Gaia attitude and residuals

I had long conversations with Anthony Brown (Leiden), Daniel Risquez (Leiden), and Giorgia Busso (Leiden) about Gaia data processing and catalog output. I learned a lot, not limited to: Risquez is looking at the changes in attitude modeling precision as the model complexity is changed, where the model complexity is set by the time spacing in knots of a bspline model. The Gaia attitude model is completely data-driven (it has no sense of torques or moments of inertia, it just knows about transit times of stars). It is also not trying to know the true attitude, but the effective attitude averaged over 4.4-second intervals (because that is the CCD drift-scan time). That leads to interesting subtleties and constraints, one of which is that knot spacings much smaller than 4.4 seconds (or maybe half that) can never be useful.

Busso is working on the charge-transfer-inefficiency model for the CCDs. As the CCDs get damaged, they develop traps which delay a small fraction of the charge in the CCD drift-scan. This slightly moves the stellar centers, by much less than a CCD pixel but much more than the required precision (and in a stellar-brightness-dependent way)! It sounds impossible, but because Gaia cuts through every star at 80 or more different angles at 80 or more different times, the magnitude-dependent and time-dependent CTI effects can in fact be modeled and fit to restore the precision of the instrument. The nice thing is that the collaboration hopes to be able to make an empirical model of the CTI and its evolution; that reassures me because the theoretical models of CTI are both young and simplistic.

Brown, who I think (looking from the outside) has had lots of great influence on the Gaia collaboration, told me that early data releases from Gaia are now more-or-less promised. That's a big and important thing; if you think about how much SDSS learned from its early (and often wrong) data releases and how much Hipparcos benefitted from its complete post-final-release reanalysis, early data release is essential to the production of the best possible catalog. Brown also intrigued me by saying that it was likely that the releases would include all the timing residuals for every star at every transit. This is exciting to me because these residuals could be used to create an approximate Gaia likelihood function as I have been trying to imagine for some time now.