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Differential Photometry With Variable Reference Stars

Differential Photometry With Variable Reference Stars. Christopher Broeg , AIU Jena / MPE Garching, Germany Matilde Fernández , Instituto de Astrofisica de Andalusia, CSIC, Spain Ralph Neuh äuser , AIU Jena, Germany. Outline. Motivation Differential photometry revisited Using weighted CS

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Differential Photometry With Variable Reference Stars

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  1. Differential Photometry With Variable Reference Stars Christopher Broeg, AIU Jena / MPE Garching, Germany Matilde Fernández, Instituto de Astrofisica de Andalusia, CSIC, Spain Ralph Neuhäuser, AIU Jena, Germany 5th COROT week

  2. Outline • Motivation • Differential photometry revisited • Using weighted CS • Getting a Good Value for  • Examples & Comparison • Summary 5th COROT week

  3. Motivation • Often no standard stars in the FOV • Main sequence stars are not constant in brightness down to arbitrary accuracy levels • It is always better to check for constant brightness than to assume it 5th COROT week

  4. Differential Photometry Revisited • Extinction by a constant factor can be cancelled by subtraction of a constant comparison star (CS) due to the logarithmic scale • This works as well for an arbitrarily averaged CS 5th COROT week

  5. Using Weighted CS • What is the best choice for the weights? • Any choice of weights is fine choose weights so such the error of the artificial CS gets minimized choose weights • This gives the lowest error for CS while still compensating for extinction 5th COROT week

  6. Getting a Good Value for  • A robust estimator s of  can be determined directly from the measurements, without relying on the instrumental errors • How? • Compare each CS with remaining CS • Calculate  of the time series of CS brightnesses • This  is used for weights of new calculation • Repeat until convergence 5th COROT week

  7. Differential Photometry start over calculate initial weights: User input: calculate differ- ential magnitudes: new weights no calculate new magnitudes yes no yes Start Iterative Algorithm: Sequence of operations output results 5th COROT week

  8. Examples: Output 5th COROT week

  9. Comparison With Ordinary Method (1) - Lightcurve Weights ~ m-3 New method No weights 5th COROT week

  10. Comparison With Ordinary Method (2) – Phased Lc Weights ~ m-3 New method No weights Please notice the different scales!!! 5th COROT week

  11. Comp. cont. – only best 6 CS for comparison Weights ~ m-7 New method Weights ~ m-3 6 brightest CS only Again, please notice the different scales!!! 5th COROT week

  12. Summary • Algorithm automatically gets best S/N possible for the CS • Variable CS are detected and quantified • Well defined error bars are calculated • The method is clearly superior to unweighted mean even when using the same CS (bad CS already rejected) 5th COROT week

  13. Addendum • A paper presenting this method will be published in Astronomical Notes.If you are interested and want further information please contact:Christopher Broeg, Schillergässchen 2-3, 07745 Jena, Germanybroeg@astro.uni-jena.de • Thank you for your time! 5th COROT week

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