I spent the afternoon at Leiden chatting with the locals. Among other things, I learned that Kenworthy (my host) has figured out how to use the real-time output from an adaptive-optics wavefront sensor to predict the point-spread function in the science data. This is a big deal. It means we could potentially have a model of the PSF before we even read out the data. The wavefront-sensing data are used to control the adaptive optics system but of course they also contains scientific information of great value. That point is simultaneously obvious and brilliant.
Labbé and I discussed alternatives to
stacking—simple adding or averaging of data—when there are signals too weak to see in any one datum. He has written some nice things about this issue but also has the pragmatic view that doing a better job only makes sense if you get a better answer. We discussed contexts for making that point.
On the way to dinner, Kuijken described his approaches to weak lensing problems, which involve finding the shear map that distorts the image into one in which there is no net ellipticity. He made the nice point that it is much easier to show that a group of galaxies shows no net ellipticity than it is to quantitatively measure the departure from zero ellipticity. That's an interesting point and worth contemplating. He still has to deal with the point-spread function, which has not been lensed, so his methods are not trivial; we discussed the problem of the PSF because we have some disagreements there.