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