Band-Frame Distortion

H.L. McCallon 08-07-00

I. Distortion Checks at 6-Month Intervals:

Using a technique previously described, band-frame level distortion has been checked at 6 month intervals for both the Northern and Southern telescopes. The range of nights going into each check point was limited to one week. Due to differences in source density and weather the number of points available varied. Each check point required restoring the necessary pipeline files from several nights of processing and running GenDXY on the Cal scans from each night to generate the needed dxys files of bias-removed USNOA differences. The dxys files for each set of nights were then combined and the distortion was fitted using FitDXY.

Northern telescope model-fit distortion results at the 6 month check points are presented for x-scan_J, in-scan_J, x-scan_H, in-scan_H, x-scan_Ks and in-scan_Ks. In each figure, mean distortion as per the model fit is plotted as a function of x-scan frame position (xf) in the upper panel and in-scan frame position (yf) in the lower panel. One line is plotted for each of the 6 month check points, coded as follows:

05/97 = black line with "x" symbols

11/97 = red line with open square symbols

05/98 = green line with open triangle symbols

11/98 = blue line with open star symbols

05/99 = magenta line with solid triangle symbols

11/99 = orange line with solid square symbols

05/00 = light grey line with solid star symbols

Southern telescope model-fit distortion results at the 6 month check points are presented for x-scan_J, in-scan_J, x-scan_H, in-scan_H, x-scan_Ks and in-scan_Ks. In these plots the check points are coded as follows:

03/98 = black line with "x" symbols

09/98 = red line with open square symbols

03/99 = green line with open triangle symbols

09/99 = blue line with open star symbols

03/00 = magenta line with solid triangle symbols

II. Global Fit- Assuming the Distortion Remains Constant:

The preceding plots have shown very little change in distortion with time for either the Northern or Southern telescopes. Combining the data from all the 6 month check points with data now being produced during pipeline production runs, overall band-frame distortion fits were done for each hemisphere. These fits benefited from very large sample sizes of 2.2, 1.9 and 1.5 million for J, H and Ks, respectively, in the north; and 3.9, 3.4 and 2.3 million for J, H and Ks, respectively, in the south.

Two-dimensional plots for the Northern telescope showing the uncorrected J-band distortion, the model-fit J-band distortion and the residual J-band distortion after correction are presented (plotted at 100 times actual size). Corresponding plots for the uncorrected H-band distortion, the model-fit H-band distortion and the residual H-band distortion after correction are also presented. Followed by plots for the uncorrected Ks-band distortion, the model-fit Ks-band distortion and the residual Ks-band distortion. Note that in all three cases the distortion model does a fine job of removing the observed distortion.

The same two-dimensional plots for the Southern telescope showing the uncorrected J-band distortion, the model-fit J-band distortion and the residual J-band distortion after correction are presented. Corresponding plots for the uncorrected H-band distortion, the model-fit H-band distortion and the residual H-band distortion after correction are also presented. Followed by plots for the uncorrected Ks-band distortion, the model-fit Ks-band distortion and the residual Ks-band distortion. The distortion model does even better in the south.

III. Global Fit Residuals at 6-Month Check Points:

It has been demonstrated that the global fits do an excellent job of removing distortion as determined from mean differences taken over the whole data set. How well do the global fits handle each of the 6-month check points? To address this question, two-dimensional plots of mean residuals after the global distortion corrections were generated at each of the check points. The results presented below show that:

1) Compared to the original distortion, the global fit residuals are small at all check points.

2) Check points with more noticeable global fit residuals typically have fewer measurement points "npts" available.

For the northern j-band, plots for the 05/97 (npts=169523), 11/97 (npts=327565), 05/98 (npts=446188), 11/98 (npts=279113), 05/99 (npts=167882), 11/99 (npts=179375) and 05/00 (npts=219644) check points are presented. For the northern h-band, plots for the 05/97 (npts=148518), 11/97 (npts=274887), 05/98 (npts=399476), 11/98 (npts=229990), 05/99 (npts=142100), 11/99 (npts=153987) and 05/00 (npts=190703) check points are also presented. And for the northern k-band, plots for the 05/97 (npts=104997), 11/97 (npts=194789), 05/98 (npts=372287), 11/98 (npts=166210), 05/99 (npts=90899), 11/99 (npts=103297) and 05/00 (npts=147598) check points are presented as well.

For the southern j-band, plots for the 03/98 (npts=449011), 09/98 (npts=200980), 03/99 (npts=767384), 09/99 (npts=299924) and 03/00 (npts=759791) check points are presented. For the southern h-band, plots for the 03/98 (npts=407909), 09/98 (npts=178710), 03/99 (npts=695391), 09/99 (npts=274099) and 03/00 (npts=689207) check points are also presented. And for the southern k-band, plots for the 03/98 (npts=272569), 09/98 (npts=118956), 03/99 (npts=453939), 09/99 (npts=211580) and 03/00 (npts=468258) check points are presented as well.

It appears likely the increased residuals at some check points are due to limitations on the number of available measurements and therefore may not reflect actual changes in the distortion. This is supported by the fact that the model-fit results from Section I (which are less sensitive to measurement counts) show little change between check points.

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http://spider.ipac.caltech.edu/staff/hlm/2mass/distrvst/distrvst.html
Comments to: Howard McCallon
Last update: 10 August 2000