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gaia data release 3 documentation

5.1 Introduction

5.1.3 Properties of the input data

Author(s): Francesca De Angeli

The PhotPipe system running at DPCI is currently registered as consumer of about 250 different data types. Even though not all types are currently in use, this figure gives a fair idea of the complexity of the data handling and of combining all the various inputs to produce the PhotPipe final products. Here follows a list of the main inputs, without these no processing could be performed:

  • Raw observations (Astro- and PhotoObservations, AO and PO respectively), containing the decoded telemetry. These are produced on a daily basis in IDT (see Section 3.4.2) for all FoV transits.

  • Object logs (see Section 1.1.3), containing a minimal record of the detection features, such as time, coordinate, FoV, acquisition mode, etc. and the brightness of the source observed in a transit. These are needed for the application of the bias calibration which is done in PhotPipe for the BP/RP data.

  • CCD PEM NU library (see Section 3.4.7), produced off-line in Edinburgh, required for the application of the bias calibration. Full mitigation is applied in PhotPipe.

  • IDT (Section 3.4.2) and IDU (Section 3.4.2) data products, in particular AstroElementaries (AE), containing the results of the Image Parameter Determination, IPD. For Cycle 03 processing, only IDU results were used. IPD performs a fit to the samples for each CCD transit to produce an estimate of the centroid position (AL only for 1D windows) and of the integrated flux using a library of calibrated LSF/PSFs (Section 3.3.5). In the latest processing run, this library was vastly improved to include detailed dependencies on AC motion, colour and time.

  • Cross-match information, linking each transit either to a source existing in the current source catalogue (i.e. the catalogue of the sources detected by Gaia) or to a new source. There are two sources of cross-match data: IDT, which runs a cross-match on a daily basis, and IDU, which runs at the end of each Data Segment covering all data since the start of nominal operations. IDU has the advantage of being able to use multi-epoch information to improve the results and to filter more efficiently spurious observations. So far, PhotPipe has always relied on the most recent IDU cross-match results.

  • Satellite attitude, required to reconstruct the satellite position and pointing direction at any time. There are three systems producing attitude: IDT does a daily attitude reconstruction (OGA1, Section 3.4.5), ODAS (Section 3.4.5) improves this on a daily basis while AGIS runs in the cyclic processing to provide the best attitude reconstruction (OGA3, Section 4.3.5). PhotPipe requires the accuracy reached by OGA3 for the prediction of centring errors (for the flux loss calibration) and predicted positions (for the geometric calibration of the BP/RP instruments and for the special treatment of observations in crowded areas).

  • Source astrophysical coordinates, required to compute predicted positions of the sources onto the focal plane (for measuring centring errors and calibrating the geometry of the BP/RP instruments). PhotPipe relies on AGIS astrometry for these.

  • AF geometric calibration, required for the computation of predicted positions of sources on the BP/RP CCDs (see the description of the previous item in this list).

The careful reader will notice that the above list does not include an external source catalogue. This is because PhotPipe does not rely on any bootstrap source information. No external catalogue is ever used for the processing of the data, except of course for the external calibration step that ties the internal photometric system to the absolute system (see Section 5.4.1). External catalogues are currently also used for validation purposes.