The Jargon Glossary

40 terms, two honest sentences each — the same definitions that power the tooltips inside Astryx, so what you learn here follows you into the tools.

ADU: Analog-to-Digital Units — the raw brightness numbers the camera records per pixel.
Airmass: How much atmosphere you shoot through: 1.0 straight up, ~2.0 at 30° altitude. More airmass = softer stars and dimmer signal — why altitude matters so much.
Astronomical darkness: When the Sun is more than 18° below the horizon — the only truly dark part of the night, and the window all planning here is measured against.
Background level: The brightness of 'empty' sky in your subs — driven by light pollution, moon, and haze. A slow upward trend can also mean dew or a failing dark library.
Bias frames: Zero-length exposures that capture the sensor's readout signature. Cheap to shoot, long shelf life.
Bortle scale: A 1–9 rating of your sky's darkness: 1 = pristine wilderness, 5 = suburban, 9 = inner city. Narrowband filters claw a lot of it back for nebulae.
Broadband: Full-spectrum color imaging (LRGB or one-shot color). Ideal for galaxies, clusters and reflection nebulae — but it wants dark, moonless skies.
Dark frames: Calibration shots with the cap on, matching your lights' exposure, gain and temperature. They subtract sensor heat signal and hot pixels. Libraries age — reshoot every few months.
Dark-flats: Darks matched to your flats' (short) exposure — they calibrate the flats themselves. Used instead of bias on many CMOS cameras.
Dithering: Nudging the mount a few pixels between subs so hot pixels and pattern noise land in different places and vanish when stacking.
Eccentricity: How stretched your stars are (0 = perfectly round). A rising trend usually means sensor tilt, flexure, or guiding trouble — not bad luck.
Flat frames: Shots of an evenly lit surface (panel or dawn sky). They remove dust shadows and vignetting — and must be reshot after any focus, rotation, or camera re-seat.
Focuser steps: The motorized focuser's position in motor steps. It drifts with temperature as the telescope shrinks and grows — hence refocusing through the night.
FOV — field of view: The patch of sky your telescope + camera combination captures, set by focal length and sensor size.
FWHM — Full Width at Half Maximum: How wide a star appears at half its peak brightness. Smaller = sharper; set by seeing, focus and optics together.
Gain: Camera signal amplification. Higher gain = lower read noise but less dynamic range; most cameras have a documented sweet spot. Calibration frames must match it.
Guiding: A second camera watches a star and sends tiny corrections to the mount, keeping stars round through long exposures.
Guiding RMS: Root-mean-square guiding error in arcseconds — the average wobble while your mount tracks. Total RMS under ~1″ is solid for most setups; under ~0.5″ is excellent.
HFR — Half-Flux Radius: The radius (in pixels) holding half a star's light — the standard focus metric. Lower = tighter stars; watch the trend across a session, not one frame.
Image scale: Arcseconds of sky per camera pixel — how finely you sample detail. ~1–2″/px suits typical seeing; far below that is oversampling.
Integration: Total exposure time stacked on a target — the sum of every kept sub. More integration = smoother, deeper images; hours matter more than any single setting.
Keeper rate: The share of subs good enough to stack; the rest get rejected for clouds, guiding wobbles, or satellite trails. 80%+ is a good night.
Magnitude: Astronomical brightness — counterintuitively, LOWER numbers are brighter (each 5 steps = 100×). Mag 6 is the naked-eye limit under dark skies.
Meridian flip: A German equatorial mount must swap sides when a target crosses due south (the meridian). Automated rigs handle it; it just pauses imaging a few minutes.
N.I.N.A.: Nighttime Imaging 'N' Astronomy — the free Windows acquisition suite that runs many amateur rigs. Astryx imports its profiles, sequences, and session telemetry.
Narrowband: Filters passing only specific emission lines (Ha, OIII, SII). They cut through moonlight and light pollution — the great equalizer for city-bound nebula imaging.
OAG — Off-Axis Guider: Picks the guide star through the main telescope with a small prism — no separate guide scope, no flexure between the two.
Offset: A small pedestal added so no pixel ever reads zero. Set once, keep consistent — calibration must match.
Parfocal offsets: Filters rarely reach focus at exactly the same focuser position; the offsets are the small per-filter corrections. Astryx measures the real ones from your focus history.
PHD2: The free guiding application most rigs run ('Push Here Dummy'). Astryx reads its live status and analyzes its logs.
Plate solving: Matching a photo's stars against a catalog to compute exactly where the telescope points — how modern rigs center targets without you squinting.
Read noise: Electronic noise added each time the sensor reads out. Expose subs long enough that sky noise 'swamps' it, and it stops mattering — that's what the sub-length math here optimizes.
Seeing: Atmospheric steadiness, in arcseconds — how much stars shimmer. It, not your telescope, usually limits fine detail. Lower is better; 2″ is average, 1″ is a gift.
SNR — signal-to-noise ratio: How far your target's signal stands above the noise floor. It grows with the square root of integration: 4× the hours ≈ 2× the SNR.
SQM — Sky Quality Meter: Sky darkness in magnitudes per square arcsecond. Higher = darker: ~21.8 is a pristine site, ~18.5 a bright city.
Sub (subframe): One individual exposure. Your final image is dozens or hundreds of subs stacked together.
Surface brightness: How spread-out a target's light is (mag/arcsec²). Higher numbers = fainter per patch of sky — a SB-23 galaxy is a dark-site project, whatever its headline magnitude says.
Trail limit: Comets move against the stars, so subs longer than this smear the comet by more than a pixel at your image scale. Keep subs shorter, or track the comet itself.
Transit: The moment a target crosses your meridian at its highest altitude — the least atmosphere, the sharpest data. Plan the heart of a session around it.
Transparency: How clear the air is — haze, humidity, smoke. Poor transparency dims faint targets even when the sky 'looks' clear.