One- and two-dimensional images

The data stream created by observing some 1 billion sources on average 80 times over 5 years can become very large. Transmitting it over limited time spans from the Lagrangian point L2 a distance of 1.5 million km makes this an even more challenging task. In order to reduce the total amount of data transmitted, while still maintaining the scientific goals of the mission, Gaia will collapse images of faint stars from the measured two-dimensional image to a one-dimensional, 1D image. The collapse is along the across-scan direction, which will still create a 1D image from which a transit time can be derived. For the brighter images the full two-dimensional image is preserved. This provides essential information on the across-scan pointing of the telescopes, and largely avoids saturation of the image.

All images are obtained by accumulating responses on the CCD detectors as the images pass across those detectors. As an image passes, a “window” on the detector will follow the image, and the image will gradually accumulate within this window (see also Edinburgh contribution for more information and illustrations). For the brightest images an accumulation over the full width of the CCD would lead to saturation. To avoid this, so-called gates are activated on the CCD. When a gate is activated, the accumulation of the image will take place over a fraction of the CCD only. This fraction can vary from around 50 per cent to only few per cent. All images observed this way will be transmitted in 2D.