What I am working on at the moment …

Image in the red photometer not saturated (left) and saturated (right).

Image in the red photometer not saturated (left) and saturated (right).

CCD images can have saturated pixels as you may have experienced with your digital camera already when taking photos where a bright light or lamp is included. This means that the pixels actually have reached to maximum pixel level that can be obtained technically. This is the same for Gaia CCDs: if a bright star is observed then the central pixels of the image are saturating. Obviously, this is not a positive thing if you want to measure the position and brightness of the star with very high precision. Therefore Gaia CCDs have a special operation mode with reduced exposure times for bright stars. We call this observations with “gates activated”. There are different “gates” with different exposure times available ranging from 0.01 seconds to 4.3 seconds. These “gates” will be activated depending on the brightness of the stars determined on-board the satellite with the star mapper (SM) CCDs which first will observe all stars with the same “gate” activated. A few seconds after these observations with the SM CCDs the same detected objects (mainly stars) will be observed by 11 more CCDs in the row of one of the seven CCD rows available. If the star is bright then the “gates” will be activated to avoid the saturation of the images. If an image of a star is saturating depends on many factors as for instance the colour of the star, the scan motion of the satellite, and even how the image center is located in pixel space. Therefore you need a lot of simulations to determine how many images will saturating. You need to know at which pixel level the CCDs are saturating. These saturation levels are different from CCD to CCD, there are even variations of the saturation levels within one CCD chip. Therefore we will adopt the ranges of the magnitudes (brightness) for the activation of the “gates” for 16 different areas of each CCD. We are determing these parameters at the moment with a huge number of simulations. These parameters will be uplinked to Gaia after launch before the first observations are starting in December. When the first real Gaia observations are downlinked  we will have to check if our simulations were close to reality and only a very limited fraction of images are saturated.


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