Measurement of the subpixel structure of AXAF CCD's

We present a method to measure the subpixel structure of a charge-coupled device (CCD), information necessary to accurately determine (<1% uncertainty) the absolute detection efficiency of the device. Our approach uses a thin metal film with periodically spaced holes (small, compared to the pixel size) to localize incident X rays to a particular region of the pixel. The mesh is rotated to create a small angular misalignment between the grid holes and the CCD pixels, producing a moire effect in the data. The resultant moire pattern is compared to a CCD model, and a best fit minimization technique is used to constrain the parameters that describe the subpixel structure. This technique was developed to measure and calibrate the X-ray CCD's that will comprise one of the two focal plane instruments on-board AXAF: but it is applicable for measuring the structure of any pixelated solid state device.