The experience of BEST
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The experience of BEST. Heike Rauer and the BEST Team Institut für Planetenforschung Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) and Zentrum für Astronomie und Astrophysik Technische Universität Berlin +. The experience of BEST. Berlin Exoplanet Search Telescope System.

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The experience of best

The experience of BEST

Heike Rauer and the BEST Team

Institut für Planetenforschung

Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR)

and

Zentrum für Astronomie und Astrophysik

Technische Universität Berlin

+


The experience of best

The experience of BEST

Berlin Exoplanet Search Telescope System

Goals of BEST: - support for CoRoT

- detect large planets

- variable stars, additional science


Berlin exoplanet search telescope

Berlin Exoplanet Search Telescope

Specifications:

TelescopeSchmidt-Cassegrain

Aperture20 cm

Focal ratiof/2.7

InstrumentAP-10 CCD

Size2048 x 2048 pixels

Pixel size14 µm

Pixel scale5.5 arcsec/pixel

Field of view 3.1° x 3.1°

2001 - 2004 Thüringer Landessternwarte

Tautenburg (TLS), Germany

Since end 2004 Observatoire de Haute Provence (OHP), France


The experience of best

BEST I

TEST at TLS

BEST II


The experience of best

Observatorio Cerro Armazones, Chile

Instituto de Astronomía - Universidad Católica del Norte (UCN) in Antofagasta, Chile

Astronomisches Institut- Ruhr-Universität Bochum (RUB), Germany.


The experience of best

BEST II

Specifications:

TelescopeBRC - 250

Aperture25 cm

Focal ratiof/5.0

InstrumentFLI IMG-1680 CCD

Size4096 x 4096 pixels

Pixel size9 µm

Pixel scale1.5 arcsec/pixel

Field of view 1.7° x 1.7°

Precision < 1% V=15-16

smaller FoV for BEST II is compensated by less stars influenced by crowding


The experience of best

Modes of operation

  • BEST I at TLS:

  • - obervations by observer at TLS

  • - data reduction at DLR

  • BEST I at OHP:

  • - observations via remote control from Berlin

  • - data reduction at DLR

  • BEST II at OCA

  • - „robotic“ observations (regular remote monitoring, manual interaction in case of alarm)

  • - basic calibration at OCA, full data reduction at DLR


The experience of best

Performance

  • The two critical factors for a transit search system are:

  • High duty cycle: full coverage of planetary orbits by observations.

  • 2. Large number of high quality lightcurves (e.g. rms < 1%).


The experience of best

Duty cycle

Need: High duty cycle, full coverage of planetary orbits by observations of sufficient quality.

BEST experience: duty cycle is the major limiting factor from central Europe (not a surprise  ). Next:

 place BEST II at OCA, Chile

 start building a network (NEST)

 participate to ASTEP


The experience of best

Performance

  • The two critical factors for a transit search system are:

  • High duty cycle, full coverage of planetary orbits by observations

  • 2. Large number of high quality lightcurves (e.g. rms < 1%)

  • - correction for detector effects (dark, bias, flats, hot/cold/defect pixels,…)

  • - correction of atmosphere (extinction, seeing, scientillation, …)

  • - accurate photometry (aperture/image subtraction/PSF fitting, crowding)


The experience of best

Detector effects

For example: a star moves across a hot pixel during the night due to imperfect guiding of the telescope….

  • Causes transit-like signal which has to be evaluated by comparison with the original data.

  • adds work-load on transit candidate evaluation


The experience of best

Detector effects

  • Correction for detector effects (dark, bias, flats, hot/cold/defect pixels,…)

  • Low-quality CCD: need to check transit events for detector effects, check position of the star on CCD

  • varying bias, dark, etc., adds to systematic noise residuals

  • Recommendation:

  •  buy good h/w

  •  adapt the observing sequence to calibration needs


The experience of best

Atmosphere effects

Correction of atmosphere (extinction, seeing, scintillation, …)

- Airmass correction is critical (no filter, large FOV)

 restriction in airmass, depending on site and target field

 adapt reduction method, e.g. work on sub-fields

 implement filter if possible

- Effect of seeing variations on crowding


The experience of best

Photometry

Accurate photometry (aperture/image subtraction/PSF fitting, crowding)

- Crowding can be a major problem

 improve photometric method (image subtraction ok)

 match the pixel scale of h/w


The experience of best

B.E.S.T. Candidate 3

BEST

POSS-I

BEST Magnitude of host star12.1

Depth [%]2.5

Duration [h]3.0

Orbital period [d] 423.10/n

Number of detections2


The experience of best

BEST candidate 5

depth [%]1.0

duration [h]4.5

orbital period [d] > 10 ?

semi mayor axis[AU]?

number of detections1

target field No.8

host starM dwarf

magnitude(BEST)12.56

V magnitude14.73

reference star

BEST 18.3‘ x 18.3‘


Crowded target fields lead to further reduction of the photometric accuracy

Crowded target fields lead to further reduction of the photometric accuracy

  • diluted signals

  • - neighboring stars are resolved

  • - but a neighbor contributes flux within the PSF or photometric aperture

  • b) unresolved stars

  • - neighboring stars are not resolved

  • c) a combination of a) and b)

  • - due to varying seeing the resolution of stars changes over the night

  •  a transit signal is weakend

  •  noise is added if the neighbor is variable

The photometric data reduction algorithm needs to be adopted.


Comparison of photometric methods

Comparison of Photometric methods

* Source-Extractor, SExtractor (Bertin & Arnouts 1996)

Performs different kinds of aperture photometry

* Multi Object Multi Frame photometry, MOMF (Kjeldsen & Frandsen 1992) Combination of aperture and PSF photometry

* Image Subtraction, ISIS (Alard 2000)

Subtraction of a convolved reference frame from all frames.

 implementation of parallel approach in data pipeline (SExtractor, ISIS)

Karoff et al. 2005


The experience of best

SExtractor used in less crowded fields

SExtractor

TLS


The experience of best

Comparison of both methods for the COROT field

Data from 4 nights in spring 2005 obtained at OHP (COROT winter field)

SExtractor

ISIS

Karoff et al. 2005

 a reduction routine able to deal with very crowded target fields is important


The experience of best

Performance of BEST I at OHP after evaluation of its first regular observing season at OHP in summer 2005.


The experience of best

BEST at OHP: Variable stars in the CoRoT center field

37 nights/142 hours observations from OHP in summer 2005.

  • Search for variable stars and eclipsing binaries:

  • 83 periodic variable stars identified: 76 new discoveries

  • 11 variable stars with period < 120 days known in GCVS, 8 confirmed variables, for 3 stars no variability found

  • 37 of the variables are eclipsing binaries

Karoff et al. 2006


The experience of best

Location of the variables in the center field

  • Periodic variable stars are detected over the whole magnitude range.

  • There are many more stars with high rms: real but non-periodic variables and distorted lightcurves

See Karoff et al. 2006


How complete is our search for variables from ohp

How complete is our search for variables from OHP?


The experience of best

Number of detected eclipsing binaries as a function of period

Eclipsing binaries in Hipparcos data

Eclipsing binaries in BEST observations

Söderhjelm & Dischler 2005

Karoff et al. 2006

The BEST survey is complete only up to 10 – 20% for

periods > 1 day.

But: we have indications that the detection algorithm for periodic variables is not perfect…


The experience of best

BEST – basic lessons from TLS and OHP

  • Strong points:

    • Simple robust system with good capability for long-term photometric surveys of a substantial number of stars

    • Well suited to catalog bright stars in the COROT fields

    • Demonstrated ability to reach the accuracy limit needed to discover Jupiter-sized planets

    • Cost efficient way to characterize stellar variability over long time periods

  • Weak point:

    • Transit detections limited by crowding and duty cycle (i.e. to few nights and/or to few stars)


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