Correction for Small Satellite Motion on the COROT Asteroseismology Channel

Paper number

IAC-05-B5.2.10

Author

Ms. Rachel Drummond, K.U. Leuven, Belgium

Coauthor

Dr. Bart Vandenbussche, K.U. Leuven, Belgium

Coauthor

Mr. Michel Auvergne, Observatoire de Paris, France

Year

2005

Abstract

COROT
COnvection, ROtation and planetary Transit
is a French CNES initiative, with contributions from Belgium, Brazil, Germany, Spain, Austria and ESA. The satellite will be launched in June 2006 into a 900km polar orbit. COROT will observe stellar fields for up to 6 months in two channels: the exoplanet and asteroseismology CCDs. Typical exposure times are 32s on the exoplanet channel and 1s for asteroseismology. In order to detect low amplitude
6ppm
stellar oscillations, and planetary transits, the satellite must maintain a very high accuracy pointing. The Proteus bus offers a stability of 10 arcseconds rms rotation around the line of sight and 0.14 arcseconds rms rotation about the other two spacecraft axes. This angular motion can be equated to a motion of 0.1 pixels on the CCDs used for the photometry.

Due to limited downlink bandwidth, COROT cannot download every pixel of data from 4 CCDs. The data downlinked from the 10 stars (maximum) observed on the asteroseismology channel will therefore only be the total flux within a mask, and the barycentre of the star. The mask is a fixed area on the CCD, determined per star on the ground. Its size depends on the magnitude of the star and the location of the stellar image on the CCD, since the PSF changes as a function of position in the focal plane. If the satellite moves, the star moves on the CCD, and flux is lost outside the mask. The topic of this paper is the methodology developed to use the barycentre data to correct for the flux lost. Several different methods for a jitter correction algorithm are proposed and developed.

The paper will outline these methods and describe in detail the method that has been chosen for implementation on the COROT data. We have confronted the different strategies with simulated data and obtained the best results with a method that explicitly reconstructs the mean spacecraft attitude per exposure based on a least squares fit of the barycentre displacements. The reconstructed attitude allows us to determine the fraction of the PSF that falls outside the mask per integration interval and per star. This, in turn, allows us to correct the measured fluxes.

This paper will present results that examine the improvement in the frequency spectrum, demonstrating that stellar oscillations (if present) are preserved, while jitter frequencies are removed. This Fourier analysis allows the user to see how much the signal to noise ratio has improved at the frequencies of interest.

The algorithms developed are specific to COROT, but the jitter problem has implications for all photometric space missions, such MOST (2003) or Kepler (2007) and we will show that the method can be applied to other such satellites.

The work presented is part of the PhD work of R. Drummond, under supervision by Prof. C. Aerts and Dr B. Vandenbussche in Leuven and M. Auvergne in Paris. R. Drummond is supported by the Research Council of the University of Leuven under grant GOA/2003/4.

The above authors hereby certify that they will be available to present this paper during IAC and that the findings of this paper have never previously been presented.

Abstract document

IAC-05-B5.2.10.pdf

Manuscript document

IAC-05-B5.2.10.pdf (🔒 authorized access only).

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