Glossary

Glossary

A

• Absolute magnitude
  Is the real brightness of an object that an observer would measure at a uniform distance.

• Achromat
  Lens telescope in which not all wavelengths meet in one focal point.

• ADC (Analog-Digital-Converter)
  Converts the voltage generated by the photons in the chip into a digital number.

• ADU (Analg-Digital-Unit)
  Digital number generated by the ADC, as a value for brightness.

• Alignment
  Aligning the telescope's right ascension axis parallel to the Earth's rotation axis.

• Altitude (elevation, pole height angle)
  Angle of a point above the horizon.

• Altitude axis
  Axis to rotate a telescope in the vertical in an azimuthal mount.

• Amici prism
  Accessory used to redirect light sideways out of the telescope tube for more convenient observation with an eyepiece, imaging the object sideways rather than upside down.

• Amplifier glow
  Glow in the image occurring with aging CCD sensors, which is caused by infrared heat radiation from the outsourced amplifier and excites the pixels sensitive in this wavelength range.

• Angular diameter distance
  Is the ratio between the extent and the solid angle at which an astronomical object appears to an observer.

• Aperture
  Optical component for limiting the cross-section of the beam.

• Aperture number (f-number)
  Is the reciprocal of the aperture ratio and a measure for the light intensity of the telescope.

• Aperture ratio
  Is the ratio of telescope aperture to telescope focal length.

• Apochromat
  Lense telescope in which the wavelengths of visible light meet at a focal point, and the color error of the achromat is thus corrected.

• Apparent magnitude
  Indicates how bright objects appear to an observer in comparison.

• APS-C (Advanced Photo System)
  A medium-size camera chip format (usually 3:2 ratio) that is 1.5 to 1.6 times smaller than full-frame (analog film).

• ASCOM
  Driver platform by means of which the various devices can communicate with each other.

• Autumn equinox
  On this day, the sun passes the intersection of the celestial equator and the plane of the earth's orbit around the sun, making day and night of equal length in all places on earth. This happened the same way at the vernal equinox (half a year before).

• Azimuth
  Angle along the horizon, starting from the meridian plane.

• Azimuth axis
  Axis to rotate the telescope parallel to the horizon in an azimuthal mount, or to align the mount parallel to the Earth's rotation axis /meridian plane in an equatorial mount, depending on the location.

• Azimuth error
  Horizontal deviation of the right ascension axis from the Earth's rotation axis.

• Azimuthal mount
  Device for holding a telescope with the possibility to adjust it in azimuth and elevation angle.

• Azimuth peg
  Stop pin on the tripod to allow the mount to be rotated horizontally.

B

• Back focus
  Distance from the telescope exit to the focus point, or when using auxiliary optics, the working distance from their screw-in support to the camera chip.

• Back focus of the camera
  Distance from the outer edge of the camera aperture to the camera chip.

• Background limited capturing
  Capturing that has been exposed long enough that the background noise of the sky glow is so great that the readout noise can be neglected.

• Background noise
  Noise of the signal generated by the sky glow.

• Backlash
  Backlash caused by backlash in the gearbox of the mount when switching from one direction to the other and no movement can take place in this interim period.

• Bahtinov mask
  Component which is placed on the telescope aperture and which is an aid to finding the correct focus point by varying the star image.

• Barlow lense
  Auxiliary optics, which sits in the beam path, and extends the focal length of the telescope over one or more lenses.

• Baryonic matter
  Any matter built up from atoms in the universe.

• Bayer-matrix
  Color filter that is applied over a monochrome sensor and with the means of which a certain color can be assigned to each pixel.

• Bias-frame (offset-frame)
  An image taken with the shortest possible exposure time to determine the offset.

• Binning
  Changing the pixel size by combining neighboring pixels.

• Blocking filter
  Filter that blocks a certain wavelength range.

• Blooming
  A bright spot/stripe that usually occurs on CCD sensors when saturated pixels release their excess charge carriers to neighboring pixels.

• Blueshift
  Is the shift of spectral lines in the emission and absorption spectrum of astronomical objects towards the blue wavelength range and is associated with the object getting closer to the observer.

• Burning out
  Term for a fully saturated pixel (memory of the pixel is full), which then assumes the color white.

C

• Calibration (images)
  Correction of the individual images with dark-frames (subtraction from the sub-frame) and then with flat-frames (division from the sub-frame) freed from offset and readout noise.

• Camera chip
  Surface sensitive in a certain wavelength range, which converts incoming photons into an electric charge.

• Catadioptric telescope
  Telescope that uses both lenses and mirrors for imaging.

• CCD Sensor (Charge-coupled Device)
  Camera chip in which vertical and horizontal charge transport takes place and the charge/voltage conversion of all pixels takes place outside the sensor.

• Celestial equator
  Thought line of the Earth's equator projected onto the celestial sphere.

• Celestial sphere
  An imaginary hollow sphere in the sky on which all visible celestial bodies lie.

• Chip (camera chip)
  Area sensitive in a certain wavelength range that converts incoming photons into an electric charge.

• CMOS sensor (Complementary metal-oxide-semiconductor)
  Camera chip in which the charge/voltage conversion takes place in each pixel of the sensor.

• Collimator
  Alignment laser used to align the two mirrors in a Newtonian telescope.

• Coma, coma error
  Imaging error that occurs due to rays that are not parallel to the optical axis, giving objects a tail at the edge.

• Coma corrector
  Auxiliary optics, which sits in the optical path, and corrects the coma error in Newtonian telescopes.

• Comoving distance
  Real distance of an astronomical object to the observer at the respective time of measurement.

• Crop factor
  Ratio of a sensor image diagonal to the diagonal of the full-frame sensor.

• Crosshair eyepiece
  Eyepiece with included crosshairs for centering and holding objects.

D

• Dark current
  Current generated by electrons randomly decoupling in the silicon chip due to thermal effects.

• Dark current noise
  Noise of the dark current generated in the silicon chip by thermal effects.

• Dark energy
  Hypothetical form of energy used to explain the accelerated expansion of the universe.

• Dark-frame
  An image created with the camera aperture closed and the same exposure time as the light frame to determine the dark current. In addition, the sensor glow as well as hot and dead pixels can be removed from the light- or sub-frame.

• Dark matter
  Postulated form of matter that is not directly visible but interacts gravitationally.

• Darkflat-frame (Flatdark-frame)
  An image created with the camera aperture closed and the same exposure time as the flat-frame, to remove the offset and dark current from a longer exposed flat-frame. In the case of longer exposure flat frames, they replace bias-frames.

• Data Depth
  Number of bits available to generate the gray values.

• Dead pixel
  Due to technical problems, there is no or a permanent current flow, so that the pixels are either black or white (burned out) as a result.

• Declination
  Angle of a point above or below the celestial equator.

• Declination axis
  Axis to adjust the distance of the object to the celestial equator.

• Decoupling
  Detachment of an electron from its orbit around the atomic nucleus.

• Dew cap, dew protection cap
  Cylindrical body (usually also heated) mounted around the front telescope aperture to prevent dew from forming on the optics.

• Diffraction disk
  Structure of a light beam that is refracted through an aperture (tube aperture) resulting in a central bright spot with surrounding rings.

• Dithering
  A software shifting (after one or more capturing) of the image field by a few pixels in order to locate the last hot pixels, since these do not change the pixel position.

• Doppler effect
  Temporal compression or elongation of waves during a change in distance, caused by a change in travel time.

• Drizzle
  Software method to split a pixel into 2x2 or 3x3 pixels to make the image look softer when the pixels are large, or when objects are zoomed in and should not look pixelated.

• Duo Narrowband filter
  Narrowband filter that passes two different wavelength ranges.

• Dynamic range
  Number of gray levels in an image as a function of the data depth of the camera chip. Is largely responsible for the depth of detail of an image.

E

• Electromagnetic wave spectrum
  Totality of all emitted electromagnetic wavelengths of an object.

• Elevation (Altitude, Pole height angle)
  Angle of a point above the horizon.

• Emission nebula
  Gas cloud which is excited by high energy particles from neighboring stars and thereby emits light.

• Equatorial mount
  Device for holding a telescope with the possibility of adjusting it in right ascension and declination with parallel alignment to the Earth's rotation axis.

• Erecting lense
  Accessory used to image the object on a Newtonian telescope laterally correct and not upside down.

• Exit pupil
  Is the ratio of telescope aperture to magnification and describes the diameter of the beam leaving the eyepiece.

• Extinction
  Measure of the attenuation of light as it passes through the atmosphere.

• Eyepiece
  Optical system that projects an image onto the retina of the eye.

• Eyepiece focuser
  Device used to finely adjust a camera or eyepiece to produce a sharp image.

F

• f-number (aperture number)
  Is the reciprocal of the aperture ratio and a measure of the light intensity of the telescope.

• Fast optics
  Light-intensive optical system with small aperture number.

• Filter wheel
  Auxiliary component that allows automatic filter change during the capturing sequence without changing the position of the camera chip or unintentionally defocusing.

• Filter drawer
  Additional component which enables a manual filter change during the exposure sequence without changing the position of the camera chip or unintentionally defocusing.

• Fixed Pattern Noise (spatial noise)
  Deviations caused by production-related differences between pixels.

• Flat-frame
  An image captured in the average brightness value range (ADU) to correct vignetting and uneven image illumination caused by dust or dirt in the light-frame.

• Flatfield mask
  Component used to create an evenly illuminated area to capture flat-frames.

• Flattener
  Auxiliary optics, which is located in the beam path and levels the image, so that stars are displayed up to the corners without aberrations.

• Focal length
  Distance from the principal plane of a lense or mirror to the focal point (focus point). The greater the focal length, the greater the magnification of the object.

• Focal point (also focus point)
  Point on the optical axis at the distance of the focal length, which is bundled by the optical system from the parallel incident rays.

• Focus point (also focal point)
  Point on the optical axis at the distance of the focal length, which is focused by the optical system from the parallel incident rays.

• Fork mount
  Variant of the azimuthal mount for holding a telescope with the possibility to adjust it in azimuth and elevation angle.

• Full-frame sensor
  Designation for a camera chip that is the size of the formerly common 35 mm analog film with an image ratio of 3:2.

• Full Well Capacity
  Number of charge carriers that can be stored in the pixel memory until the memory is full.

• FWHM (Full Width at Half Maximum)
  Describes the half width of the brightness distribution of the diffraction disk of a star.

G

• Gain
  Mostly used term in astro cameras for the amount of amplification for the signals generated by the pixel.

• Galaxy
  A large gravitationally bound collection of stars and nebulae.

• Geographic North Pole
  Point at the North Pole through which the Earth's rotation axis passes.

• Global shutter process
  Simultaneous exposure of all pixels.

• Gray value
  Indicates how light or dark the pixels appear to the human eye.

• Guiding
  Monitoring and centering one or more guide stars to give correction commands to the mount's tracking motors.

• Guiding Camera
  Takes a picture of one or more guide stars to detect changes in the position of these stars.

• Guide star
  Bright star aimed with camera or eyepiece, used in Scheiner-method or guiding.

• Guiding telescope
  Small telescope that is parallel to the telescope and captures guide stars via the guiding camera attached to it.

H

• H-alpha-filter
  Filter, which only allows a narrow wavelength spectrum around 656 nm to pass through to the chip.

• Half-width
  Is the difference of two values of a function with a "maximum mountain". The two values are on the left and on the right of the maximum and correspond from the value of the half of the maximum. Graphically, this is the width of the function at half its height.

• Hand control box
  Remote control for manual control of the mount.

• Heating band
  Heated cloth band mounted around the front telescope aperture to prevent dew from forming on the optics.

• Helical focuser
  Short add-on component in which focus can be achieved by rotating an outer ring, often without rotating the camera chip.

• Histogram
  Graphical representation of the frequency distribution of the tonal values.

• Hot pixel
  Formed by single electrons jumping from one pixel to the neighboring pixel and reacting disproportionately to the additional energy. As a result, they glow red, yellow or sometimes green or blue in the final image.

• Hour angle (right ascension)
  Angle along the celestial equator starting from the vernal equinox (right ascension) or from the meridian plane (hour angle).

• Hour axis (right ascension axis)
  Axis of rotation by which celestial objects can be tracked at the speed of the Earth's rotation in an East-West direction.

• Hubble parameter (Hubble constant).
  Describes the present expansion rate of the universe.

I

• Image dynamic range
  Number of gray levels in an image.

• Image field
  Section of the sky that is imaged on the chip.

• Image field rotation
  Rotation of the image during tracking with an azimuthal mount.

• Image scale
  Measure of the size of the section of sky that can be imaged on a pixel.

• Image sensor
  Surface sensitive in a certain wavelength range that converts incoming photons into an electrical charge.

• Infrared light (IR, photographic range)
  Electromagnetic waves with a wavelength of approx. 680 to 1000 nm.

• Integration (images)
  Combines the prepared individual images to improve the signal-to-noise ratio and to suppress interfering image structures pixel by pixel (stacking process).

• ISO
  Term mostly used in Digital single-lens reflex camera (DSLR) for the degree of amplification for the signals generated by the pixel.

J

K

• Kochab-method
  Procedure for aligning the telescope.

L

• Lens
  Light-gathering optical system that focuses parallel incident rays into a focal point (focus point).

• Light-frame (sub-frame)
  Capturing light from the object with a specific exposure time.

• Light pollution
  Glow of the night sky caused by artificially generated light scattered in the atmosphere.

• Light travel time distance
  A time-based distance that indicates how long it took light to travel from an astronomical object to the observer.

• Lucky Imaging
  Taking a large number of extremely short-exposure captures and then selecting from them the images where seeing is just absent or extremely weak.

• Luminance image
  Image of brightness taken with a monochrome camera (usually without any filters), in which all image details are included except the color information.

• Luminosity distance
  Object distance as a function of the space expansion, resp. the power density and the luminosity (for distant objects) or as a function of its apparent magnitude related to its absolute magnitude (for near objects).

• Luminosity
  Is the radiated energy per unit time.

M

• Magnetic North Pole
  Variable point in the North where the magnetic field lines of the earth's magnetic field enter the earth's surface perpendicularly.

• Matter density parameter
  A value for all matter (dark and baryonic) in the universe.

• Maximum useful magnification
  Results from the double telescope aperture.

• Meridian
  Line imaginary on the celestial sphere running from the North to the South pole vertically above the observer.

• Meridian flip
  Changing the telescope from the West to the East side, when the viewing direction of the telescope crosses the meridian.

• Meridian plane
  Plane perpendicular to the Earth's surface from the meridian.

• Minimum useful magnification
  Ratio of telescope aperture to pupil width.

• Modified DSLR (Digital single-lens reflex camera)
  Removal of the filter built into commercially available DSLRs, which is intended to lower the red tones in the silicon chips that are particularly sensitive in the red wavelength range.

• Monochrome
  Term used in pictures to describe an image with pure gray shades.

• Mount
  Central component in which the axes of rotation are located.

N

• Narrowband filter
  Optical filter that passes only a certain range of wavelengths.

• Newton telescope
  A type of reflecting telescope developed by Isaac Newton.

• North celestial pole
  Thought point (near Polaris) on the celestial sphere through which the Earth's ration axis passes.

• Nyquist-Shannon sampling theorem
  Statement from signal and control engineering that an analog frequency f must be digitally sampled at a frequency greater than 2*f in order to accurately reconstruct the output signal.

O

• O-III-filter
  Filter, which only allows a narrow wavelength spectrum around 496 nm resp. 501 nm to pass through to the chip.

• Obstruction
  Shading in the optical path caused by a secondary mirror (Newtonian telescope) or corrective lenses in the Schmidt plate (Schmidt-Cassegrain telescope).

• Off-axis-guider
  Additional component in the optical path which redirects light via a prism to a guiding camera in order to determine the position of guide stars.

• Offset
  A defined electronic value so that a pixel never reaches values below its sensitivity threshold, and thus would output the value zero.

• Offset-frame (Bias-frame)
  An image taken with the shortest possible exposure time to determine the offset.

• One Shot Camera (Single Shot Camera)
  In astrophotography, the term for color cameras because, in contrast to monochrome cameras, all color channels are captured with one capturing.

• Optimal pixel size
  In this case, two closely spaced stars can still be displayed separately on the chip by the telescope camera system.

• Optimum magnification
  Is present, if it succeeds to be in the range of the resolving power of the telescope (considering the maximum resolving power of the human eye). This corresponds to the telescope aperture divided by 0.7 mm.

• Oversampling
  The light of a star is distributed to too many pixels.

P

• Parallax angle
  Apparent shift in the position (in the background) of a close object when viewed from two different positions.

• Payload
  Indication of the maximum weight the mount may be loaded with.

• Peltier element
  An electrothermal transducer that cools the sensor chip in astro cameras to a fixed value by flow of current.

• Photoelectric effect
  Interaction of photons with material in which an electron is released from the material by the photon.

• Photon
  Quantum particle of the electromagnetic field. Particle emitted by the object at a specific frequency, which interferes with the camera chip.

• Photon noise (Shot noise)
  The standard deviation of the number of photons from the expected average of photons incident per pixel.

• Pixel
  Single small picture element of a camera chip divided into picture elements.

• Pixel memory
  Memory device present for each pixel, in which the electrons generated by photons are stored.

• Planetary nebula
  Nebula, consisting of gas and plasma, which has been ejected at the end of a star's life.

• Plate Solving
  Comparison of an image with a database to determine the exact viewing direction of the telescope.

• Polaris
  Latin short form for the polar star.

• Pole error
  Alignment error of the right ascension axis to the North celestial pole.

• Pole finder
  Optical accessory of an equatorial parallactic mount to align the right ascension axis parallel to the Earth's rotation axis.

• Pole height angle (Altitude, Elevation)
  Angle of a point above the horizon.

• Pole height axis
  Axis to adjust the telescope's height angle in an azimuthal mount, or to align the mount's orientation parallel to the Earth's rotation axis in an equatorial mount, depending on the location.

• Pole height error
  Vertical deviation of the right ascension axis from the Earth's rotation axis.

• Power density
  Is the energy that hits a surface per unit of time.

Q

• Quantum efficiency
  Ability of the chip / pixel to actually convert an incoming photon into an electron.

R

• Rayleigh criterion
  Method for determining the angular distance between two neighboring objects in order to be able to display them separately.

• Readout current
  Complete output signal, including all additional error currents and their noise components.

• Readout noise
  Noise of the readout current, which occurs in all components used in signal readout due to randomly generated electrons.

• Redshift
  Is the shift of the spectral lines in the emission and absorption spectrum of astronomical objects towards the red wavelength range and is associated with the moving away of the object from the observer.

• Reducer
  Auxiliary optics, which is placed in the optical path, and reduces the focal length of the telescope via one or more lenses.

• Reflection nebula
  Nebulae made of dust that scatter the light of neighboring stars.

• Reflector
  Term for a telescope that uses only mirrors for imaging the object.

• Refraction
  Deflection of light by the earth's atmosphere.

• Refraction error
  Incorrect interpretation of the height of an object above the horizon caused by the atmospheric refraction of light.

• Refractor
  Term for a telescope that uses only lenses for imaging the object.

• Registration (images)
  Alignment of individual images to a common reference to allow them to be stacked without offset.

• Resolving power
  Ability of the telescope to display two neighboring objects still separately.

• RGB-filter set
  Filters used for monochrome cameras that allow only red, green, or blue wavelength spectra to pass through each.

• Right ascension (hour angle)
  Angle along the celestial equator starting from the vernal equinox (right ascension) or from the meridian plane (hour angle).

• Right ascension axis (hour axis)
  Axis of rotation used to track celestial objects at the speed of the Earth's rotation in an East-West direction.

• ROI (Region of Interest)
  Selection of a small pixel area.

• Rolling shutter effect
  Blurring of moving objects in the image data by exposing the pixels column-wise or line-wise in the rolling shutter process.

• Rolling shutter method
  Exposure of pixels in columns or rows.

S

• Satellite track
  The sunlight captured during a long exposure that is reflected from the solar panels of the orbiting satellite.

• Saturated pixel
  Pixel where the memory is full, and thus no more photon-generated charge carriers can be accommodated.

• Scale factor a
  Is a measure for relative expansion of the universe.

• Scheiner-method
  Method/procedure according to Julius Scheiner to align the telescope via guide stars with the help of a crosshair eyepiece.

• Secondary mirror
  Small mirror at the tube aperture of a Newtonian telescope that redirects light reflected from the primary mirror to the camera sensor or eyepiece.

• Seeing
  Air turbulence due to swirling of air layers.

• Sensor glow
  Unwanted additional signals during long exposures, generated by infrared heat radiation from components located next to the chip, which excite sensitive pixels in this wavelength range.

• Shot Noise (Photon Noise)
  The standard deviation of the photon count from the expected average of photons incident per pixel.

• Signal to Noise Ratio (SNR)
  Ratio of the signal generated by the object to the noise generated by the signal. The signal-to-noise ratio should be as high as possible.

• Single Shot Camera (One Shot Camera)
  In astrophotography, the term for color cameras because, in contrast to monochrome cameras, all color channels are captured with one capturing.

• Sky glow
  Additional signal by shining of the sky by e.g. moonlight, light pollution or reflecting dust.

• Slow optics
  Low light optical system with large aperture number.

• Smear effect
  Bright image fringes caused by the typical readout process of CCD sensors, when photons generate additional charge carriers during readout.

• Spatial noise (Fixed Pattern Noise )
  Deviations caused by manufacturing differences between pixels.

• SQM (Sky Quality Meter)
  Device for determining the brightness of the night sky.

• ST-4 port
  Interface to transmit correction signals from guiding to the mount.

• Stacking
  Mathematical procedure to combine several images to a complete image.

• Star-alignment
  Moving to bright stars stored in the hand control box to tell the mount the position of the telescope.

• Star cluster
  Collection of very many stars in one area.

• Stretching
  Stretching out the tone value curve in the histogram.

• Sub-frame (Light-frame)
  Capturing from the object with a certain exposure time.

T

• Telephoto lens
  Camera lens with comparatively long focal length, or refractor with comparatively short focal length.

• Telescope aperture
  Responsible for the light gathering capacity of the object photons.

• Telescope focal length
  Distance from the main plane of a lense or mirror to the focal point (focus point). The larger the focal length, the greater the magnification of the object.

• Telescope tube
  Cylindrical body of the telescope.

• Theoretical magnification
  Is the ratio of telescope focal length and eyepiece focal length.

• Tone value
  Brightness value as a function of the existing data depth.

• Tone value curve
  Curve progression over the number of all tone values within the histogram.

• Tracking
  Compensating for the Earth's rotation during observation to keep the object always in the center of the image.

• Tripod
  Used for stable setup and holding of the mount and telescope.

• Tube Seeing
  Air turbulence inside the telescope tube caused by temperature differences between the telescope tube and the environment.

U

• Undersampling
  The light of a star is imaged on only one pixel and the star looks angular.

V

• Vacuum energy density parameter
  Measure for the dark energy present in the universe.

• Vernal equinox
  On this day the sun passes the point of intersection of the celestial equator and the plane of the earth's orbit around the sun, making day and night of equal length in all places on earth. This happens again at the autumn equinox (six months later).

• Vignetting
  Shadowing at the edges of an image, usually caused by several components in the optical path.

W

• Walking Pattern Noise
  Neighboring pixels that have different sensitivities or are defective (e.g. hot pixels) create a stripes effect during stacking if a very small drift occurs during the exposure. A workaround is to force a large drift (dithering), so that the software can actually recognize these differences and calculate them out.

• Wobble
  Periodic oscillation back and forth.

X

Y

Z

• Zenith
  Vertical point above the location on the celestial sphere.

• Zenith mirror
  Accessory to redirect light sideways out of the telescope tube for more convenient observation with an eyepiece.

• Zenith prism
  Accessory used to redirect light sideways out of the telescope tube for more convenient observation with an eyepiece.