Capturing Photons: Digital Sensors in Astronomy Quiz

If the sensor functions by moving electrical charges from one pixel to another in a sequence to be read, and if these charges are "coupled" as they move toward the output amplifier, then the device is correctly named a Charge-Coupled Device.

If Einstein’s photoelectric effect states that incoming photons can knock electrons loose from a material like silicon, and if these liberated electrons are then trapped in "pixel wells," then this effect is the fundamental physics behind digital imaging.

If we want to measure how "good" a sensor is at catching light, and if we compare the number of photons hitting the surface to the number of electrons captured, then this ratio is defined as quantum efficiency.

If heat generates "fake" electrons (dark current), and if the process of reading the data adds static (read noise), and if light itself is statistically random (shot noise), then these are all valid types of noise found in astronomical data.

If the sensor outputs an analog electrical signal based on the number of electrons, and if a computer can only process digital 1s and 0s, then the ADC is the specific component required to translate that voltage into a numerical value.

If a sensor is linear, then doubling the exposure time or the brightness of the star will result in exactly double the amount of signal recorded; this predictability is why digital sensors are superior to film for scientific measurement.

If heat causes silicon atoms to vibrate and release electrons that mimic starlight, then reducing the temperature stops this error; if a small electronic device is used to chill the sensor without moving parts, it is a thermoelectric (Peltier) cooler.

If a pixel has a maximum capacity for electrons (well depth), and if more light hits that pixel once it is full, then no more data can be recorded; this state is known as saturation and can cause "blooming" or white streaks in the image.

If film only catches about 1% of light while CCDs catch 90%, and if digital data allows for instant processing and precise linear measurement, then digital sensors provide a massive scientific advantage over chemical film.

If every electronic component has a tiny amount of unpredictable current, and if that current gets mixed in with the signal as it is converted to a number, then that specific error is defined as read noise.

If a sensor needs to see a bright star and a faint nebula in the same photo, and if it can measure both without the bright one saturating or the dim one getting lost in static, then it has a high dynamic range.

If the ADC converts a voltage to a number, then that number represents a unit of measured light; in technical software, these are referred to as Analog-to-Digital Units.

If you want to increase the sensitivity of a camera at the cost of resolution, and if you add the signal from 4 pixels (2x2) together before reading the data, then you are performing binning to create a stronger signal.

If we want to isolate the true starlight, we must remove the effects of heat (dark), electronic baselines (bias), and dust on the lens (flat); if we perform these mathematical subtractions and divisions, we are calibrating the image.

If CMOS technology allows for "on-chip" processing and high frame rates at a lower cost than traditional CCDs, then they have become the standard for most consumer and amateur-grade astronomical cameras.

If we imagine a pixel as a bucket catching rain (photons), and if the bucket can only hold a certain volume, then that capacity limit is termed the full-well capacity.

If a pixel is large, it catches more light but sees a larger area of the sky (less detail); if it is small, it has better resolution but catches fewer photons; therefore, astronomers must balance pixel size with the telescope's focal length.

If different materials respond to different energy levels of light, then silicon is best for the visible range, while complex alloys like InGaAs and HgCdTe are engineered to capture the lower energy photons of the infrared spectrum.

If a specific pixel on the silicon chip is physically damaged or disconnected, then it cannot capture or transfer electrons; if it fails to function, it will always appear as a black spot (zero value) in the raw data.

If the purpose of a digital sensor is to turn light into information for scientists, and if scientists need the most accurate data possible, then the sensor produces a digital file (Flexible Image Transport System - FITS) that stores the exact brightness value for every pixel.