LesvePhotometry an automatic photometry solution Pierre de Ponthière AAVSO – GEOS –CBA member www.dppobservatory.net Geos meeting at OHP - Easter 2011
Automatic Photometry Why? Classical programs LesvePhotometry 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
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
FITS Header
Database : Variables stars SQLite.Net Database engine Embedded in the program
Database : Comparison stars
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.
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 Log10 (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 CBA Excel sheet 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 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)
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