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LesvePhotometry an automatic photometry solution

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  1. LesvePhotometry an automatic photometry solution Pierre de Ponthière AAVSO – GEOS –CBA member www.dppobservatory.net Geos meeting at OHP - Easter 2011

  2. Automatic Photometry • Why? • Classical programs • LesvePhotometry structure • Results examples

  3. Automatic PhotometryWhy? • better reliability • less prone to mistakes • comparison stars in a database • no need to open star chart • faster • easy • avoid tedious tasks

  4. Photometry with classical programs • Manually • Identify the stars • Position the aperture • Provide magnitude of reference star(s) • You need • Sky chart • Comparison magnitudes

  5. LesvePhotometry structure • Inputs • Sky images • Image FITS header (all needed information) • Star database (Variables and Comparison stars) • Star catalog (GSC, usnoa2) • Star location in Sky image : by astrometry • Differential aperture photometry • Calculation of magnitudes, snr, airmass, … • Automatic generation reports (AAVSO, CBA, xls) • Interactive tool to determine extrema


  6. FITS Header

  7. Database : Variables stars • SQLite.Net Database engine • Embedded in the program

  8. Database : Comparison stars

  9. Astrometry • Done with PinPoint of dc3.com ($149) • Solve the image • Detection of stars • Matching with star catalog (GSC, usnoa2) • List of all detected stars RA, DEC and pixels coordinates of Centroids • Pixel coordinates are used to position the aperture circles.

  10. Aperture measurement • Aperture : pixel value = star + background + pedestal • Annulus : pixel value = background + pedestal • Flux = Sum ( Aperture – Annulus )

  11. Aperture measurement • Sum the ADU of pixels inside the circle • Calculate the distance : pixel center to circle center • Bresenham’s algorithm - Midpoint circleto 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 1/2 • Error 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 flux • FRef = Comparison star flux • Mref from star database • mobj = Mref - 2.5 Log10 (Fobj/Fref) • More than one star canbeused for the reference • called ensemble photometry • fluxes of reference stars are added

  14. Error estimation • Poisson noise • Background noise • Errors on Variable and reference stars -> added in quadrature

  15. Reports • AAVSO • upload to AAVSO database • not user friendly • CBA • Comparison stars • JD, magnitudes, airmass, a.s.o. • Excel sheet • Graph of the light curve • Graphical interface to estimate the extrema • Reinsch spline algorithm to obtain the

  16. Software installation • LesvePhotometry • Available as freeware • www.dppobservatory • OS • XP • Win 7 • Office 2003 or later • PinPoint (astrometry) • See dc3.com website • $149 • Sky catalogs • GSC • Usnoa2 • Nbr of users : 4 (March 2011)

  17. THE END