angle now reads +7.63 degrees! Not sure what happened there, but I changed my FIX ALT REF to that and it seemed to work fine.
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I gather you were using AUTO ADJUST ON when you performed the FIX ALT
REF step and two-star alignment?
The ALT REF point is with respect the mount. For example, visualize the mount's
Az axis standing vertically. The zero degree ALT REF point is when the OTA
is 90 degrees away from this vertical Az axis. If the mount happened to be
level, this would correspond to when the tube were level and pointing
toward the horizon.
If the mount were level and the tube in the lowest 'rest' position looks below
the horizon, the ALT REF point at that 'rest' position should indeed be
a negative value.
The AUTO ADJUST ON feature is very tolerant to the initial guess of the
ALT REF point. It simply takes the initial value as an estimate and after the
two-star alignment, adjusts it on the basis of the expected separation of the two
alignment stars compared to the angular separation was based on the encoder motions.
When picking the two alignment stars, make sure the two not only have
differing Azimuths, but differing Alts as well.
Once the encoder installation has been verified by the Daytime Encoder
Test and ballpark pointing established when using AUTO ADJUST OFF,
we recommend to then use AUTO ADJUST ON on subsequent observing
sessions.
After performing the FIX ALT REF with AUTO ADJUST ON and the
two-star alignment, if one then brings the OTA back to the rest position
(commonly the vertical stop on a Dob) and DIALS up MODE ENCODER,
it is instructive to note the right-hand displayed ALT encoder value.
This should approximately correspond to the ALT REF point that AUTO ADJUST
has computed.
AUTO ADJUST ON can do an amazing job of refining the ALT REF point.
However, it only uses the two alignment stars as inputs into this process.
When one samples at least four stars and fits a model using the IE
term (which should always be used) and then puts that model in use,
TPAS now takes over the job of AUTO ADJUST ON. Rather than just use
two stars to determine the ALT REF point, TPAS uses as many stars
as you have sampled and will take into account any other error terms you have
defined into the model as well. In other words, the IE term, which stands
for Index Error in Elevation has an association with the ALT REF point.
TPAS with the IE term in place therefore becomes a 'super AUTO ADJUST'
feature.
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
