To improve jointcal's photometric fitting, we need a way to represent a spatially-varying photometric model. Our current Calib object is a constant flux/magnitude zero point (fluxMag0) per ccd/visit. We expect to want to handle a spatially varying component (whether due to sky, optical system, filters, a shift across a CCD, or some combination thereof), and jointcal should be able to fit such a model. As an example, HSC's meas_mosaic fits a 7th order polynomial on the focal plane, plus a fluxMag0 per CCD.

I propose a replacement for lsst::afw::image::Calib that is built on top of an lsst::afw::math::BoundedField, defined in terms of the conversion from counts to flux in maggies. It will have countsToMaggies() and countsToMagnitudes() methods (replacing the current getMagnitude()) that take counts and/or a point, as well as a SourceRecord or SourceCatalog. The new PhotoCalib will behave the same as the old Calib for spatially invariant scaling (i.e. only fluxMag0 is defined).

The class docstring for the new PhotoCalib object is as follows:

/**

 * @brief      The photometric calibration of an exposure.

 *

 * A PhotoCalib is a BoundedField (a function with a specified domain) that converts between calibrated

 * counts-on-chip (ADU) to flux and magnitude. It is defined in terms of "maggies", which are a linear

 * unit defined in SDSS: http://www.sdss.org/dr12/algorithms/magnitudes/#nmgy

 *

 * PhotoCalib is immutable.

 *

 * The spatially varying flux/magnitude zero point is defined such that,

 * at a position (x,y) in the domain of the boundedField zeroPoint

 * and for a given measured source counts:

 *     zeroPoint(x,y) * counts = flux (in maggies)

 * while the errors (constant on the domain) are defined as:

 *     sqrt(countsSigma^2 + zeroPointSigma^2) = fluxSigma (in maggies)

 */

You can see the full API for the new PhotoCalib on this gist.