Southern Distortion as Determined from 980801s/Stone(Field m)

H.L. McCallon 10-27-98

Herein are figures presenting distortion with the Southern instrument for all three bands as determined from 24 scans of Stone's deep/dense field "m" on 980801s. First, scans 010-021 and 068-079 were reconstructed using the Stone sources as reference stars. Although each scan was only approximately one degree long, the field is dense and all 24 scans are totally contained therein. After any remaining biases were removed a band at a time, the Stone source positions were mapped into individual band-frame coordinates. These were matched to 2MASS extractions with high quality positions and position differences were computed. 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, which has the most distortion, 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 the averaged fits track the average observed differences very well in all cases.

The improvement with the distortion model, although largely swamped by larger random errors, can be seen in the before/after histograms shown in Figure 4 for J-band, Figure 5 for H-band, and Figure 6 for K-band. As before, the solid black lines are without the fit and the dotted red lines are the fit. As expected, the K-band improvements stand out.

The next set of plots show a two-dimensional representation of the measured distortion. Figure 7 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. The vector is plotted at 100 times its actual length. Figure 8 does the same for H-band and Figure 9 repeats for K-band. In all three plots a 0.1 pixel long horizonal measuring stick is presented at plot center. Note that for K-band the worst-case distortions an the corners approach 0.2 pixels. Although worst-case distortions of ~0.2 pixels were also seen in the North, fewer of the 100 grid squares had such large values.

This analysis should be repeated in the future to see if there's any significant change with time or telescope orientation.