[LSST|dm-users 684] Re: How to Use meas_modelfit to Measure Sersic Properties

[Will Dawson]

Robert,

Following your reference it takes the flux of a zero magnitude object (units: ADUs), REF <http://lsst-web.ncsa.illinois.edu/doxygen/x_masterDoxyDoc/classlsst_1_1afw_1_1image_1_1_calib.html#a823f25da4fabd26b51a83855a4284d88>. You can see an example of setting it here <http://lsst-web.ncsa.illinois.edu/doxygen/x_masterDoxyDoc/scale_zero_point_8py_source.html#l00139>.

Thank you,
Will


> On May 27, 2015, at 4:53 PM, Russell Owen <rowen at uw.edu> wrote:
> 
> The C++ source code or Doxygen documentation is the place to look for documentation on wrapped C++ code. Unfortunately. We’re planning to fix that at some point. Meanwhile:
> 
> http://lsst-web.ncsa.illinois.edu/doxygen/x_masterDoxyDoc/search.php?query=Calib <http://lsst-web.ncsa.illinois.edu/doxygen/x_masterDoxyDoc/search.php?query=Calib>
> 
> returns the information you need. The first link goes to the Calib class which has two overloaded versions of setFluxMag0, the first of which certainly looks relevant (I’m not sure if the second has been exposed in Python).
> 
> — Russell
> 
> On May 27, 2015, at 4:32 PM, Will Dawson <will at dawsonresearch.com <mailto:will at dawsonresearch.com>> wrote:
> 
>> Jim et al.,
>> 
>>> On May 26, 2015, at 9:06 AM, Jim Bosch <jbosch at astro.princeton.edu <mailto:jbosch at astro.princeton.edu>> wrote:
>>> 
>>> In this case, "Calib" object refers to "afw.image.Calib", which is the object we attach to an Exposure to store its magnitude zeropoint (and other photometric calibration information).  CModel needs it (as well as a Wcs) because it works in absolute units so it can use Bayesian priors (though it just uses flat priors by default).  So if you can construct a afw.image.Calib object, give it a reasonable zeropoint, and add it to the Exposure (with exposure.setCalib(...)), that should solve your problem.
>> 
>> Follow up question regarding setting the zero point with lsst.afw.image.Calib. I see a function called lsst.afw.image.Calib.setFluxMag0() but it isn’t clear from the title of this function what input I should give it, and the doc string returns:
>> 
>> Help on method setFluxMag0 in module lsst.afw.image.imageLib:
>> 
>> setFluxMag0(self, *args) unbound lsst.afw.image.imageLib.Calib method
>>     setFluxMag0(Calib self, double fluxMag0, double fluxMag0Sigma=0.0)
>>     setFluxMag0(Calib self, double fluxMag0)
>>     setFluxMag0(Calib self, pairDoubleDouble fluxMag0AndSigma)
>> 
>> So I am left wondering if I should input:
>> — zero point magnitude (e.g. 31) or,
>> — flux for a zeroth mag object (e.g. 4760 Jy; Schneider, Gunn, & Hoessel (1983)). If so what units and I guess I should specify other telescope specific properties. Or,
>> — something else?
>> 
>> For coding simplicity I am guessing it is just the zero point magnitude, but the function name is making me nervous about simply assuming that.
>> 
>> Thanks,
>> Will
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