Distortion North - Oct/Nov '98

H.L. McCallon 11-24-98

Herein are figures presenting distortion for all three bands as determined from 9 full nights of calibration scans using USNO-A2.0 measurements. The nights used for this analysis included 981023n, 981025n, 981028n, 981029n, 981030n, 981101n, 981103n, 981107n, and 981111n. As was pointed out previously, USNO-A2.0 can be used to compute distortions provided biases are carefully removed and sufficient measurements are included to compensate for the increased uncertainties. The x-scan (dx) and in-scan (dy) differences were fitted separately for each band to the following polynomial:

         del =c1*x^2 +c2*y*x^2 +c3*x*y +c4*x*y^2 +c5*y^2 +c6*x +c7*y +c8 +c9*x^3 +c10*y^3

Figure 1 plots the average x-scan distortion in J-band as a function of x-scan frame position in the upper-left panel and as a function of in-scan frame position in the lower-left panel. Note that the units are pixels. The in-scan distortion is plotted in the two panels to the right. The same presentation is made for H-band in Figure 2 and for K-band in Figure 3. In each plot the solid black lines refer to the measured distortion and the dotted red lines to the polynomial fit. Note that for all three bands the results track rather closely the previous results using Stone field measurements from the night of 980113n.

Agreement with the previously determined distortion model can also be seen in a set of plots showing a two-dimensional representation of the measured distortion. Figure 4 breaks the J-band field up into a 10x10 grid and plots arrows showing the direction and magnitude of the distortion in each of the 100 grid squares. Each vector is plotted at 100 times its actual length and a 0.1 pixel horizonal scale bar is shown for reference at the center of the grid. Figure 5 does the same for H-band and Figure 6 repeats for K-band.

These results show that there has been very little change in distortion for the Northern telescope/camera over a 10 month time period. They furthermore confirm the feasibility of using USNO-A2.0 measurements to model distortion.