HSC PSFs truncated -> create ringing in LowResObservation

[pmelchior]

The multi-resolution tutorial uses a HSC PSF with 31 pixels. This is apparently too small and creates ringing in the oversampled PSF (which defines the diff kernel):

plt.imshow(obs_hsc._diff_kernels.image[4], vmax=1e-6)

yields

Other HSC bands are similar. This should affect the results of the resampling+convolution operation.

This issue addresses two aspects:

• Confirm whether the ringing is detrimental for the convergence or accuracy of multi-observation fitting
• If so, create code to check for this problem before we run the fitter

[fred3m]

HSC PSFs are usually 41x41 or 43x43 pixels. So you should be able to test with the full HSC PSF to see if truncation is the issue.