Details

Type: RFC

Status: Implemented

Resolution: Done

Component/s: DM

Labels:None
Description
To improve jointcal's photometric fitting, we need a way to represent a spatiallyvarying 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

* countsonchip (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.
Attachments
Issue Links
 is triggering

DM9192 Implement new spatiallyvariable PhotoCalib model
 Done
 relates to

DM10153 Replace Calib with finished PhotoCalib
 Done

DM11905 Fix PhotoCalib defintion to use multiplication
 Done

RFC573 Backwards compatibility API for Calib>PhotoCalib transition
 Implemented

DM9184 Higher order models for jointcal photometric solutions
 Done

DM9193 design new Calib object and interface
 Done

RFC549 Use nanojansky for calibrated fluxes in DM code and intermediate data products
 Implemented
I'm proposing this now, because it's the minimum thing I need now, and we don't have a way to represent such a calib.
As I said above re:LDM151, this is the proposal for a new PhotoCalib that has room to be extended in the future. We already know there needs to be an SED term eventually, and as Russell Owen says, one could make a composite BoundedField to represent a more complicated set of transforms.