8), with the advantage that transient sources such as supernovae and near-Earth asteroids are observed too.

Every object crossing the focal plane is first detected either by SM strip 1 (SM1) or SM strip 2 (SM2).

These CCDs exclusively record, respectively, the objects from the preceding or from the following telescope.

This is achieved through a physical mask that is placed in each telescope intermediate image, at the M4/M4’ beam-combiner level (Sect. 3.3.1).

The SM CCDs are read out in full-frame TDI mode, which means without windowing.

Read samples, however, have a reduced spatial resolution with an on-chip binning of 2 pixels along-scan × 2 pixels across-scan per sample.

Windows are assigned to detected objects and transmitted to ground; they measure 40 × 6 samples of 2 × 2 pixels each for stars brighter than G = 13 mag and 20 × 3 samples of 4 × 4 pixels each for fainter objects.

The SM CCD has the longest TDI gate, with 2900 TDI lines (2.85 s) effective integration time, permanently active to reduce image degradation caused by optical distortions (which are significant at the edge of the field of view), and to reduce the CCD effective area susceptible to false detections generated by cosmic rays and solar protons.

The astrometric data acquired in the 62 CCDs in the AF field are binned on-chip in the across-scan direction over 12 pixels, except in the first AF strip (AF1) and for stars brighter than 13 mag.

For these stars, unbinned, single-pixel-resolution windows are often used in combination with temporary TDI-gate activation, during the period of time that corresponds to the brightstar window length, to shorten the CCD integration time and avoid pixel-level saturation.

In AF1, across-scan information is maintained at the CCD readout, but later binned by the onboard software before transmission to ground; this permits the measuring of the actual velocities of objects through the focal plane to feed the attitude and control subsystem, to allow along- and across-scan window propagation through the focal plane, and to identify suspected moving objects, which receive a special, additional window either right on top or right below the nominal window in the photometric instrument (Sect. 3.3.6).

The AF1 data are also used on board for confirming the presence of detected objects.

The along-scan window size in AF is 18 pixels for stars that are brighter than 16 mag and 12 pixels for fainter objects.

The astrometric instrument can handle object densities up to 1 050 000 objects deg−2 (Sect. 8.4).

In denser areas, only the brightest stars are observed.
3.3.6.

Photometric instrument The photometric instrument measures the spectral energy distribution (SED) of all detected objects at the same angular resolution and at the same epoch as the astrometric observations.

This serves two goals:
1.

The instrument provides astrophysical information for all objects (Bailer-Jones et al. 2013), in particular astrophysical classification (for instance object type such as star, quasar, etc.) and astrophysical characterisation (for instance interstellar reddenings, surface gravities, metallicities, and effective temperatures for stars, photometric redshifts for quasars, etc.). 2.

The instrument enables chromatic corrections of the astrometric centroid data induced by optical aberrations of the telescope (Sect. 3.3.1).

Like the spectroscopic instrument (Sect. 3.3.7), the photometric instrument is highly integrated with the astrometric instrument, using the same telescopes, the same focal plane (albeit using a dedicated section of it), and the same sky-mapper (and AF1) function for object detection (and confirmation).

The photometry function is achieved through two fused-silica prisms dispersing light entering the fields of view.

One disperser, called Article number, page 11 of 36

BP for blue photometer, operates in the wavelength range 330— 680 nm; the other disperser, called RP for red photometer, covers the wavelength range 640—1050 nm.

Sometimes, BP and RP are collectively referred to as XP.

Optical coatings deposited on the prisms, together with the telescope transmission and detector quantum efficiency, define the bandpasses.

The prisms are located in the common path of the two telescopes, and mounted on the CCD cold radiator, directly in front of the focal plane.

Both photometers are equipped with a dedicated strip of seven CCDs each, which cover the full astrometric field of view in the across-scan direction (see Sect. 3.3.2 for details on the photometric CCDs).