Quote:
Originally Posted by g__day
The raw pointing in each quadrant of the sky seems very consistent. I am hoping that Tpoint is smart in how in addresses point errors - by adjusting for pointing by deciding the correction required using those points closest to the target. Is this the case anyone?
|
Hi Matt,
TPOINT doesn't work in that way.
Firstly, one should separate the function of TPOINT, which is used for
data analysis and model creation, from a "pointing kernel", that can accept
TPOINT model terms and apply corrections to where the telescope points.
In other words, TPOINT does not have a pointing kernel, that is the job
of some other piece of software, specifically the piece of software that
is responsible for pointing the telescope.
Once you put a pointing model in place, a pointing kernel that accepts a TPOINT
pointing model makes no reference to the original pointing data used to create
that model.
To appreciate the mechanism, the pointing data is analyzed to create the
terms for various equations that are used to model some specific systematic
mechanical "fabrication error" of the mount/OTA. Such "fabrication errors"
also include polar misalignment. Once the terms are created, the job of the
original pointing data is done and in order to apply pointing corrections, the
pointing kernel has in-built into it essentially the same set of equations that
TPOINT used.
The TPOINT philosophy is that each of the equations models the behaviour
of some particular mechanical fabrication error of the telescope. Some of these
errors will be constant in magnitude and direction across the sky. Others, such
as Non-Perpendicular Axis Error (NP), are not. So if a term such as NP is used, then
the corrections applied might be larger in some part of the sky compared
to others. So the pointing kernel can apply a correction specific to the area
of the sky that you are currently pointing, but the magnitude and direction of that
correction is a result of the sum of all the equations in the pointing model rather
than by referencing some earlier pointing data.
TPOINT and for that matter any other pointing analysis software can only
analyze and correct for systematic errors. A sudden mirror flop is an example of
a non-systematic random error.
By the way, the two clumps of data to the East and West in your scatter diagram
hint most likely to something such as a non-perpendicularity between your optical axis
and Dec axis - what TPOINT refers to as CH - a common fabrication error in most GEMs.
Hope this is helpful.
