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PLATO PLAnetary Transits & Oscillations of stars. Data onboard treatment PPLC study February 2009 on behalf of Reza Samadi for the PLATO data treatment team. Onboard processing modes. Main functions of onboard/onground processing. Observation mode: smearing correction
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PLATO PLAnetary Transits & Oscillations of stars Data onboard treatment PPLC study February 2009 on behalf of Reza Samadi for the PLATO data treatment team
Main functions of onboard/onground processing • Observation mode: • smearing correction • weighted mask photometry / aperture photometry • kinematic aberration correction • jitter correction • Configuration mode: • measuring/modelling PSF • measuring/modelling sky background
Smearing correction • Normal telescopes: • sampling: 25s • integration time: 23s • readout time: 2s • Fast telescopes: • sampling: 2.5s • integration time: 2.25s • transfer time: 0.25s overscan rows CCD registers • measure smearing thanks to overscan rows • subtract from each image before after
PSF target star mV= 11 nearby faint star weighted mask Weighted mask photometry minimizing the impact of confusion cf CDF study • the target star can be polluted by a neighbouring star • to avoid confusion : use of a weighted mask • weights = integral of PSF over pixel • need to know the PSF • normal aperture photometry to be used for brighter stars ~ 90 % of the flux
flux time Differential kinematic aberration PLATO: • large field of view : 42° • pixel size : 12.5” (14.3”) • the effect is much more important than for CoRoT • star displacements over 1 month : ~ 7 pixels (worst case) • will induce an unacceptable decrease of the flux • thermoelastic variations of the telescope pointing direction can also induce star displacement mV=11 5 months - update the mask position frequently - avoid flux loss - introduce periodic perturbation - need to limit impact of this perturbation - update every hr (tbc) - hourly update is entirely predictible - less frequent update for telescope variations
Is jitter correction at all necessary? • CoRoT : 0.25'' rms + orbital components • PLATO : specified : 0.2'' rms with ref to photon noise • PRNU does not seam to be a problem • Depending of the jitter noise level and nature : the perturbation can be important or negligible • For bright star the contribution can be important if the jitter is ~ 0.5'' rms or more • Aperture photometry results in negligible perturbations
Jitter correction • from the knowledge of the PSF, we can predict the perturbations induced by any displacement: PSF mask mask Surface for the jitter correction Fialho et al (2007, PASP) • This method also corrects for differential aberration • The presence of polluting sources can be accounted for in the correction surface • Accurate knowledge of the star displacements: x, y is needed • Accurate PSF is needed
PSF determination (configuration mode) Assumptions, for each telescopes : • The PSF varies slowly across the field of view • We have available N (=1000) reference stars with associated image time series • We have a functional form of the PSF as a function of K parameters ai (eg. width , skewness, etc): PSF(x,y) = fa1,a2,…,aK(x-x0,y-y0) For each star, for each telescope: • We constrain the parameters using the image time-series. The fitted parameters ai (j) are then considered as a function of the position [x0(j) and y0(j)] of the star j. A 2D polynomial interpolation is then performed to derive the values of the parameters at any position across the field of the telescope. However, PSF can depend on the star colour => 3D polynomial interpolation (x,y,colour) ? Procedure to apply at TBD frequency (once a week?) PSF used to calculate mask weights and jitter correction surface
Sky background determination (configuration mode) • set 400 background windows per telescope (100 per CCD) • collect a long enough time series of background measurements • background is modeled using a 2D polynomial fit • The sky background level can then be estimated at any position, then for all stars in the FOV.
Onboard processing dimensioning: star samples • Sample P1 : mv < 9.6 - 11.15 ; noise level < 27 ppm/h • 10 000 stars : photometry @ 50s , centroids @ 600 s • Subset : N = 1000 references stars, mv= 8.6-9.6, individual light curve • Sub-images (imagettes) : n = 400 stars @ 25 s sampling • Sample P2 : mv < 12 ; noise level < 80 ppm/h • 20 000 stars @ 600s • Oversampled : 400 stars @ 50s sampling • Sample P3 (P4) : 4.75 < mv < 7.3 noise level < 27 ppm/h • 500 (1 000) stars @ 50s • Subset: 100 stars centroids @ 2.5 s • Sub-images (imagettes) : m = 100 @ 50 s • Sample P5: mv < 13.5 ; noise level 80 ppm/h ; no centroids measured • 80 000 stars @ 600s • Oversampled : 1000 stars @ 50s • Background windows : 400
Onboard processing architecture 1 DPU per telescope + 1 ICU (+1 redondant) - case 1: perform onboard average - case 2: downlink all individual LC trade-off needed very soon !
Total TM rates case 1: perform onboard average case 2: downlink all individual LC
Case 1 .vs. Case 2 trade-off • Case 1 : only 1000 LCs from Sample P1 are downloaded : • 31 Gb/day (with compression) • Case 2 : all LCs are downloaded : • 71 Gb/day (with compression) • Case 1 : jitter correction to be done onboard ! Outlier discarding and LC average to be done on board. Strong constraints on the onboard processing, no replay possible. • Case 2 : jitter correction can be done onground ! Outlier discarding and LC average done on ground.
Onboard processing H/W dimensioning CPU for one DPU LEON processor at 100 MHz CPU occupation rate = 40%
Open issues • Trade-off between Case 1 and Case 2. Case 2 is preferred, but can we afford to downlink 71 Gb/day of science data ? • Pointing performances ? Level and nature of the jitter ? Is jitter correction needed? • Exact threshold in magnitude between weighted photometry and aperture photometry ? • Model for the PSF ? • Resolution required for the jitter correction ? • Resolution required for the calculation of the weighted mask ? • Photometry of the saturated stars ? Down to which magnitude ? • Calculation of the barycenter : thresholding ? simple mask ?