Glossary

An old, simple rule: longest exposure (s) = 500 ÷ full-frame equivalent focal length (mm), so about 21 s at 24 mm. Devised in the film era, it leaves slightly elongated stars on modern high-resolution sensors. The NPF rule is preferred.

See alsoNPF rule, Star trails

How bright an object would look if it were placed 10 parsecs (32.6 light-years) away. It measures its true luminosity, regardless of distance. The Sun, dazzling at −26.7, has an absolute magnitude of only +4.8; Rigel reaches about −7.

See alsoMagnitude, Parsec

ToolsUseful formulas

A refractor whose objective combines two kinds of glass to bring red and blue to the same focus. Violet stays poorly corrected: a violet halo surrounds bright objects, more so the faster the refractor. At f/12 or slower it becomes discreet.

See alsoRefractor, Apochromat, Chromatic aberration

The thickness of atmosphere the light passes through, as a multiple of the amount at the zenith (airmass 1). It is 2 at 30° altitude and nearly 6 at 10°: the image there is dimmer, redder and more turbulent. Hence the value of observing objects near culmination.

See alsoZenith, Culmination, Seeing, Atmospheric refraction

Even perfect optics do not turn a star into a point but into a tiny disc surrounded by rings, because of diffraction. Its angular radius is 1.22 λ/D: the larger the aperture, the smaller the disc. About 84% of the light falls in the central disc.

See alsoResolving power, Rayleigh criterion, Star test, Central obstruction

ToolsUseful formulas

A mount with two axes, one vertical (azimuth) and one horizontal (altitude), like a photo tripod. Intuitive and with no polar alignment, it needs both axes moved to follow an object. In long-exposure imaging the field rotates as the minutes pass.

See alsoEquatorial mount, Dobsonian, Altitude, Azimuth

The angle between an object and the horizon, from 0° (on the horizon) to 90° (at the zenith); also called elevation. Objects are usually well placed above 30°. With azimuth it locates an object in your local sky.

See alsoAzimuth, Zenith, Horizon, Alt-azimuth mount

ToolsWhat to see tonight?, Celestial fix, Interactive atlas

The gap between two objects measured in degrees on the sky. Your hand at arm’s length is a handy ruler: the little finger covers about 1°, three fingers 5°, a fist 10°, an outstretched hand about 20°. The two front stars of the Plough are a little more than 5° apart.

See alsoDegree, Arcminute, Star hopping

ToolsInteractive atlas, Celestial fix

The clear diameter of the main lens or mirror, in millimetres: the single most important figure of a telescope. The larger it is, the more light it gathers and the finer the detail it can show. A 200 mm telescope collects four times as much light as a 100 mm one.

See alsoLight-gathering power, Resolving power, Focal ratio

ToolsEyepieces, Useful formulas

A refractor whose objective uses special glass (ED, fluorite), often in three lenses, to bring at least three colours to the same focus. Chromatic aberration becomes almost invisible, even at f/6 or f/7. It is the instrument of choice for wide-field imaging.

See alsoRefractor, Achromatic refractor, Chromatic aberration, Field flattener

The angle the eyepiece’s circle of view covers for the eye, as stated by the maker: about 50° for a Plössl, 68° to 82° for a wide-angle design. The wider it is, the more it feels like looking through a big window. It is used to work out the true field.

See alsoTrue field, Eyepiece, Wide-angle eyepiece

ToolsEyepieces, Useful formulas

One sixtieth of a degree, written ′. The Moon is about 31′ across; a good eye separates two points about 1′ apart, like Mizar and Alcor (12′). Nebulae and galaxies measure a few arcminutes.

See alsoDegree, Arcsecond, Angular distance

ToolsUseful formulas

One sixtieth of an arcminute, or 1/3,600 of a degree, written ″: the apparent size of a coin about 2.5 cm wide seen from about 5 km away. Jupiter spans 30 to 50″, Mars 25″ at most. Seeing in western Europe often spreads stars over 2 to 3″.

See alsoArcminute, Resolving power, Seeing, Sampling

ToolsUseful formulas

The unit of sampling. Too large (undersampling) and stars turn square and detail is lost; too small (oversampling) and light is spread over too many pixels, raising noise. For planets, aim for a focal ratio of about five times the pixel size in µm.

See alsoSampling, Lucky imaging, Barlow lens

ToolsUseful formulas

An easily recognised star pattern that is not an official constellation. The Plough (Big Dipper) in Ursa Major, the Summer Triangle (Vega, Deneb, Altair) and the W of Cassiopeia are examples. Asterisms are handy signposts for finding your way.

See alsoConstellation, Star hopping

ToolsInteractive atlas

A small rocky body orbiting the Sun, most of them between Mars and Jupiter. In a telescope it looks like a star, but you can see it move from one night to the next. Vesta can reach magnitude 5.3 and become barely visible to the naked eye under a very dark sky.

See alsoComet, Planet

A flaw that turns a defocused star into an ellipse or a cross whose orientation swings by 90° from one side of focus to the other. It comes from pinched or badly figured optics, poor collimation, or the eye itself. If it rotates when you turn your head, it is your eye.

See alsoStar test, Collimation

The mean distance between the Earth and the Sun, fixed at 149,597,870.7 km, or about 150 million kilometres. Light covers it in 8 min 20 s. Jupiter is about 5.2 au from the Sun, Neptune 30 au.

See alsoLight-year, Parsec

ToolsUseful formulas

The atmosphere bends starlight and makes objects look higher than they are: about 0.5° on the horizon, barely 1′ at 45° altitude. When the Sun seems to touch the horizon it has in fact already set. Celestial navigation corrects for it.

See alsoHorizon, Airmass, Altitude

ToolsCelestial fix

Looking slightly to the side of a faint object instead of straight at it. The centre of the retina has few rods, the cells that sense dim light; shifting your gaze by about ten degrees makes the object show up better. The gain is about one magnitude.

See alsoDark adaptation, Limiting magnitude

An object’s direction along the horizon, measured from north towards east: 0° north, 90° east, 180° south, 270° west. With altitude it tells you where to look. Some older books count it from the south: check the convention.

See alsoAltitude, Alt-azimuth mount, Horizon

ToolsWhat to see tonight?, ISS passes, Celestial fix

A diverging lens placed before the eyepiece that multiplies the telescope’s focal length, usually by 2 or 3. A 10 mm eyepiece with a 2× Barlow on a 1,000 mm telescope gives 200× instead of 100×. It doubles your range of eyepieces while keeping their eye relief.

See alsoMagnification, Focal length, Focal reducer

ToolsEyepieces, Useful formulas

The shortest possible exposure, shutter closed. It measures the sensor’s electronic baseline, present in every image even without light. It is subtracted in particular from the flats.

See alsoCalibration, Dark frame, Flat frame

Two small telescopes side by side, labelled magnification × aperture: 10×50s magnify 10 times with 50 mm lenses. They make an excellent first instrument: wide field, upright image, both eyes in use. They show lunar craters, Jupiter’s moons and dozens of Messier objects.

See alsoMagnification, Exit pupil, Messier catalogue

A 1 to 9 scale describing the quality of a night sky, proposed by John Bortle in 2001. In class 1 the sky is so dark that the Milky Way looks textured and full of detail; class 9 is a city centre where only a few stars show. Typical countryside in France or the UK is class 4 or 5.

See alsoSQM, Light pollution, Limiting magnitude

The step that corrects the light frames with darks, flats and bias frames before stacking. It removes hot pixels, vignetting and dust shadows. Done well, it makes processing the image much easier.

See alsoLight frame, Dark frame, Flat frame, Bias frame, Stacking

The projection of the Earth’s equator onto the sky, halfway between the two celestial poles; its declination is 0°. It rises due east and sets due west everywhere on Earth. Orion’s Belt lies very close to it.

See alsoDeclination, Celestial poles, Ecliptic

ToolsInteractive atlas

The two points in the sky around which all stars seem to turn: the extension of the Earth’s axis. The north celestial pole lies very close to Polaris; the south pole has no bright star. The pole’s height above the horizon equals your latitude.

See alsoPolaris, Polar alignment, Circumpolar, Precession

ToolsPolar alignment, Interactive atlas

An imaginary sphere centred on the observer, onto which every object is projected as if they were all at the same distance. It seems to turn around us once per sidereal day. It is the frame for all celestial coordinates and sky charts.

See alsoCelestial equator, Celestial poles, Right ascension, Declination

ToolsInteractive atlas

The fraction of the aperture blocked by the secondary mirror, as a percentage: a 50 mm secondary in front of a 200 mm primary is 25%. It does not make a hole in the image but slightly lowers the contrast of fine planetary detail. Visual Newtonians aim for 20 to 25%; Schmidt-Cassegrains often exceed 30%.

See alsoSecondary mirror, Airy disc, Schmidt-Cassegrain

ToolsNewtonian calculator

A small marker, often a ring-shaped sticker, placed at the exact centre of a Newtonian’s primary mirror. It is the target for collimation. Because it sits in the shadow of the secondary mirror, it has no effect on the image.

See alsoCollimation, Primary mirror, Cheshire eyepiece

ToolsCollimation

An eyepiece-shaped collimation tool with no lenses: a small peephole and a 45° reflective face lit from the side. It shows a bright ring that you centre on the primary mirror’s centre spot. Simple and battery-free, it is often more reliable than a cheap laser.

See alsoCollimation, Centre spot, Laser collimator

ToolsCollimation

A lens bends each colour slightly differently, so they do not all come to focus at the same place. It shows as a violet halo around the Moon, Venus or Jupiter in fast achromatic refractors. Mirrors are free of it; apochromatic refractors correct it almost entirely.

See alsoAchromatic refractor, Apochromat, Refractor

Said of an object that never sets from a given place: it circles the pole without dipping below the horizon. That happens when its declination exceeds 90° minus your latitude, so +38.5° at 51.5° north. The Plough and Cassiopeia are circumpolar from the UK and France.

See alsoCelestial poles, Declination, Culmination

ToolsInteractive atlas

The adjustment that lines up a telescope’s mirrors on a single axis. A Newtonian drifts a little each time it is moved and should be checked before every session, especially if it is fast (f/5 or below). A poorly collimated telescope gives soft images, even with excellent optics.

See alsoCheshire eyepiece, Laser collimator, Centre spot, Star test

ToolsCollimation, Newtonian calculator

A flaw of parabolic mirrors: away from the centre of the field, stars take the shape of a little comet, tail pointing outwards. It grows quickly as the focal ratio drops: faint at f/8, obvious at f/4. Poor collimation shifts it and makes it visible in the centre.

See alsoComa corrector, Focal ratio, Collimation

ToolsNewtonian calculator, Collimation

A lens group placed in a Newtonian’s focuser to remove coma at the edge of the field. It is almost essential for imaging at f/5 or faster, and welcome visually with wide-angle eyepieces. It too needs a precise spacing to the sensor.

See alsoComa, Newtonian telescope, Field flattener

ToolsNewtonian calculator

A block of ice and dust a few kilometres across which, as it nears the Sun, grows a coma and tails. The curved, yellowish dust tail and the straight, bluish gas tail point away from the Sun. Their brightness is hard to predict.

See alsoMeteor shower, Asteroid, Messier catalogue

The apparent meeting of two objects in the sky, such as the Moon and Venus, while they stay far apart in space. A planet is also said to be in conjunction with the Sun when it is lost in its glare and invisible. Close planetary conjunctions are lovely naked-eye sights.

See alsoOpposition, Elongation, Occultation

ToolsWhat to see tonight?

One of the 88 regions that have officially divided the sky since 1922–1930, each with its own borders, like counties on a map. The stars of a constellation are often unrelated, at very different distances. Their Latin names are used to label stars, such as Alpha Lyrae for Vega.

See alsoAsterism, Zodiac, Star hopping

ToolsInteractive atlas, What to see tonight?

The time a telescope brought out of a warm house needs to reach the air temperature. While the mirror is warmer, currents of warm air rise in the tube and blur the image. Allow 30 minutes to an hour for a 200 mm mirror, more for a Maksutov; a fan speeds things up.

See alsoSeeing, Maksutov-Cassegrain

The weight fixed on a bar opposite the tube on a German equatorial mount. Slide it until the polar axis stays still when unlocked. Good balance spares the motors and improves tracking.

See alsoEquatorial mount, Tracking

An object’s crossing of the meridian, at the highest point of its path; the best time to observe it. For an object south of the zenith, its altitude then is 90° − latitude + declination. From London (51.5°), the Orion Nebula (−5.4°) culminates at about 33°.

See alsoMeridian, Altitude, Declination

ToolsWhat to see tonight?, Celestial fix

The time the eye takes to become sensitive in the dark: the pupil opens within seconds, but the retina gains most of its sensitivity over 20 to 30 minutes. A single bright white light undoes it all. That is why observers use a dim red light.

See alsoRed light, Averted vision, Exit pupil

An exposure taken with the shutter closed or the tube capped, with the same duration, gain and temperature as the lights. It records thermal signal and hot pixels, which software then subtracts. Usually 20 to 50 are taken.

See alsoCalibration, Flat frame, Bias frame

A dust cloud dense enough to hide the stars and glowing gas behind it. It shows up in silhouette, like the Horsehead in Orion or the Coalsack near the Southern Cross. The dark rifts in the summer Milky Way are made of them.

See alsoNebula, Milky Way

A rule of thumb found in the 19th century by William Dawes from double star observations: minimum separation (″) = 116 ÷ aperture (mm). A 150 mm telescope thus splits two equal stars 0.77″ apart. It holds for two stars of similar brightness; if one is much fainter, a wider gap is needed.

See alsoResolving power, Rayleigh criterion, Double star

ToolsUseful formulas

The celestial “latitude”: an object’s angle north (+) or south (−) of the celestial equator, from +90° to −90°. Vega is at +38.8°, Sirius at −16.7°. From a given latitude, declination tells you whether an object is always up, never up, or rises and sets.

See alsoRight ascension, Celestial equator, Circumpolar

ToolsInteractive atlas, Celestial fix

The basic unit of angle on the sky: a full turn of the horizon is 360°, and there are 90° from horizon to zenith. The full Moon is about 0.5° across; the Plough about 25°. A degree divides into 60 arcminutes.

See alsoArcminute, Arcsecond, Angular distance

ToolsUseful formulas

The moisture that settles on optics when they cool below the dew point by radiating heat to the sky. It strikes exposed surfaces first: Schmidt corrector, refractor lens, finder, eyepieces. Do not wipe it: a dew shield or a dew heater prevents it, a hair dryer clears it.

See alsoDew shield, Dew heater

A heating strap wrapped around an optic and run from 12 V, keeping it a few degrees above the dew point. Essential for a whole night in damp weather, especially when imaging. Set it to the lowest useful level so it does not create air currents.

See alsoDew, Dew shield

A tube extending the front of a refractor or Schmidt-Cassegrain, about 1.5 to 2 times as long as the aperture. It limits how much the optics cool towards the sky and delays dew. It also blocks stray light from street lamps.

See alsoDew, Dew heater

The rays of light that stick out of bright stars in a telescope with a spider. Four straight vanes give four spikes; three vanes give six. They are normal and do not point to any fault; a curved spider replaces them with a faint halo.

See alsoSpider, Airy disc

A Newtonian telescope on a simple box base that swings up and down and turns round, popularised by John Dobson in the 1960s and 1970s. You nudge it by hand to follow objects. Quick to set up and very steady, it gives a beginner the largest aperture.

See alsoNewtonian telescope, Alt-azimuth mount, Equatorial platform, Push-to

ToolsNewtonian calculator

Two stars very close together on the sky. Some are true pairs bound by gravity (binaries); others only line up by chance (optical doubles). Albireo in Cygnus shows a lovely orange and blue contrast from 30×; Mizar and Alcor split with the naked eye.

See alsoResolving power, Dawes limit, Variable star

ToolsUseful formulas, Interactive atlas

The profiled bar under the tube that clamps into the mount’s saddle. The two standards are Vixen style (about 44 mm wide) and Losmandy style (about 75 mm), stiffer for heavy instruments. A longer bar gives room to balance the load.

See alsoMount, Counterweight

The slow sliding of a star across the field when tracking is not perfect. Drift in declination betrays imperfect polar alignment; drift in right ascension usually comes from the mount’s speed or gears. In imaging it stretches the stars.

See alsoDrift alignment, Guiding, Polar alignment

ToolsPolar alignment

A very accurate polar alignment method that watches stars drift in declination. You adjust the axis in azimuth with a star near the meridian and the equator, then in altitude with a star low in the east or west. It takes patience but no polar scope.

See alsoDrift, Polar alignment, Polar scope

ToolsPolar alignment

The faint glow on the dark part of the Moon when it is a thin crescent. It is sunlight reflected by the Earth onto the Moon, then back to us. Leonardo da Vinci had already explained it.

See alsoMoon phases

ToolsThe Moon tonight

The Sun’s apparent path among the stars over the year, mirroring the plane of the Earth’s orbit. It is tilted by 23.4° to the celestial equator. The Moon and planets always stay close to it, which helps to recognise them.

See alsoZodiac, Equinox, Solstice, Planet

ToolsInteractive atlas

The angle between a planet and the Sun as seen from Earth. Mercury and Venus, being closer to the Sun, never stray far from it: at most about 28° for Mercury and 47° for Venus. They are best seen around greatest elongation, in the west after sunset or in the east before dawn.

See alsoConjunction, Planet, Twilight

ToolsWhat to see tonight?

A gas cloud lit up by hot young stars that make it glow, like a neon sign. Its hydrogen emits a red light that photos reveal; the eye mostly sees grey or pale green. The Orion Nebula (M42), about 1,350 light-years away, is visible to the naked eye.

See alsoNebula, Nebula filter, Reflection nebula

ToolsInteractive atlas

Tables giving the positions of celestial objects and the times of coming events: rising and setting, Moon phases, conjunctions, satellite passes. Once printed in books, they are now computed in an instant. They are the basis for planning a night out.

See alsoMoon phases, Conjunction, ISS (International Space Station)

ToolsWhat to see tonight?, The Moon tonight, ISS passes

The date to which celestial coordinates refer, since they slowly change with precession. J2000 means 1 January 2000 at 12:00; it is the reference for today’s charts and catalogues. GoTo mounts convert these to coordinates “of date”.

See alsoPrecession, Right ascension, Declination

A mount with one axis (the right ascension, or polar, axis) parallel to the Earth’s axis of rotation. Once polar aligned, turning that single axis follows the stars, by hand or with a motor. The “German” type, with counterweights, is the most common.

See alsoPolar alignment, Counterweight, Tracking, Meridian flip

ToolsPolar alignment

A motorised base slipped under a Dobsonian, whose motion mimics an equatorial mount for about an hour. The telescope then follows objects on its own, which makes high power and sharing the view much easier. It is then reset to its starting point.

See alsoDobsonian, Tracking, Equatorial mount

The moment the Sun crosses the celestial equator, around 20 March and 22 September; day and night then last about 12 hours everywhere. The point the Sun crosses in March, the vernal equinox point, is the zero of right ascension. On that day the Sun rises due east.

See alsoSolstice, Right ascension, Ecliptic

The small disc of light that leaves the eyepiece and enters the eye; its diameter is the aperture divided by the magnification. A 200 mm telescope at 40× gives a 5 mm exit pupil. Between 5 and 7 mm the image is at its brightest (nebulae); around 0.5 to 1 mm you reach the maximum useful magnification.

See alsoMagnification, Minimum magnification, Eye relief

ToolsEyepieces, Useful formulas

How long the sensor collects light for one frame. For deep sky, take many exposures of 30 s to a few minutes and stack them; for planets, exposures of a few milliseconds freeze the seeing. Without tracking, exposure is limited by the NPF rule.

See alsoStacking, NPF rule, Gain (ISO), Signal-to-noise ratio

The distance between the eyepiece’s last lens and the spot where your eye must be to see the whole field. Glasses wearers need at least 15 to 20 mm. Short focal length eyepieces of older designs often have very short, uncomfortable eye relief.

See alsoEyepiece, Exit pupil

ToolsEyepieces

The small interchangeable magnifier you look through; it sets the magnification. Its focal length is engraved on it: the shorter it is, the higher the power. The standard barrels are 1.25 inch (31.75 mm) and 2 inch (50.8 mm) wide.

See alsoMagnification, Apparent field, Plössl, Wide-angle eyepiece, Eye relief

ToolsEyepieces

A telescope’s sharp image is not flat but slightly curved: when the centre is in focus the edges are not, and the other way round. The eye copes, but a flat sensor shows it. A field flattener corrects it.

See alsoField flattener

A lens placed in front of the camera that corrects a refractor’s field curvature, so stars stay sharp to the edge of the frame. It must sit at the right distance from the sensor (often 55 mm). At the eyepiece the eye compensates and it is not needed.

See alsoField curvature, Focal reducer, Coma corrector

The ring inside the eyepiece that gives the field its sharp edge. Its diameter gives the exact true field: field (°) = field stop (mm) ÷ telescope focal length (mm) × 57.3. A 27 mm field stop on a 1,000 mm telescope thus shows 1.55°.

See alsoTrue field, Eyepiece

ToolsEyepieces, Useful formulas

The small scope fixed on the tube, with low power and a wide field (for example 8×50, about 6°), that helps you aim. Align it with the main telescope in daylight on a distant object. In a straight-through finder the image is upside down.

See alsoRed-dot finder, Telrad, Star hopping

ToolsInteractive atlas

An exposure of an evenly lit surface (light panel, twilight sky) taken without touching focus or camera orientation. It records vignetting and dust shadows, which software corrects by dividing the lights. Flats are essential for a smooth background.

See alsoCalibration, Dark frame, Bias frame

The distance between the main lens (or mirror) and the point where it brings a star to focus, in millimetres. A long focal length gives a larger image and a narrower field. A 1,200 mm telescope magnifies 60 times with a 20 mm eyepiece.

See alsoMagnification, Focal ratio, Eyepiece

ToolsEyepieces, Useful formulas

The focal length divided by the aperture, written f/5: a 200 mm telescope with a 1,000 mm focal length works at f/5. A “fast” instrument (f/4 to f/6) gives wide fields and short photo exposures; a “slow” one (f/10 to f/15) suits the planets. For the eye, at the same magnification, it does not change how bright the image looks.

See alsoAperture, Focal length, Coma

ToolsUseful formulas

The opposite of a Barlow: a lens that shortens the focal length, for example ×0.63 on a Schmidt-Cassegrain, taking it from f/10 to f/6.3. The field widens and photo exposures get shorter (2.5 times here). Many reducers also flatten the field.

See alsoBarlow lens, Focal ratio, Field flattener

ToolsUseful formulas

The part that holds the eyepiece and moves it in and out to focus. Rack-and-pinion models are sturdy; friction-driven Crayford focusers are smoother with no backlash. A good focuser with a fine-focus knob helps a lot at high power and for imaging.

See alsoEyepiece, Newtonian telescope

The amplification of the sensor’s signal, set as ISO on a camera. Raising the gain does not capture more light: it makes the image brighter but reduces dynamic range. A medium to high gain lowers read noise, which helps with short exposures and planetary work.

See alsoExposure, Sensor, Signal-to-noise ratio

A vast system of billions of stars, gas and dust held together by gravity. The Andromeda Galaxy (M31), 2.5 million light-years away, is visible to the naked eye as an elongated smudge. In a telescope most look like faint grey glows: their light set off millions of years ago.

See alsoMilky Way, Surface brightness, Messier catalogue

ToolsInteractive atlas, What to see tonight?

Jupiter’s four large moons, Io, Europa, Ganymede and Callisto, discovered by Galileo in 1610. Small, steadily held binoculars show them as points lined up on either side of the planet. Their dance changes hour by hour: eclipses, transits across the disc, shadows.

See alsoPlanet, Transit, Occultation

ToolsWhat to see tonight?, Eyepieces

A very dense ball of hundreds of thousands of stars, among the oldest in the Galaxy, orbiting its centre. M13 in Hercules, about 22,000 light-years away, is the finest in the northern sky. A 150 to 200 mm telescope starts to resolve it into stars.

See alsoOpen cluster, Messier catalogue, Milky Way

ToolsInteractive atlas, What to see tonight?

A motorised, computer-controlled mount that slews by itself to objects chosen from its database, then tracks them. After switching on, you align it on two or three bright stars. It saves time, but learning the sky still helps when it gets lost.

See alsoPush-to, Tracking, Plate solving

A small camera, on a guide scope or an off-axis guider, watches a star and sends corrections to the mount several times a minute. This makes up for tracking errors and imperfect polar alignment. Exposures of several minutes then keep stars round.

See alsoTracking, Polar alignment, Drift

ToolsPolar alignment

The circle where the sky seems to meet the ground, at 0° altitude. The astronomical horizon is a perfect plane; the real one is hidden by trees, hills and buildings. Near the horizon, thick air and light pollution degrade the view.

See alsoAltitude, Atmospheric refraction, Airmass

ToolsCelestial fix, What to see tonight?

The time since an object crossed the meridian, in hours: hour angle = local sidereal time − right ascension. It is 0h on the meridian and grows towards the west; an object at 2h crossed the meridian two hours ago. Polar scopes and equatorial mounts rely on it.

See alsoSidereal time, Meridian, Right ascension

ToolsPolar alignment, Celestial fix

The largest crewed satellite, orbiting about 400 km up and circling the Earth in about 90 minutes. It looks like a very bright, steady point crossing the sky in a few minutes. It is only visible around dawn or dusk, when it is sunlit while the observer is in darkness.

See alsoTransit, Twilight

ToolsISS passes

A tool that shines a laser beam into the telescope: you adjust the secondary so the dot lands on the primary’s centre spot, then the primary so the beam returns to the centre of the laser. Quick and handy in the dark. It must itself be well aligned, or it will mislead you.

See alsoCollimation, Cheshire eyepiece, Centre spot

ToolsCollimation

A slight apparent rocking of the Moon, caused by its elliptical, tilted orbit. Although it always keeps the same face towards us, over time we can see about 59% of its surface. Features near the edge are best seen when libration tips them our way.

See alsoMoon phases, Terminator

ToolsThe Moon tonight

An image of the object itself, straight from the camera. It holds the signal, but also thermal noise, vignetting, dust shadows and the sensor’s electronic offset. Calibration frames (darks, flats, bias) are used to correct these flaws.

See alsoCalibration, Dark frame, Flat frame, Bias frame

Artificial lighting that brightens the night sky and drowns faint stars, nebulae and the Milky Way. A dome of light spreads for tens of kilometres around towns. It also harms wildlife and health; some towns now switch off street lights in the middle of the night.

See alsoBortle scale, SQM, Nebula filter, Limiting magnitude

How much light an instrument collects compared with the eye; it grows as the square of the aperture. Against a 7 mm pupil, a 200 mm telescope gathers (200 ÷ 7)² ≈ 800 times more light. That is what brings galaxies invisible to the naked eye into view.

See alsoAperture, Limiting magnitude

ToolsUseful formulas

The distance light travels in a year, at 300,000 km/s: about 9.46 trillion kilometres. It is a unit of distance, not time. The nearest star after the Sun, Proxima Centauri, is 4.2 light-years away.

See alsoParsec, Astronomical unit (au)

ToolsUseful formulas

The brightness of the faintest stars you can see. With the naked eye it ranges from 3 or 4 in a city to 6.5 or more under a very dark sky. A telescope of aperture D (in mm) pushes it further by about 5 log₁₀(D ÷ 7) magnitudes: a 200 mm telescope reaches roughly magnitude 14.

See alsoMagnitude, Light-gathering power, Bortle scale, Transparency

ToolsUseful formulas

The technique behind planetary and lunar photos: record thousands of very short frames, then software keeps only the sharpest, taken in the moments when the air is calm. Often 5 to 20% of the frames are stacked. It goes well beyond what the seeing lets you see at the eyepiece.

See alsoSeeing, Stacking, Arcseconds per pixel

The Moon passes through the Earth’s shadow, at full Moon. It can be penumbral (barely visible), partial or total; during totality the Moon turns coppery red, lit by light filtered through all the Earth’s sunsets. It is perfectly safe to watch with the naked eye.

See alsoSolar eclipse, Moon phases

ToolsThe Moon tonight

How many times the instrument enlarges the object: telescope focal length divided by eyepiece focal length. A 1,000 mm telescope with a 10 mm eyepiece magnifies 100 times (100×). More is not always better: past a limit the image becomes dim and blurred.

See alsoMaximum useful magnification, Minimum magnification, Exit pupil, Barlow lens

ToolsEyepieces, Useful formulas

The brightness scale of objects, running backwards: the smaller the number, the brighter the object. Five magnitudes make a factor of 100, one magnitude a factor of about 2.5. Vega is 0, Sirius −1.5, Venus up to −4.6; the eye reaches 6 under a dark sky.

See alsoAbsolute magnitude, Limiting magnitude, Surface brightness

ToolsInteractive atlas, What to see tonight?, Useful formulas

A compact telescope closed at the front by a thick, meniscus-shaped lens; the secondary is an aluminised spot on that lens. Its long focal ratio (f/12 to f/15) in a short tube makes it excellent on the Moon, the planets and double stars. It takes a long time to cool down.

See alsoSchmidt-Cassegrain, Cooldown, Focal ratio

The magnification beyond which nothing new is revealed: about twice the aperture in millimetres, so 400× for a 200 mm telescope. That limit assumes perfect optics: with real-world mirrors and lenses and imperfect collimation, the image often softens from about 1× per millimetre (200× for a 200 mm). And the steadiness of the air usually caps it at 150× to 250×, whatever the instrument. The “600×” printed on boxes of small telescopes is therefore misleading.

See alsoMagnification, Seeing, Exit pupil

ToolsEyepieces, Useful formulas

The imaginary line from north to south through the zenith. When an object crosses it, it reaches its highest point: its culmination, the best time to observe it. The Sun crosses it at solar noon, around 1 pm in winter and 2 pm in summer in France, close to noon or 1 pm in the UK.

See alsoCulmination, Zenith, Hour angle, Meridian flip

ToolsCelestial fix

On a German equatorial mount, the tube can hit the tripod once the object crosses the meridian. The mount then swings the tube over to the other side to keep tracking. In imaging the frame turns by 180° and the object must be re-centred.

See alsoMeridian, Equatorial mount, Culmination

The list of 110 deep-sky objects, M1 to M110, that the comet hunter Charles Messier published from 1774 so as not to mistake them for comets (the last entries were added later). It gathers the finest clusters, nebulae and galaxies visible from mid-northern latitudes. It is the ideal observing programme for beginners.

See alsoNGC and IC, Open cluster, Galaxy, Nebula

ToolsInteractive atlas, What to see tonight?

The streak of light from a speck of space dust, often smaller than a grain of sand, burning up as it enters the atmosphere around 80 to 100 km high, at 11 to 72 km/s. A very bright meteor is called a fireball. A piece that sometimes reaches the ground is a meteorite.

See alsoMeteor shower, Radiant, Comet

When the Earth passes through the dust trail left by a comet, meteors multiply for a few nights. The Perseids peak around 12 August and the Geminids around 14 December, with up to 100 to 150 meteors an hour in ideal conditions. Watch them with the naked eye, lying down, away from lights.

See alsoMeteor, Radiant, ZHR (zenithal hourly rate), Comet

ToolsWhat to see tonight?, The Moon tonight

Our galaxy, a disc about 100,000 light-years across holding more than 100 billion stars; the Sun lies about halfway out. Seen from inside, the disc forms the milky band across dark night skies. It is brightest in summer towards Sagittarius, where the galactic centre lies.

See alsoGalaxy, Light pollution, Dark nebula

ToolsInteractive atlas

The lowest magnification that still sends all the light into the eye: aperture divided by 7 (a young eye’s dark-adapted pupil is about 7 mm wide). For a 200 mm telescope that is about 29×. Below it, the exit pupil is wider than the eye’s pupil and some light is wasted.

See alsoExit pupil, Magnification, Dark adaptation

ToolsEyepieces, Useful formulas

The Moon’s changing look depending on how much of its sunlit side we see: new Moon, crescent, first quarter, gibbous, full Moon, then back again. The full cycle, the lunation, lasts 29.5 days. For relief, prefer the quarters to the full Moon, which looks flat.

See alsoTerminator, Earthshine, Libration

ToolsThe Moon tonight, What to see tonight?

The support that carries the instrument and lets you point it. It matters as much as the optics: at 100× the slightest wobble is magnified 100 times. Mounts are alt-azimuth or equatorial, manual or motorised.

See alsoAlt-azimuth mount, Equatorial mount, GoTo, Tracking

A cloud of gas and dust in space. Depending on how it shines, or does not, it is called an emission, reflection, dark or planetary nebula. The word once also covered galaxies, whose nature was unknown.

See alsoEmission nebula, Reflection nebula, Dark nebula, Planetary nebula, Supernova remnant

ToolsInteractive atlas

A filter that passes only the few colours emitted by nebulae, notably ionised oxygen (OIII, around 500 nm) and hydrogen. It darkens the sky background and brings out emission and planetary nebulae, such as the Veil Nebula. It does not help with galaxies or star clusters.

See alsoEmission nebula, Planetary nebula, Light pollution

The telescope invented by Isaac Newton in 1668: a concave primary mirror at the bottom of the tube and a small flat mirror at 45° that sends the image out to the side, near the top. With no lenses it is free of chromatic aberration and offers a lot of aperture for its simplicity. It needs regular collimation.

See alsoPrimary mirror, Secondary mirror, Dobsonian, Collimation, Coma

ToolsNewtonian calculator, Collimation

The New General Catalogue, published by John Dreyer in 1888, lists 7,840 deep-sky objects; its two supplements, the Index Catalogues (IC), add 5,386 more. One object can have several names: M31 is also NGC 224. The brightest NGC objects are within reach of a 150 mm telescope.

See alsoMessier catalogue, Galaxy, Nebula

ToolsInteractive atlas

A formula by Frédéric Michaud (Société astronomique de France) for the longest exposure without star trails on a fixed tripod: t (s) = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel size in µm and f the focal length in mm. At f/2.8 with 4 µm pixels and a 24 mm lens it gives about 9 s. It is more reliable than the old 500 rule with today’s sensors.

See also500 rule, Star trails, Exposure, Star tracker

ToolsUseful formulas

The notebook where you record each observation: date, time in UT, place, instrument, magnification, seeing, transparency and what you saw. It shows your progress and helps you find good settings again. Well recorded observations can also be useful to amateur societies.

See alsoSketching, Universal Time (UT), Seeing

One object passing in front of another and hiding it completely, most often the Moon in front of a star or planet. A star vanishes instantly at the Moon’s edge, since the Moon has no atmosphere. Timing these moments precisely is a useful amateur contribution.

See alsoTransit, Conjunction, Moon phases

ToolsThe Moon tonight

A group of a few dozen to a few thousand young stars, born together from the same gas cloud. The Pleiades (M45), about 440 light-years away, are the best-known example. Many look superb in binoculars or at low power.

See alsoGlobular cluster, Messier catalogue, Asterism

ToolsInteractive atlas, What to see tonight?

The moment an outer planet (Mars, Jupiter, Saturn…) lies opposite the Sun in the sky. It rises at sunset, is highest around midnight and is at its closest to Earth, so at its largest and brightest. Mars only reaches opposition about every 26 months.

See alsoConjunction, Planet, Culmination

ToolsWhat to see tonight?

The distance at which a star shows a parallax of one arcsecond when the Earth moves by one astronomical unit; its name comes from “parallax second”. One parsec is 3.26 light-years, or 206,265 astronomical units. Professional astronomers use it more than the light-year.

See alsoLight-year, Astronomical unit (au), Absolute magnitude, Arcsecond

ToolsUseful formulas

A body that orbits the Sun and shines by reflecting its light. Mercury, Venus, Mars, Jupiter and Saturn are visible to the naked eye; they move slowly among the stars, near the ecliptic, and twinkle less than stars. Uranus (magnitude 5.7) and Neptune (7.8) need binoculars or a telescope.

See alsoOpposition, Conjunction, Elongation, Ecliptic

ToolsWhat to see tonight?, Eyepieces

The shell of gas thrown off by a Sun-like star at the end of its life, lit by the hot core left behind. Nothing to do with planets: the name comes from their small disc, which reminded early observers of a planet. The Ring Nebula (M57) and the Dumbbell (M27) are classics.

See alsoNebula, Nebula filter, Supernova remnant

ToolsInteractive atlas

Software recognises the star patterns in a photo and works out, in seconds, the exact coordinates of its centre, its orientation and scale. The mount can then correct its pointing, or you can check your polar alignment. ASTAP and astrometry.net are examples.

See alsoGoTo, Polar alignment, Sampling

ToolsPolar alignment

A classic four-lens eyepiece with an apparent field of about 50°, sharp and contrasty. It is often the eyepiece supplied with telescopes, and a very good starting point. Its eye relief is about 70% of its focal length, which makes it uncomfortable below 10 mm.

See alsoEyepiece, Wide-angle eyepiece, Eye relief

ToolsEyepieces

The adjustment that points the polar axis of an equatorial mount at the celestial pole. Its angle above the horizon must equal your latitude (51.5° in London), and it must aim at true north, not magnetic north. Good polar alignment keeps stars from drifting, visually and in photos.

See alsoEquatorial mount, Polar scope, Celestial poles, Drift alignment, Polaris

ToolsPolar alignment

A small scope built into the polar axis of an equatorial mount, with a reticle. Since Polaris is about 0.65° from the pole, you place it at a precise spot on the reticle that depends on the time. An app or calculator tells you where.

See alsoPolar alignment, Polaris, Hour angle

ToolsPolar alignment

Alpha Ursae Minoris, a magnitude 2 star about 0.65° from the north celestial pole. It barely seems to move and marks north to within a degree. Find it by extending the front edge of the Plough (Big Dipper) by about five times its length.

See alsoCelestial poles, Polar alignment, Polar scope, Precession

ToolsPolar alignment, Interactive atlas, Celestial fix

The Earth’s axis slowly sweeps out a cone, like a spinning top, in about 26,000 years. The celestial poles therefore wander among the stars: in about 12,000 years Vega will be near the north pole. Star coordinates slowly change, which is why an epoch such as J2000 is given.

See alsoEpoch (J2000), Celestial poles, Equinox

The large concave mirror at the bottom of a telescope, which collects the light and forms the image. In a Newtonian it is parabolic and sits in a cell with adjustment screws for collimation. Its diameter is the telescope’s aperture.

See alsoSecondary mirror, Newtonian telescope, Collimation

ToolsNewtonian calculator, Collimation

A set of encoders on the axes of an unmotorised telescope, often a Dobsonian, linked to a handset or phone. The screen shows which way to push the tube to reach the chosen object. You keep the simplicity of a manual mount, with help to find things.

See alsoGoTo, Dobsonian, Setting circles

The point in the sky from which the meteors of a shower seem to stream, a perspective effect like railway lines meeting in the distance. It gives the shower its name: the Perseid radiant is in Perseus. The higher it is, the more meteors you see; no need to stare at it, though.

See alsoMeteor shower, ZHR (zenithal hourly rate)

Two points count as resolved when the centre of one Airy disc falls on the first dark ring of the other: separation = 1.22 λ/D. In green light (550 nm) this gives 138 ÷ aperture (mm) arcseconds, a little wider than the Dawes limit.

See alsoAiry disc, Dawes limit, Resolving power

ToolsUseful formulas

A dim red torch for reading a chart without losing your dark adaptation: the retina’s rods are almost blind to red. Keep it as dim as possible. A phone screen should also be switched to red and minimum brightness.

See alsoDark adaptation, Sketching

A finder with no magnification that projects a small red dot onto the sky through a window. You place the dot on the target star with both eyes open. Simple and intuitive, it suits beginners well, as long as you keep a spare battery.

See alsoFinder scope, Telrad

A dust cloud that does not glow by itself but scatters the light of nearby stars. Just as our sky scatters sunlight, it looks blue in photos. The haze around the Pleiades is an example; nebula filters do not help with them.

See alsoNebula, Emission nebula, Dark nebula

An instrument whose objective is a group of lenses at the front of a closed tube. With no obstruction it gives very contrasty images, cools quickly and almost never needs collimating. For the same aperture it is harder to make and longer than a reflector.

See alsoAchromatic refractor, Apochromat, Star diagonal, Chromatic aberration

The smallest separation between two stars that an instrument can show as distinct, in arcseconds. It depends only on the aperture: about 1.2″ for 100 mm and 0.6″ for 200 mm. Seeing often prevents you from reaching it.

See alsoDawes limit, Rayleigh criterion, Airy disc, Double star

ToolsUseful formulas

The celestial “longitude”, counted eastwards along the celestial equator from the vernal equinox point, in hours from 0h to 24h (1h = 15°). With declination it fixes a star’s position on the celestial sphere. Abbreviated RA.

See alsoDeclination, Hour angle, Equinox, Sidereal time

ToolsInteractive atlas, Celestial fix

The angle of sky covered by one pixel, in arcseconds per pixel: 206.265 × pixel size (µm) ÷ focal length (mm). 3.76 µm pixels behind a 500 mm focal length give 1.55″/px. For deep sky, aim for about a third to a half of the seeing (often 1 to 2″/px).

See alsoArcseconds per pixel, Sensor, Seeing, Focal length

ToolsUseful formulas

A compact telescope closed by a thin corrector plate, with a spherical primary and a convex secondary, usually at f/10. Versatile and portable, it often comes on a GoTo mount. Focusing moves the primary mirror, which can shift the image slightly.

See alsoMaksutov-Cassegrain, Focal reducer, Dew, Central obstruction

The small mirror that sends the light from the primary to the eyepiece. In a Newtonian it is flat, elliptical and tilted at 45° near the top of the tube; it is also called the “diagonal”. Its size sets the central obstruction and the fully illuminated field.

See alsoPrimary mirror, Central obstruction, Spider

ToolsNewtonian calculator, Collimation

The restlessness of the atmosphere that makes images dance and boil; it is also what makes stars twinkle. It is measured in arcseconds (the size of a star in a photo) or on scales from 1 to 10 (Pickering) or I to V (Antoniadi). It limits high magnification far more often than the instrument does.

See alsoTransparency, Maximum useful magnification, Cooldown, Lucky imaging

The electronic chip in a camera or astronomy camera, made of millions of pixels that turn light into a signal. Its key figures are the pixel size (in micrometres), the sensor dimensions and its noise. Astronomy cameras are often cooled to reduce thermal noise.

See alsoSampling, Gain (ISO), Dark frame

Graduated dials on the axes of an equatorial mount, in right ascension and declination. Once the mount is polar aligned and set on a known star, you dial in an object’s coordinates to find it. The digital encoders of push-to systems are their modern version.

See alsoRight ascension, Declination, Push-to

Star time: local sidereal time is the right ascension currently on the meridian. A sidereal day lasts 23 h 56 min 4 s, about 4 minutes less than a solar day. That is why stars rise 4 minutes earlier each night, or 2 hours per month.

See alsoHour angle, Meridian, Tracking, Right ascension

ToolsCelestial fix, Polar alignment

The strength of the useful signal (the object’s light) compared with the random fluctuations that make the image grainy. It is the measure of an astrophoto’s quality. It grows as the square root of total exposure time: four times as many frames give an image half as noisy.

See alsoStacking, Exposure, Dark frame

Drawing what you see at the eyepiece, in pencil on white paper, under a red light. It forces you to look long and carefully: you notice far more detail. Galaxy and nebula sketches are often made in negative, then inverted.

See alsoObserving log, Red light, Averted vision

The Moon passes in front of the Sun, at new Moon. It is total if the Moon hides it completely, annular if the Moon is too far to cover it (a ring stays visible), partial elsewhere. Never look at the Sun without certified eclipse glasses, except during the brief total phase.

See alsoLunar eclipse, Solar filter, Moon phases

ToolsThe Moon tonight

The only safe way to look at the Sun: a certified film or glass fixed securely over the front of the instrument, blocking more than 99.99% of the light. Never use a filter that screws onto the eyepiece: it can crack from the heat. A single glance through an unfiltered instrument causes blindness in a fraction of a second.

See alsoSunspot, Solar eclipse, Transit

The moment the Sun reaches its greatest declination, +23.4° around 21 June and −23.4° around 21 December. In the northern hemisphere June brings the longest day and December the longest night. Around the June solstice in France and the UK, true darkness is short or missing.

See alsoEquinox, Ecliptic, Twilight

A spherical mirror or lens does not bring rays from the edge and from the centre to the same point: the image stays a little soft whatever the focus. Newtonian mirrors are therefore parabolised. With a small, slow mirror (114 mm at f/8, for example), a spherical one is still acceptable.

See alsoPrimary mirror, Star test, Newtonian telescope

The holder of the secondary mirror, made of thin vanes (often three or four) stretched across the tube. The vanes diffract light and produce the spikes around bright stars. It carries the secondary’s adjustment screws.

See alsoDiffraction spikes, Secondary mirror

ToolsNewtonian calculator

The Sky Quality Meter, a small device that measures the brightness of the sky background in magnitudes per square arcsecond. The higher the number, the darker the sky: about 21.7 to 22 under a perfect sky, 19 in the suburbs, 17 or less in a city. It allows sites to be compared objectively.

See alsoBortle scale, Surface brightness, Light pollution

Adding or averaging many images of the same object, aligned on the stars, to reduce noise. Stacking 16 frames improves the signal-to-noise ratio by a factor of 4. Software such as Siril or DeepSkyStacker does the job.

See alsoSignal-to-noise ratio, Calibration, Lucky imaging

A 90° mirror or prism placed between a refractor (or Schmidt-Cassegrain) and the eyepiece, so you can look near the zenith without straining your neck. The image is then upright but reversed left to right, as in a mirror. Keep this in mind when reading a chart.

See alsoRefractor, Schmidt-Cassegrain, Eyepiece

The method of starting from an easy bright star and hopping from star to star, chart in hand, to the target. A finder or Telrad and the known size of its field help. It is slow at first, but it is how you learn the sky.

See alsoTelrad, Finder scope, True field, Asterism

ToolsInteractive atlas, Eyepieces

You point at a moderately bright star at high power and defocus it slightly on each side of focus. With good, well collimated optics you see identical concentric rings on both sides. It is the final judge of collimation and optical quality.

See alsoCollimation, Airy disc, Seeing, Astigmatism

ToolsCollimation

A small motorised equatorial mount designed for a camera and lens. It follows the stars and allows exposures of several minutes without trails with a 14 to 200 mm lens. It fits in a bag and needs careful polar alignment.

See alsoPolar alignment, Tracking, NPF rule

ToolsPolar alignment

The streaks stars leave on a photo taken from a fixed tripod, because of the Earth’s rotation. Unwanted in a sharp image, they become an artistic effect when hours of exposures are stacked: the stars then draw circles around the celestial pole.

See alsoNPF rule, Celestial poles, Tracking

A dark area on the Sun’s surface, cooler (about 4,000 °C against 5,500 °C) because of an intense magnetic field. Some are larger than the Earth. Their number follows a cycle of about 11 years; observe them only with a proper solar filter.

See alsoSolar filter, Transit

The explosion of a star, which can shine for a few weeks as brightly as a whole galaxy. Many are found every year in distant galaxies, sometimes by amateurs, at magnitude 12 to 16. The last one seen in our own Galaxy was in 1604.

See alsoSupernova remnant, Variable star, Galaxy

The expanding debris of a massive star that exploded. The Crab Nebula (M1) is what remains of the supernova seen in 1054; the Veil Nebula in Cygnus is far older and very large. An OIII filter helps a lot with the latter.

See alsoSupernova, Nebula, Nebula filter

The brightness of an extended object spread over its area, in magnitudes per square arcsecond. A large magnitude 9 galaxy can be harder to see than a small magnitude 11 nebula. It is what matters when judging how visible a deep-sky object will be.

See alsoMagnitude, Galaxy, SQM

In English, “telescope” covers both mirror instruments (reflectors) and lens instruments (refractors). In French, “télescope” means only a reflector; a refractor is a “lunette”. The main families of reflector are the Newtonian, the Schmidt-Cassegrain and the Maksutov-Cassegrain.

See alsoNewtonian telescope, Refractor, Schmidt-Cassegrain, Maksutov-Cassegrain

A finder with no magnification that projects three red circles of 0.5°, 2° and 4° onto the sky. The circles act as a scale for hopping from a bright star. Many star atlases print these circles to make finding objects easier.

See alsoRed-dot finder, Finder scope, Star hopping

ToolsInteractive atlas

The line between day and night on the Moon (or a planet). The Sun is low there and shadows are long, so craters and mountains stand out in relief. That is always where to look in a telescope, and it moves from one evening to the next.

See alsoMoon phases, Libration

ToolsThe Moon tonight

The motion that cancels the Earth’s rotation to keep an object in view. At the sidereal rate the mount makes one turn in 23 h 56 min, about 15 arcseconds per second. At 200× with a 50° eyepiece, an untracked star near the celestial equator crosses the field in about a minute.

See alsoSidereal time, Equatorial mount, GoTo, Equatorial platform

A small object passing in front of a larger one without hiding it: Mercury across the Sun (next on 13 November 2032), one of Jupiter’s moons across the planet, or the ISS across the Sun or the Moon. For the Sun, a solar filter is essential.

See alsoOccultation, Galilean moons, ISS (International Space Station), Solar filter

ToolsISS passes

How clear the air is: free of haze, moisture, dust and thin cloud. Good transparency shows faint stars and contrasty nebulae. It does not always go with good seeing: hazy, still nights are often excellent for the planets.

See alsoSeeing, Limiting magnitude, Light pollution

The patch of sky actually visible in the eyepiece. As a first approximation it is the apparent field divided by the magnification: a 52° eyepiece at 50× shows about 1°, twice the width of the full Moon. The exact figure comes from the field stop.

See alsoApparent field, Field stop, Magnification

ToolsEyepieces, Useful formulas, Interactive atlas

The period when the sky darkens after sunset. It is civil until the Sun is 6° below the horizon (you can read outside), nautical until 12° (the horizon fades) and astronomical until 18°. Only then is the night truly dark for deep-sky objects.

See alsoSolstice, Dark adaptation

ToolsWhat to see tonight?, The Moon tonight

The time on the Greenwich meridian, used by astronomers worldwide to time events. UK winter time equals UT; summer time is UT + 1h (France: UT + 1h in winter, UT + 2h in summer). Always write UT in an observing log.

See alsoSidereal time, Observing log

A star whose brightness changes over time. Algol in Perseus fades from magnitude 2.1 to 3.4 every 2.87 days when a companion eclipses it; Mira in Cetus swings from 3 to 10 over about 332 days. Amateurs follow thousands of them and share their estimates.

See alsoMagnitude, Double star, Supernova

An eyepiece with an apparent field of 65° to 100° or more, using more lenses. At the same magnification it shows more sky, which helps when finding objects and following a planet with a Dobsonian. It is heavier and bulkier than a Plössl.

See alsoApparent field, True field, Plössl

ToolsEyepieces

The point in the sky directly overhead, at 90° altitude. That is where you look through the least air, so images are at their sharpest and brightest. Its opposite, below your feet, is the nadir.

See alsoAltitude, Meridian, Airmass

ToolsCelestial fix

The number of meteors one observer would see in an hour under a perfect sky (limiting magnitude 6.5), with the radiant at the zenith. It is a theoretical maximum: in practice you often see two or three times fewer. For the Perseids it is about 100.

See alsoMeteor shower, Radiant, Limiting magnitude

The band of sky along the ecliptic where the Sun, Moon and planets move. The ecliptic actually crosses 13 constellations, including Ophiuchus. Astrology’s “signs” no longer match the constellations, shifted by precession: astrology is not a science.

See alsoEcliptic, Constellation, Precession

A faint cone of light rising along the ecliptic, in the west after dusk in spring and in the east before dawn in autumn. It is sunlight scattered by dust in the Solar System. You need a very dark sky to see it.

See alsoEcliptic, Light pollution, Twilight

An eyepiece with a variable focal length, for example 24 to 8 mm, so you can change magnification without swapping. Handy to find the best power for the night’s seeing. The trade-off is a narrow apparent field at the long end (about 40°).

See alsoEyepiece, Magnification, Seeing

ToolsEyepieces