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 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.

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 dc3.com website
$149
Sky catalogs
GSC
Usnoa2
Nbr of users : 4 (March 2011)

17. THE END