Gaia's photometric instrument is based on a dispersive-prism approach such that starlight is not focused in a PSF-like spot but dispersed along the scan direction in a low-resolution spectrum. Gaia features two low-resolution fused-silica prisms dispersing all the light entering the field of view. One disperser - called BP for Blue Photometer - operates in the wavelength range 330-660 nm; the other - called RP for Red Photometer - covers the wavelength range 650-1000 nm. Both prisms have broad-band filters for blocking unwanted light. The photometric instrument is integrated with the astrometric and spectroscopic instruments and telescopes; the photometric CCDs are located in the Gaia focal plane. Two CCD strips of 7 CCDs each are dedicated to photometry, one for BP and one for RP. Both strips cover the full astrometric field of view in the across-scan direction.
The sampling of the spectra is a function of wavelength as a result of the natural dispersion curve of fused silica; the dispersion is higher at short wavelengths. The BP and RP dispersers have been designed in such a way that BP and RP spectra have similar sizes (on the order of 36 samples along scan). BP and RP spectra will be binned on-chip in the across-scan direction; no along-scan binning is foreseen. For bright stars (G ~ 13 mag), single-pixel-resolution windows are used, in combination with TDI gates. Window extensions of 12 samples on either side of the 36-sample source area, meant to measure the sky background, are available.
The image shows a dispersed RP image for an M6V star with magnitude V = 17.3 mag, corresponding to G = 15 mag. The colour scale shows the photon counts in each pixel. The white contour indicates where the sky-background level starts. The red rectangle is the window around the dispersed image that will be read out and transmitted to ground. The window consists of 12 + 36 + 12 = 60 samples of 1 X 12 pixels (AL X AC) each. Normally, the flux inside the rectangle is summed in the vertical (across-scan) direction to obtain the 1D spectrum.
Further details are available from "Interface document for ad-hoc simulations of prism spectra for the EADS-Astrium Gaia design", by A.G.A. Brown (GAIA-C8-SP-LEI-AB-006-1, available from Livelink).
Image courtesy Anthony Brown