Location accuracy limitations for CCD cameras

The accurate measurement of the position of celestial objects is a fundamen tal step for several astrophysical investigations. For ground based instrum ents, the atmosphere is considered the basic limiting factor; in space, the knowledge of the instrumental parameters and/or of their stability define the performance limits, but CCD cameras operated in time delay integration may take advantage of their operating mode to reduce significantly the cali bration problem. We implemented a low-cost laboratory experiment aimed at a ssessing the precision achievable in the location determination with a CCD camera, by evaluating the measurement repeatability throughout a set of ima ges of a simulated stellar field. Our experiment provides an initial locati on dispersion of the order of 1/100 of the CCD pixel, with clear evidence o f dominant common mode effects. After removing such terms with straightforw ard numerical procedures, we achieve a final location precision of 1/700 pi xel on individual images, or 1/1300 pixel on co-added images. The scaling o f precision with target magnitude is in quite good agreement with theoretic al expectations. The initial common mode systematics appear to be induced b y the thermal control of the CCD camera head, which degrades the structural stability. In actual implementations, such problems can be greatly reduced by proper design. Finally, our results show that residual effects, which c ould hamper the final astrometric accuracy, can be calibrated out with simp le procedures.