Effect of CCD Readout Noise in Astronomical Speckle Imaging

A theoretical and experimental comparison of photon-countingcameras and CCD's for use in astronomical speckle imaging wasperformed. Photon-counting cameras able to detect single-photonevents typically exhibit a lower quantum efficiency (QE) and suffersaturation effects at high light levels. In contrast, CCD's offera high QE and virtually unlimited photon-count rate. However CCD'sare limited at lower light levels by noise associated with the readoutprocess. Speckle-imaging performance was quantified by derivationof the signal-to-noise ratio (SNR) of the power spectrum and theKnox-Thompson product to include CCD readout noise. Ground-basedtelescope observations at various light levels were obtained with anadvanced, high-speed, low-noise CCD camera to verify SNRexpressions. The useful operating ranges for these two camera typeswere compared by consideration of the effects of QE, readout noise, andmaximum photon-count rate. Although photon-counting camerascontinued to dominate low-light-level applications, CCD's are shown tooffer significant improvements over photon-counting cameras for a widerange of light levels. Future reductions of readout noise will further improve CCD speckle-imaging performance.