Changes between Initial Version and Version 1 of PS1_GPC1_Rotator_20080912

Timestamp:

Mar 1, 2009, 4:54:36 PM (17 years ago)

Author:

eugene

Comment:

--

PS1_GPC1_Rotator_20080912

@@ +1 @@
+2008.09.12
+
+I have spent far too much time this week trying to understand exactly
+what is happening in my astrometry model, and I think I have finally
+converged.
+
+A little background:  psastro (IPP astrometry fitting program) can use
+a reference model which describes the layout of the chips and the
+optical distortion.  I generate this model from observations for which
+I have an astrometric solution.  One part of the solution is to take a
+sequence of images obtained at different rotator positions, but with
+the same nominal pointing, and fit for the motion of the claimed
+boresite coordinate, in the assumption that the reported location is
+rolling around the true boresite.
+
+In looking at the rotator sequences from 2008.08.27, I got concerned
+by the observation that the boresite motion was not primarily a
+rotation but a more linear motion.  That meant that the model was
+invalid, so I adjusted the program to be able to write a model in
+which I simply assert the coordinate of the boresite (in pixels
+relative to some chip), allowing me to generate a model based on a
+single astrometrized image.
+
+I discovered some low-level errors in coordinate transforms between
+defining the model and applying the model which resulted in an
+inconsistency: I was unable to generate a model from an image, and
+then apply the model back to that image and correctly predict the
+location of the chips.  After a lot of digging, I fixed all of the
+small bugs that caused this problem.  I also added the capability to
+the astrometry analysis program to compare the model-predicted
+locations of the chip 0.,0 pixels to the measured positions, and use
+this information to measure the boresite offset, the rotation, and the
+scaling difference relative to the predictions of the model.
+
+I now am able to generate an astrometry model which can then be
+applied back to images and correctly predict their chip positions.
+Using this fixed up code, I get a much more rotational boresite motion
+for the rotator sequences.  Attached are three plots, equivalent to
+ones I sent out last week, but using the new analysis of the boresite
+offsets.  The + marks the measured offset between the boresite and the
+model prediction for a given image; the line is a fit to the motion as
+an elliptical path; the o marks the prediction on the path for each
+image.  There are three plots, one for each of the altitudes 30, 50,
+70.  For these plots, I have forced the boresite to be located at the
+midpoint of chips 33 and 44.
+
+These now look more like the measurement of a flexure in the
+x-direction plus an offset error in the boresite location: the
+x-displacement is maximized at POS angle 217, which corresponded in
+these sequences to ROT = -89.
+
+aloha
+gene