Lesve Photometry an automatic photometry solution Pierre de

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

Automatic Photometry • Why? • Classical programs • Lesve. Photometry structure • Results examples

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

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

Lesve. Photometry structure • Inputs – – • • • Sky images Image FITS header (all needed information) Star database (Variables and Comparison stars) Star catalog (GSC, usnoa 2) 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

FITS Header

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

Database : Comparison stars

Astrometry • Done with Pin. Point of dc 3. com ($149) • Solve the image – Detection of stars – Matching with star catalog (GSC, usnoa 2) • List of all detected stars RA, DEC and pixels coordinates of Centroids • Pixel coordinates are used to position the aperture circles.

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

Aperture measurement • Sum the ADU of pixels inside the circle • Calculate the distance : pixel center to circle center • Bresenham’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 1/2 • Error on circle surface less than 0. 2% (radius 5 pixels)

Close stars? • Astrometry provides a list of stars • If a star is in the annulus, program doesn’t use this part of annulus

Differential photometry • FObj = Variable flux • FRef = Comparison star flux • Mref from star database • mobj = Mref - 2. 5 Log 10 (Fobj/Fref) • More than one star can be used for the reference – called ensemble photometry – fluxes of reference stars are added

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

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

Software installation • Lesve. Photometry – Available as freeware – www. dppobservatory • OS – XP – Win 7 • Office 2003 or later • Pin. Point (astrometry) – See dc 3. com website – $149 • Sky catalogs – GSC – Usnoa 2 • Nbr of users : 4 (March 2011)

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