difference image concerns:

 * normalizations:
   * are they correct?
   * significant coupling between normalization and other kernel terms
 * ringing -- there is significant ringing left behind in difference of images with PS1_V1 PSF shapes
   * is the basis function able to represent the actual observed differences sufficiently?
   * is the observed ringing the result of the coupling to the normalization?
   * is the observed ringing due to the degeneracy between basis terms?
 * basis function orthogonality
   * does the non-orthogonal nature of the Alard-Lupton kernels increase sensitivity to noise and false positives?
   * can we choose a more-orthogonal basis set?
 * span of basis set
   * how can we choose a set of kernels that spans a sufficient range?
   * can the Alard-Lupton basis set cover a wide enough range of structures?
 * weighting?
 * windowing?
 * dual convolution -- strange results with power going to non-radial terms

Some specific bugs identified

 * normalizations were failing for 1D convolutions

 * the wrong preCalc element was being used in a certain context related to Dual

 * the peak pixel is rediscovered, changing the center by 1 pixel for well-centered stars (this is not technically a bug, but 
   makes it difficult to interpret the chi-square images)