I'm following the correspondence on T-Point and hope to be able to
contribute something useful over the next couple of days. Here are
a few preliminary remarks:

* As one subscriber said, you can't make a silk purse out of a sow's
ear. The results can't be any better than the encoders and any
hysteresis effects (for example mirror slop). What T-Point will
do is get the best out of whatever telescope you've got.

* Poor results are often because something untoward happened during
the test run - something shifted, or it was necessary to resync
for example. The best you can do with such a test is to chop out
the good bit and just use that.

* You should only use enough terms to match the quality of the data
and the size of the run. Including more and more terms, especially
powerful harmonics and polynomials, will reduce the RMS but make the
model worse. If there's trouble, cut right back to the 6 geometric
terms (IH ID NP CH ME MA for an equatorial) and look at the
residuals for signs of two populations etc.

* When you've changed your setup, fix those terms which ought not
to have changed and only fit the ones that are likely to be
different. For example, if you've got a fixed mounting and no
big changes of weight, fit only IH ID and CH. All the other terms
in the model, such as the polar axis terms ME and MA and any flexure
or runout terms, should be included but fixed at the values you got
from a previous full-scale test. That way, the start of night
procedure needs only to be 3-5 stars typically, to absorb the
resync-ed zero points and any collimation changes.

* If your model is sensible (not too many terms) and you don't seem
to get as good a result in practice as the PSD figure claims, suspect
the implementation of the model in the telescope control system.
There's an easy way to test this - do a "dummy pointing run". Do
a normal T-Point run, but leave the telescope where it is each time:
don't recenter the image. When you reduce the run, you should get a
small PSD (as limited by the encoder resolution) and all the
coefficients should come out to whatever you told your control
system to use.

More in due course.

Patrick Wallace

View index by [date] [author] [subject]
Previous message: Re: [M]: Re: Ooooops: ACP did NOT crisp my scope., Mark Taylor
Next message: [M]: LX200 Control Pad Enter Key, Email address hidden
Next message in thread: [M]: Re: [M] 12"LX200 with T-Point, Ric L Ecker
Previous message in thread: Re: [M]: Re: [M] 12"LX200 with T-Point, Frank Loch