The words nobody
stops to explain.

An old shutter estimate: 500 divided by focal length.

Fine for a small web image on a low-resolution sensor and optimistic for anything else. Useful as mental arithmetic in the field, not as a specification.

See alsoNPF rule

Faint natural emission from the upper atmosphere.

Green or red banding that shows up in long exposures from genuinely dark sites. People routinely mistake it for light pollution or a sensor problem, then try to remove it. It is real sky, and removing it flattens the image.

See alsoGradientBortle scale

Sensor electronics leaking light into a corner of the frame.

A brightening, usually in one corner, caused by the sensor's own readout electronics. Dark frames matched to the same temperature, gain and exposure length remove it cleanly. Modern cooled cameras mostly avoid it.

See alsoDark frameCalibration frame

The sun between 12 and 18 degrees below the horizon.

True darkness only begins when the sun is more than 18 degrees down. Above about 48 degrees of latitude that never happens in midsummer, which is why deep-sky season has a hard stop in northern Europe and Canada.

See alsoNautical twilightBlue hour

The exact distance from a corrector to the sensor.

Field flatteners and reducers specify a working distance, usually 55mm for photographic imaging trains. Being a millimetre out shows up as elongated corner stars that look exactly like tilt or coma.

See alsoTiltComa

The shortest possible exposure with the shutter closed.

Records the sensor's read noise floor and offset. Fast to collect, and needed by most calibration workflows unless your dark frames already contain the bias signal.

See alsoCalibration frameDark frame

A nine-class rating of night sky brightness.

John Bortle's scale runs from class 1, where the Milky Way casts a shadow, to class 9, an inner-city sky. It is descriptive rather than instrumental, so pair it with an SQM reading or a light pollution atlas.

See alsoSQMAirglow

Bias, dark and flat frames used to correct the light frames.

Calibration removes what the camera and optics added rather than what the sky sent. Getting this right is worth more than any processing plugin, and it is the step beginners skip most often.

See alsoFlat frameDark frameBias frame

Stars smearing into comet shapes toward the frame corners.

An optical aberration common in fast newtonian telescopes and wide camera lenses. A coma corrector fixes it in a telescope; stopping down a lens by a stop or so usually helps a great deal.

See alsoBackfocusTilt

An exposure with no light, matched to your light frames.

Captures thermal signal and hot pixels at the same temperature, gain and duration as your real exposures. Uncooled cameras make this awkward because sensor temperature drifts through the night.

See alsoCalibration frameAmp glow

Telling the viewer how an image was constructed.

Composites, tracked skies blended with static foregrounds, time blends, replaced skies and generative fill should be labelled where people see the picture. It costs nothing and protects the credibility of everyone doing the work honestly.

See alsoSHO palette

Shifting the frame slightly between exposures.

Small random moves between subs mean fixed-pattern noise and walking noise land in different pixels each time, so stacking averages them out. It costs a few seconds per frame and saves hours of processing pain.

See alsoWalking noiseGuiding

A stacking method that recovers detail from undersampled data.

Developed for the Hubble Space Telescope. It only helps when your data is genuinely undersampled and well dithered, and it will happily amplify noise if it is not.

See alsoSamplingDithering

How much sky your sensor and focal length actually cover.

Set by sensor dimensions and focal length. Checking this before a session is the difference between framing a target properly and discovering at 1am that half of it is outside the frame.

See alsoSamplingPlate solving

An evenly lit exposure that maps vignetting and dust.

Shot through the exact same optical train, focus position and rotation as the light frames. Move anything and the flats stop matching, which usually shows up as dark rings where dust motes used to be.

See alsoCalibration frameVignetting

An uneven background caused by sky glow, moonlight or a light dome.

Large-scale brightness variation across the frame. Removing it early makes colour calibration and stretching behave. Removing it aggressively erases real nebulosity, so work gently and check faint structure afterward.

See alsoAirglowBortle scale

A second camera correcting the mount in real time.

A guide camera watches a star and sends small corrections to the mount, which allows exposures far longer than the mount could hold on its own. It adds cables, software and failure modes, so add it only once unguided tracking is reliable.

See alsoDitheringPeriodic error

Total exposure across every frame in a stack.

Twenty 5-minute frames is 100 minutes of integration. Signal to noise improves with the square root of total time, so doubling quality means roughly quadrupling the hours.

See alsoStackingSub-exposure

A filter that blocks common artificial emission lines.

Broadband versions help modestly. Dual-band and narrowband filters, which pass only hydrogen-alpha and oxygen-III, transform emission nebula work from a suburban location. They do nothing useful for galaxies, star clusters or reflection nebulae.

See alsoNarrowbandBortle scale

A German equatorial mount swapping sides as a target crosses south.

Necessary to avoid the telescope hitting the tripod. It rotates the field by 180 degrees, so framing, guiding calibration and sometimes cable routing all need attention on either side of it.

See alsoGuidingTransit

Imaging through filters that pass a few nanometres of light.

Hydrogen-alpha, oxygen-III and sulphur-II filters isolate specific emission lines. They cut light pollution dramatically and let you shoot through moonlight, at the cost of much longer exposures and false-colour decisions in processing.

See alsoLight pollution filterSHO palette

A shutter estimate that accounts for aperture and pixel pitch.

More honest than the old 500 rule because it knows that a 60-megapixel sensor shows trailing a 12-megapixel sensor hides. Still a starting point: check the actual file at 100 percent.

See also500 ruleSampling

Repeating tracking error from a mount's worm gear.

Shows up as a slow oscillation in guiding graphs with a period matching one worm rotation. Periodic error correction or guiding handles it. A large amount often means a mount needs adjustment rather than more software.

See alsoGuidingPolar alignment

Software identifying exactly where a frame is pointing.

It matches star patterns against a catalog and returns real coordinates, which lets a mount centre a target precisely and repeat framing across nights. Once you have used it, manual framing feels absurd.

See alsoField of viewMeridian flip

Aligning a mount's rotation axis with the celestial pole.

The single biggest determinant of how long an unguided exposure can be. Getting within a few arcminutes is usually enough for wide fields; long focal lengths want better than that.

See alsoPeriodic errorField rotation

How many arcseconds of sky land on one pixel.

Calculated as 206.265 times pixel size in microns, divided by focal length in millimetres. Somewhere between 1 and 2 arcseconds per pixel suits most seeing conditions. Going finer usually just magnifies atmospheric blur.

See alsoSeeingDrizzle

Atmospheric turbulence blurring fine detail.

Independent of transparency: a crystal clear night can have terrible seeing. It sets the real resolution limit for planetary and long focal length deep-sky work, and no amount of extra integration fixes it.

See alsoTransparencySampling

Mapping sulphur, hydrogen and oxygen to red, green and blue.

Also called the Hubble palette. It is a deliberate false-colour choice, not a natural rendering, and it should be described as such when the image is published.

See alsoNarrowbandDisclosure

Sky quality meter reading in magnitudes per square arcsecond.

A direct measurement of sky brightness. Higher numbers mean darker skies. Readings above about 21.5 become unreliable with common handheld meters.

See alsoBortle scale

Averaging many frames to raise signal above noise.

Random noise partially cancels when frames are averaged while real signal reinforces. This is how a camera that shows nothing in one exposure produces a detailed nebula from two hundred.

See alsoIntegration timeDithering

One individual frame in a stack.

Sub length is a trade: long enough to swamp read noise, short enough to survive tracking errors, satellites, wind and clipping bright cores. Sky brightness usually decides more than anything else.

See alsoIntegration timeStacking

The sensor sitting slightly non-perpendicular to the optical axis.

Produces stars that are sharp in one corner and elongated in the opposite corner. Easy to confuse with backfocus error, and diagnosed by rotating the camera and seeing whether the bad corner follows.

See alsoBackfocusComa

How much light the atmosphere is absorbing.

High thin cloud, dust and humidity cut transparency without necessarily hurting seeing. Bad transparency dims faint targets and inflates star halos.

See alsoSeeing

Corners darker than the centre.

Present in almost every optical system, worse at wide apertures. Flat frames correct it during calibration, which is why flats matter even on a clean sensor.

See alsoFlat frame

Diagonal streaks of correlated noise in a stack.

Caused by small consistent drift between frames combined with fixed-pattern noise. Dithering between subs is the fix, and no amount of noise reduction afterward works as well.

See alsoDithering