differential photometry with variable reference stars
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
5th COROT week 1
Differential Photometry With Differential Photometry With Variable Reference StarsVariable 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 2C. Broeg
OutlineOutline
MotivationDifferential photometry revisitedUsing weighted CSGetting a Good Value for Examples & ComparisonSummary
5th COROT week 3C. Broeg
MotivationMotivation
Often no standard stars in the FOVMain sequence stars are not constant in
brightness down to arbitrary accuracy levelsIt is always better to check for constant
brightness than to assume it
5th COROT week 4C. Broeg
Differential Photometry RevisitedDifferential 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 5C. Broeg
Using Weighted CSUsing 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
21
i
iCS
w
5th COROT week 6C. Broeg
Getting a Good Value for 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
C. Broeg 75th COROT week
Iterative Algorithm: Sequence of operationsIterative Algorithm: Sequence of operationsStart
calculate newmagnitudes
output results
, ,read data: ,i io CS o CSm
calculate differ-ential magnitudes:
i
j
i i
w
CS i
w
CS CS CS j i
m m m
m m m
new weights 21
i
iCS
w
( )i i
j
i i
CS CS
w
CS CS CS j i
stddev m
m m m
new oldm m
calculate initialweights:
2i
2 2 2
1w
( )i i nI
err
err Factor
, ,CS i CS ierr
User input:
nI
CS,i ,
change and
so that
err
If for some CS this is not
possible, remove them.
CS i
Factor
Differential Photometry
5th COROT week 8C. Broeg
ExampleExampless: Output: Output
5th COROT week 9C. Broeg
Comparison With Ordinary Method (1) - LightcurveComparison With Ordinary Method (1) - Lightcurve
New method
Weights ~ m-3
No weights
5th COROT week 10C. Broeg
Comparison With Ordinary Method (2) – Phased LcComparison With Ordinary Method (2) – Phased Lc
New method
Weights ~ m-3
No weights
Please notice the different scales!!!
5th COROT week 11C. Broeg
Comp. cont. – only best 6 CS for comparisonComp. cont. – only best 6 CS for comparison
New method
Weights ~ m-7
Weights ~ m-3
6 brightest CS only
Again, please notice the different scales!!!
5th COROT week 12C. Broeg
SummarySummary
Algorithm automatically gets best S/N possible for the CS
Variable CS are detected and quantifiedWell defined error bars are calculatedThe method is clearly superior to
unweighted mean even when using the same CS (bad CS already rejected)
5th COROT week 13C. Broeg
AddendumAddendum
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, [email protected]
Thank you for your time!