9 Astrometry Done with PinPoint of dc3.com ($149) Solve the image Detection of starsMatching with star catalog (GSC, usnoa2)List of all detected starsRA, DEC and pixels coordinates of CentroidsPixel coordinates are used to position the aperture circles.
10 Aperture measurementAperture : pixel value = star + background + pedestalAnnulus : pixel value = background + pedestalFlux = Sum ( Aperture – Annulus )
11 Aperture measurement Sum the ADU of pixels inside the circle Calculate the distance : pixel center to circle centerBresenham’s algorithm - Midpoint circle to speed up the process (avoid square calculations of real numbers)Pixels straddling the circle are divided in 10 by 10 subpixels and algorithm is repeated.Straddling subpixels counted for 1/2Error on circle surface less than 0.2% (radius 5 pixels)
12 Close stars? Astrometry provides a list of stars If a star is in the annulus, program doesn’t use this part of annulus
13 Differential photometry FObj = Variable fluxFRef = Comparison star fluxMref from star databasemobj = Mref Log10 (Fobj/Fref)More than one star can be used for the referencecalled ensemble photometryfluxes of reference stars are added
14 Error estimation Poisson noise Background noise Errors on Variable and reference stars -> added in quadrature
15 Reports AAVSO CBA Excel sheet upload to AAVSO databasenot user friendlyCBAComparison starsJD, magnitudes, airmass, a.s.o.Excel sheetGraph of the light curveGraphical interface to estimate the extremaReinsch spline algorithm to obtain the
16 Software installation LesvePhotometryAvailable as freewareOSXPWin 7Office 2003 or laterPinPoint (astrometry)See dc3.com website$149Sky catalogsGSCUsnoa2Nbr of users : 4 (March 2011)