LesvePhotometry an automatic photometry solution
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Transcript of LesvePhotometry an automatic photometry solution
LesvePhotometry an automatic photometry solution
Pierre de PonthièreAAVSO – 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 starsRA, 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 – 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)
THE END