Astrophotography is a complex hobby, and as such it is riddled with many terms and acronyms that may be confusing to beginners, and some may even be unknown to intermediate imagers. This glossary will cover all of the astrophotography terms I have come to know over the years. If you know of a term that is not covered here, please feel free to contact me to have it added.

 

  • Alternating Current / AC
    • Electrical current that has an oscillation in both voltage and current
  • Analog-to-Digital Converter / ADC
    • An electronic device that converts analog information into digital information at a given bit depth
  • Analog-to-Digital Unit / ADU
    • The discrete units of information and measurement generated by an ADC unit as it converts analog signal information into digital signal information
    • Explicit within the context of a given ADC units bit depth, ADU is not a generic term (for a generic term, see DN)
  • Aperture
    • The opening in a telescope that allows light to pass through and eventually reach an eyepiece or camera sensor
  •  ASI
    • Brand of cameras manufactured by ZWO Company in SuZhou, China
  • Atik
    • English camera manufacturer of CCD (and soon CMOS) cameras and related accessories
  • Backfocus
    • Also called back focus or the back focal distance. The amount of distance from some point in a telescope to the focal point. Usually from the back flange of a telescope in reflecting telescopes, possibly from the back of the focuser for refracting telescopes (excluding the threads) , this is the distance that the sensor must be placed to achieve focus. This is also used to referr to the space within which all various imaging train components must fit, such as spacers, filters, filter wheels, OAG ports, etc. The introduction of an optical unit into the imaging train, such as a field flattener, reducer or corrector, will often change the backfocus. The optical unit itself will usually have it’s own specified back focal distance.
  • Backlash
    • Play in mechanical mechanisms arising from gaps between teeth, looseness in belts, etc.
  • Baffle
    • A structure inside of a telescope that is used to block or otherwise limit reflected light from reaching the eyepiece or camera sensor. Unbaffled telescopes are prone to background sky brightening or even reflective artifacts affecting contrast and overall image quality. Baffling can take a variety of forms, from small circular ridges within refracting telescopes, to cylindrical tubes inside reflecting telescopes, to small ridges on the inside surfaces of spacers and other optical accessories.
  • Barlow lens
    • I diverging lens used to increase the effective focal length of a telescope
  • Bit
    • The discrete unit of information used by computers, supporting only two values per place: zero or one
    • Digital switches used to store information, bits can be “on” or “off”, many bits in sequence can represent numbers, strings, any form of information
  • Bit Depth
    • The number of computer bits used to represent information in a digital form
    • Common bit depths: 8, 10, 12, 14, 16
  • CFA
    • Color Filter Array, used to describe the matrix of color filters applied over each pixel in a color image sensor
  • Dark Current / DC
    • A leakage of current through pixel structure that can leave behind additional unwanted charge
    • A form of noise in digital camera sensors
  • Direct Current / DC
    • Electrical current that has no oscillation in voltage or current
  • Digital Number / DN
    • A discrete unit of digital information and digital measurement
    • Also referred to as DU, Digital Unit
  • Dynamic Range / DR
    • The range of discernible tones allowed by a device or digital format, limited by noise and the saturation point or maximum recordable value
  • Electron
    • Elementary particle, negative charge carrier
    • The analog unit of measure of a signal detected by a pixel
  • Extender
    • Additional optical device added between a sensor and a telescope or lens to increase the effective focal length. Functions to extend focal length the same regardless of where it is placed in the imaging train
  • Extension
    • A distance of lensless separation added between the sensor and a telescope or lens
  • FLI
    • Finger Lakes Instruments, US camera manufacturer of CCD cameras and related accessories
  • Full Well Capacity / FWC
    • The maximum charge level that can be held within a sensor pixel
  • Fixed Pattern Noise / FPN
    • Noise that varies within space but not within time, thereby maintaining a fixed pattern
    • Non-random noise within a camera, often exhibiting distinct structure
  • Image Sensor
    • An electronic device comprised of many smaller sub-sensing units called pixels in a two dimensional matrix, used to detect photonic signals, convert them to electrical signals, which are ultimately converted to digital signals
  • Noise
    • Uncertainty in a signal, unexpected deviations from the expected value
  • QHY
    • Chinese camera manufacturer of CMOS and CCD cameras and related accessories
  • QSI
    • Quantum Scientific Imaging, US camera manufacturer of CCD cameras and related accessories
  • Read Noise / RN
    • Random noise generated by a camera during image readout
  • Saturation Point
    • The charge level at which a pixel will “saturate”, or reach it’s maximum value, for a given gain (not to be confused with FWC)
  • SBIG
    • Santa Barbra Imaging Group, US camera manufacturer of CCD cameras and related accessories
  • Sensor
    • An analog and/or electronic device used to detect signals
  • Signal
    • A measurable or detectable quantity of information
  • Signal to Noise Ratio / SNR
    • The ratio between a signals level/strength/power and it’s noise.
    • Simple formula: SNR = S/N = S/SQRT(S)
    • Basic formula with dark current & read noise: SNR = S/SQRT(S + DC + RN^2)
  • ZWO
    • Chinese camera manufacturer of ASI CMOS cameras and related accessories