Direct Imaging and wavefront control- Detectors Flashcards
How does a CCD work?
The CCD is divided up into a large number of light-sensitive small areas (known as pixels) which can be used to build up an image of the scene of interest. A photon of light which falls within the area defined by one of the pixels will be converted into one (or more) electrons and the number of electrons collected will be directly proportional to the intensity of the scene at each pixel. When the CCD is clocked out, the number of electrons in each pixel is measured and the scene can be reconstructed.
(Ian) CCDs have voltage wells for each pixel. On read-out, the charge accumulated in these wells is shifted out along one axis, a pixel at a time. In this way, a single amplifier handles all the reads for all pixels across many rows. This leads to much more uniform read-noise, etc across the array, but is much slower than a CCD. It also has the effect of over-saturated pixels spilling out onto neighboring voltage wells, leading to characteristic oversaturation of entire rows from bright stars, etc.
How does an infrared CMOS sensor work?
With CMOS, each pixel has its own amplifier. This can lead to much faster read rates, and allows for unique capabilities like “up the ramp” sampling, where each pixel is read multiple times, non-destructively, before the well is reset. Downside is that each pixel has it’s own read-noise characteristics that need to be carefully calibrated
Do we need photon counting and photon number resolving sensors on the ground? What are these?
Photon counting means that the read-noise is low enough that individual photon events can be distinguished. If you plotted a single pixel’s read-count vs number of photons, a typical detector would have smooth ramping line, as the read noise adds random variations to the counts. As the read noise comes down, you start to see resolvable peaks for each additional photon received.
This is very helpful for high-contrast imaging as we are chasing extremely low signal-to-noise detections
