Gaia's CCD detectors are covered with a thin film. To optimise the transmission of light from the vacuum of space into the silicon of the light detectors, the silicon is covered with a very thin anti-reflective (AR) coating. The thickness chosen for that layer depends on the wavelength range which is to be observed with a particular CCD detector: for the CCDs of Gaia's "Blue Spectrometer" (BP), designed to obtain spectra of stars in the short-wavelength, blue to ultraviolet part of the spectrum, this layer is the thinnest. For the CCDs used in the "Astrometric Field" (AF), designed to determine the position of stars from as much light as possible, all the way from ultraviolet to infrared, the anti-reflective coating is thicker. For the CCDs of Gaia's "Red Photometer" (RP) and "Radial Velocity Spectrometer", both used for producing spectra at longer wavelengths in the red and near-infrared, the film is the thickest.

As part of the Gaia commissioning activities, the estimates of the quantum efficiencies of Gaia's CCDs were revised. The model of the CCDs used for that purpose does not only allow to determine the amount of light that is absorbed by the CCDs as a function of wavelength, but also to compute the fraction of light reflected from its surface. Using the latter information, it is possible to compute the colour of the CCDs as a human being would see them, as a function of the thickness of the AR coating. All that is required is an assumption on the illumination (the standard daylight spectrum "D65" in this case), the wavelength-dependent light sensitivity of the human eye (assuming the "CIE standard observer"), and the conversion from the stimulus of the human eye to a standard colour space (here the sRGB).

The small frame in the image shows the colour in which a human being would see a Gaia CCD as a function of thickness of the AR coating. Increasing the thickness of that coating, the grey silicon turns first to a brownish colour, and then takes a purple shade before reaching a deep blue. The blue colour is getting paler and paler as the thickness of the AR layer further increases. The nominal thicknesses of the coatings for AF, BP, and RP/RVS CCDs are indicated in the figure by dashed lines. A comparison with a picture of Gaia's focal plane taken after mounting of the CCDs shows how the colours, predicted as a by-product while reproducing the CCD quantum efficiencies, match the visual impression.

Credits: ESA/Gaia/Airbus DS/DPAC/IEEC-ICCUB/M.Weiler

[Published: 02/10/2015]