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In 2002, a new plate scale calibration was performed. That work
is documented here.
We have used the method laid out in that work to perform new
calibrations using the data from 2002-2005. In brief, we performed a
Powell minimization of 10 of the 19 calibration coefficients
(leaving temperature fixed) using as our source data the expected and measured distance
between available star pairs in science observations.
We came at this problem with two goals: 1) to track changes in
the camera assembly calibration over time and 2) to compare the
calibration presently used by ground pipeline processing tools
to the present state of the camera.
Changes Over Time
To track changes over time, we generated a new "baseline" set of
coefficients using the 2001 data. These coefficients are
referred to as "2001newbase" in the tables below. For each
year, the star distance deviations calculated with newly fit
coefficients have been contrasted with the deviations consistent
with use of the 2001newbase coefficients. The newly fit
coefficients were used at an estimate of the ACA housing temperature for
the year, and the 2001newbase coefficients were always used at
14.5 deg C.
2000 : Temp = 14.15 deg C
| 2001newbase coeff | new fit coeff |
|---|
| CHISQ/N= 0.0542 | CHISQ/N= 0.0470 |
 |  |
2001 : Temp = 14.75 deg C
| 2001newbase coeff | new fit coeff |
|---|
| CHISQ/N= 0.0565 | CHISQ/N= 0.0565 |
 |  |
2002 : Temp = 14.75 deg C
| 2001newbase coeff | new fit coeff |
|---|
| CHISQ/N= 0.0711 | CHISQ/N= 0.0695 |
 |  |
2003 : Temp = 16.35 deg C
| 2001newbase coeff | new fit coeff |
|---|
| CHISQ/N= 0.0916 | CHISQ/N= 0.0795 |
 |  |
2004 : Temp = 16.55 deg C
| 2001newbase coeff | new fit coeff |
|---|
| CHISQ/N= 0.0735 | CHISQ/N= 0.0685 |
 |  |
2005 : Temp = 16.75 deg C
| 2001newbase coeff | new fit coeff |
|---|
| CHISQ/N= 0.0940 | CHISQ/N= 0.0866 |
 |  |
Conclusions
While there have been measureable changes in the camera as seen
by fitting new plate scale coefficients, even the largest expected
offsets at the corners appear to be less than .30 arcseconds when the
data from 2005 is compared to the data from a baseline generated by
the 2001 data. These changes are to be expected due to the gradual
increase in ACA housing temperature.
Evaluation of Current Ground Processing Calibration
Here we have compared each new coefficient fit by year with
the coefficients currently in use by the ground processing
system. Each calibration was redone using the ground processing
coefficients as the seed points. For each year, the ground
processing coefficients and the newly fit coefficients were used
at an estimate of the ACA housing temperature for that year.
2000 : Temp = 14.15 deg C
| ground process coeff | new fit coeff |
|---|
| CHISQ/N= 0.0503 | CHISQ/N= 0.0470 |
 |  |
2001 : Temp = 14.75 deg C
| ground process coeff | new fit coeff |
|---|
| CHISQ/N= 0.0578 | CHISQ/N= 0.0565 |
 |  |
2002 : Temp = 14.75 deg C
| ground process coeff | new fit coeff |
|---|
| CHISQ/N= 0.0714 | CHISQ/N= 0.0698 |
 |  |
2003 : Temp = 16.35 deg C
| ground process coeff | new fit coeff |
|---|
| CHISQ/N= 0.0865 | CHISQ/N= 0.0792 |
 |  |
2004 : Temp = 16.55 deg C
| ground process coeff | new fit coeff |
|---|
| CHISQ/N= 0.0950 | CHISQ/N= 0.0682 |
 |  |
2005 : Temp = 16.75 deg C
| ground process coeff | new fit coeff |
|---|
| CHISQ/N= 0.1122 | CHISQ/N= 0.0866 |
 |  |
Conclusions
The evaluation of the ground processing coefficients (compared to
newly fit values) was designed to quantify the extent, if any, that
aspect products might improve if newly calculated coefficients were
used in this ground processing. The expected-measured distance plots
and the vector difference plots do illustrate two important points: 1)
as years pass from the creation time of the ground processing
coefficients, greater improvement would be seen by implementing new
coefficients (this can most clearly be seen by the difference in
CHISQ/N for each set of plots) and 2) the current ground processing
coefficients are still mostly within .20 arcseconds of "ideal" new
coefficents even for the 2005 data.
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Last modified:12/27/13
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