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gaia data release 3 documentation

8.3 Processing steps

8.3.5 Photometry processing

Author(s): Laurent Galluccio, Marco Delbó, Alberto Cellino

The photometry of the SSOs was computed by using CCD-level epoch photometry computed by the PhotPipe system (see Chapter 5), complemented by information on the reliability of each CCD based on the results of the astrometric reduction of each SSO transit. In particular, only the fluxes from AF1-AF9, from which a position of the SSO had been derived for the individual CCDs, were used to compute the photometry. No SM fluxes were used for the computation of the photometry since, for Gaia DR3, no SM data were used to derive positions.

The adopted criteria to accept any individual recorded flux values for SSO photometric analysis as given by the PhotPipe system required, therefore, also the availability of an accurate astrometric position in the same CCD. In more detail, a photometric flux measurement was accepted also in the following cases:

  • astrometric position was derived, but the uncertainty on the attitude could not be computed because of a problem with the epoch;

  • astrometric position was derived, but the uncertainty on the attitude could not be computed because of an exception in a Gaia Tools method;

  • astrometric position was derived, but the uncertainty on the attitude could not be computed because of a problem with a covariance matrix;

  • astrometric position was derived, but the uncertainty on the attitude could not be computed because of several errors;

  • astrometric position was derived, but the geocentric position of Gaia could not be computed.

Some rejection filters were applied to individual CCDs based on information coming from PhotPipe and from the Initial Data Treatment (IDT). In particular, transit photometry was removed if the transit faced at least one of the following issues:

  • multiple gates;

  • window affected by charge injection;

  • window affected by an AOCS update;

  • first samples of window affected by gate release;

  • missing samples;

  • IPD not successful;

  • window affected by multiple peaks;

  • non-nominal window;

  • no data available;

  • predicted position computation failed or predicted position outside window;

  • no calibration;

  • calibration application failure;

  • excluded from the generation of the mean source photometry, due to pre-filtering based on available flags;

  • rejected in the source accumulation process;

  • observation affected by a satellite event.

Then, the average G-band flux was computed using the weighted average formulae in the remaining validated individual CCDs.

Let wf be the weight of the flux (AF) be characterized as

wfi=1flux_errori2, (8.13)

with the indices i=0,,8 corresponding to AF1–AF9. The G-band flux Φ is then

Φ=(iwfifluxi)(iwfi)-1. (8.14)

The information is given in the sso_observation table in the field g_flux.

The average G magnitude value was computed by using the average G-band flux and the G magnitude zero point value computed by PhotPipe:

G=-2.5log10Φ+G0, (8.15)

where G0 corresponds to the G zero point.

The information is given in the archive on the sso_observation table in the field g_mag.

A final filtering was applied based on the computed value of the magnitudes and their errors. All epoch magnitudes with values above 21 mag or errors above 0.2 mag were filtered out and so are not included in the archive. Values filtered out or not computed (due to the absence of data provided by PhotPipe) were set to NaN.

Astrometric reduction played another important role in the processing of photometric and spectroscopic data (see also Section 8.4.5). In particular, astrometric reduction was used to determine for each transit of an SSO the field angles, i.e., the angular coordinates in the focal plane, at three different epochs, namely at the reference epoch, the epoch of read-out of AF1 as given in the transitId, and at epochs 45 s, 50 s, and 55 s thereafter. These three epochs should contain the timings of the BP and RP observations so that, once the exact timings of BP (Blue Photometer) and RP (Red Photometer) observations were known, the precise field angles could be derived by interpolation.

In order to derive the field angles at the three epochs, the software first computed the SSO’s position on the sky plane at the three epochs by adding the positional differences due to the SSO’s sky-plane motion to its computed sky-plane position in Right Ascension and Declination. When the SSO was identified, its sky-plane motion was taken from the a priori ephemerides, since the motion from the ephemerides was more accurate than the motion derived from a single transit observed by Gaia.

However, when the object was not identified, the sky-plane motion was computed from the Gaia transit, i.e., by fitting a linear motion to the positions derived from the Gaia observations of that single transit, omitting positions that were removed by the filters. In the case of 1D windows, only the AL motion of the SSO could be derived with reasonable accuracy, whereas the AC motion had uncertainties larger than its actual value.

The three derived positions on the sky-plane were then converted back to field angles, using the OGA3 attitude and its geometric calibration. These field angles were then handed over to the PhotPipe system (see Chapter 5) which used them to calibrate the SSO data.