CCD linearity testing



   Lymm Observatory

     CCD linearity testing

    There has been a great deal of discussion at BAA meetings as to whether CCD cameras employing anti-blooming gates are suitable for photometry. This feature is found in many lower-cost CCDs; it is provided to prevent saturated pixels overflowing onto the rest of the image. The potential disadvatange is that it may render the camera non-linear.

    The camera I use (Starlight Xpress MX916) employs antiblooming gates, as do many of the other cameras in the Starlight Xpress range.

    I have developed a simple device for measuring the linearity of CCDs. This uses a pulsed LED to deliver precisely controlled light 'doses' to the CCD chip. The original design used a fairly complex timing circuit containing several counters and flip flops; but I have recently simplified the electronics by using a PIC microcontroller (right).

    Here is a fuller description of the device.