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PSF photometry is one of the major ways to measure the flux of sources (detections) in PanSTARRS1 data. It is based on fitting a predefined analytical shape to the counts reported for each detection, and then applying a zero point conversion to the total object count rate thus obtained.
The current description is incomplete in some details, such as the degree of variation allowed in the PSF over a given warp, the exact definition of flux uncertainty, and the treatment of deviations ("residuals") from the chosen analytical formula.
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PSF Photometry
Obtained from fitting a predefined PSF form to pointsource images. The quality of the fit can help determine whether a source is indeed a point source, it is exteded, or it is spurious. PSF photometry is performed on warps by a module called PSFPHOT. The result of the fit is reported in the Detection table. Values reported include flux, uncertainty, position, elliptical size, and quality parameters.
The PSF Model
The PSF model usually takes the form of an analytical function plus residuals. The fitted parameters and residuals vary with position.
Analytical functions tested include:
 GAUSS : exp (z)
 PGAUSS : (1 + z + z^{2}/2 + z^{3}/6)^{1}
 QGAUSS : (1 + kz + z^{2.25})^{1}
 RGAUSS : (1 + z + z^{k})^{1}
 PS1_V1 : (1 + kz + z^{1.67})^{1}
where z is the elliptical contour (akin to a radius squared):
 z = x^{2}/(2σ^{2}_{xx}) + y^{2}/(2σ^{2}_{yy}) + σ_{xy}XY
The PS1_V1 model is the current default value for PS1 analysis.
Variability over the image is formally represented as:
 PSF = F[dx,dy;ai(x,y)] + R0[dx,dy] + x Rx[dx,dy] + y Ry[dx,dy]
Existing documentation states that a global linear fit is performed in which the fluxes of all objects is fitted for simultaneously with the following considerations:
 Simultaneous fit of fluxes for all objects in the image
 Chisquare fit:
 χ^{2} = Σ(f_{i}  Σ (A_{j} F_{j)})^{2} W_{i} (i : pixels; j : objects)
 W_{i} – weighting function
 now constant (from mid2012), was inv variance
 using a constant weight removes a photometric bias found for faint sources
 minimization of A_{j} requires inversion of large square matrix
 N (number of objects) may be up to 100k
 but, highly diagonal, so inversion is actually fast
 ~ 1 second for 100k objects (unless they grow too large)
Unclear what constrains are placed on PSF parameters (other than flux) during the fitting.
How residuals are recorded and used to determine, e.g., aperture corrections is also unclear.
Photometric and astrometric parameters from PSF fitting
The Detection table contains the following parameters related to PSF photometry:
xPos  raw pixels  REAL  4  999  PSF x center location. 
yPos  raw pixels  REAL  4  999  PSF y center location. 
xPosErr  raw pixels  REAL  4  999  Error in PSF x center location. 
yPosErr  raw pixels  REAL  4  999  Error in PSF y center location. 
psfFlux  Janskys  REAL  4  999  Flux from PSF fit. 
psfFluxErr  Janskys  REAL  4  999  Error on flux from PSF fit. 
psfMajorFWHM  arcsec  REAL  4  999  PSF major axis FWHM. 
psfMinorFWHM  arcsec  REAL  4  999  PSF minor axis FWHM. 
psfTheta  degrees  REAL  4  999  PSF major axis orientation. 
psfCore  dimensionless  REAL  4  999  PSF core parameter k, where F = F0 / (1 + k r^2 + r^3.33). 
psfQf  dimensionless  REAL  4  999  PSF coverage factor. 
psfQfPerfect  dimensionless  REAL  4  999  PSF weighted fraction of pixels totally unmasked. 
psfChiSq  dimensionless  REAL  4  999  Reduced chi squared value of the PSF model fit. 
psfLikelihood  dimensionless  REAL  4  999  Likelihood that this detection is best fit by a PSF. 
This information was obtained from the Schema Browser at http://web01.psps.ifa.hawaii.edu/PSI/schema_browser.php?describeTable=Detection (requires PSPS login) after selecting SAS42PV3 in the PS1 Query Page. The STScI version is at at http://panstarrsplps3v.stsci.edu/PSI/schema_browser.php.
Note that psphot actually returns PSF_MAJOR and PSF_MINOR, whose relation to FWHM depends on the value of k for the PS1_V1 profile. For k=0, FWHM=PSF_MAJOR*2*sqrt(2)*pixel_size.
Details needed and questions
 Is a multiparameter fit used for each detection, or are the parameters (position, PSF shape) obtained from global fits (for PSF) and detection properties (position)? Above notes are not fully sufficient.
 How exactly is the flux error defined?
 How are the flags set?
Source of information
Most of the information contained here is based on the PS1 Guide:
http://ps1sc.ifa.hawaii.edu/PS1wiki/index.php/PS1Guide#The_PSF_model
Other relevant papers include:
http://adsabs.harvard.edu/abs/2007ASPC..364..153M
http://adsabs.harvard.edu/abs/2012ApJ...756..158S
http://adsabs.harvard.edu/abs/2012ApJ...750...99T
These papers provide good information on the calibration process  correction for airmass, reduction to a standard magnitude system, other instrumental effects, etc  but none describe explicitly (or even implicitly) how the *flux* itself is determined. If you are aware of such a paper, please add a comment or email stefano@stsci.edu.
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