Characterization of low light performance of a CMOS Sensor for ultraviolet astronomical applications

Abstract

CMOS detectors offer many advantages over CCDs for optical and UV astronomical applications, especially in space where high radiation tolerance is required. However, astronomical instruments are most often designed for low light-level observations demanding low dark current and read noise, good linearity and high dynamic range, characteristics that have not been widely demonstrated for CMOS imagers. We report the performance, over temperatures from 140 -240 K, of a radiation hardened SRI 4K×2K back-side illuminated CMOS image sensor with surface treatments that make it highly sensitive in blue and UV bands. After suppressing emission from glow sites resulting from defects in the engineering grade device examined in this work, a 0.077 me − /s dark current floor is reached at 160 K, rising to 1 me − /s at 184 K, rivaling that of the best CCDs. We examine the trade-off between readout speed and read noise, finding that 1.43 e − median read noise is achieved using line-wise digital correlated double sampling at 700 kpix/s/ch corresponding to a 1.5 s readout time. The 15 ke − well capacity in high gain mode extends to 120 ke − in dual gain mode. Continued collection of photo-generated charge during readout enables a further dynamic range extension beyond 10 6 e − effective well capacity with only 1% loss of exposure efficiency by combining short and long exposures. A quadratic fit to correct for non-linearity reduces gain correction residuals from 1.5% to 0.2% in low gain mode and to 0.4% in high gain mode. Cross-talk to adjacent pixels is only 0.4% vertically, 0.6% horizontally and 0.1% diagonally. These characteristics plus the relatively large (10µm) pixel size, quasi 4-side buttability, electronic shutter and sub-array readout make this sensor an excellent choice for wide field astronomical imaging in space, even at FUV wavelengths where sky background is very low.

Glow and hot pixels removal using improved robust principal component analysis

“…Equation ( 21) is the soft thresholding operator, which is mentioned in Eq. (8). Based on the hot pixel distribution of large sparsity and little connectivity between them, a top-hat filter is introduced into the soft threshold operator to preserve only the hot pixels.…”

“…6 Hence, glow intensity has become an important performance metric used to evaluate a sensor. 7,8 Hot pixels are defective pixels that are significantly brighter than circumambient pixels. They strongly depend on temperature, and their number increases during the lifetime of a sensor.…”

“…Glow also affects the Near-Infrared Camera and Multi-Object Spectrometer of the Hubble Space Telescope 6 . Hence, glow intensity has become an important performance metric used to evaluate a sensor 7 , 8 …”