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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 Geos meeting at OHP - Easter 2011

2 Automatic Photometry Why? Classical programs LesvePhotometry structure
Results examples

3 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

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 ($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 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)

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 Log10 (Fobj/Fref) More than one star can be used 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 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

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


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