Astronomy glossary

A catalogue of 86 planetary nebulae, drawn up by George Abell in 1966. These objects are very faint, and they are kept for the end of the road. The same author published a second catalogue, devoted to clusters of galaxies.

The instrument looks at the stars in front of it and recognises them. From that it works out which way it is pointing. On a smart telescope, the operation is automatic and takes a few seconds.

See also Résolution astrométrique

A ball of several hundred thousand very old stars. It orbits a galaxy. The Milky Way holds about 160 of them.

A group of young stars, born together and still loosely packed. It holds anything from a few dozen to a few thousand. About 1,100 are recorded in our galaxy. The Pleiades are the best known example.

A coloured halo in a corner of the image, produced by the heat of the sensor electronics. It grows with the length of the exposure and with temperature. Recent sensors produce little of it, and the dark takes it away.

See also Capteur CMOS Dark (image de)

The distance light travels in one year. It is used to state the distance of deep sky objects. To see an object a thousand light-years away is to see it as it was a thousand years ago.

Source: Jacques Gispert, Association Andromède, lexique d'astronomia.fr

See also Ciel profond

The first of the two coordinates that place an object in the sky. It is the equivalent of longitude, projected onto the sky. It is counted in hours, minutes and seconds, from 0 to 24h.

Source: Jacques Gispert, Association Andromède, lexique d'astronomia.fr

See also Déclinaison

A mount that moves up and down and left and right. It is simple and compact. But it makes the field rotate as the night goes on. Sub-exposures therefore have to stay short.

See also Équatoriale (monture) Rotation de champ

The English name for the offset frame, for exactly the same thing. The word circulates in French because the processing software is in English and calls that batch of exposures by that name. A second sense exists, with nothing to do with calibration: the Malmquist bias, which means that far away, only the brightest objects are still detected.

See also Offset (image d')

Neighbouring pixels are grouped in squares, most often two by two, to make one single pixel. The image loses resolution but gains sensitivity. It is useful on faint objects and under a mediocre sky.

See also Capteur CMOS Échantillonnage

The scale that says how dark the sky is, from 1 to 9. Class 1 is a sky with no trace of artificial light at all; class 9 is a city centre. It is read with the naked eye, by looking for the faintest star still made out. It is the best explanation for why two images of the same object, taken with the same device, do not look alike.

See also Pollution lumineuse Magnitude limite Duo-bande (filtre)

A catalogue of 109 bright objects, drawn up by Patrick Moore in 1995. They are objects absent from the Messier catalogue. It is the right next step once the easy Messiers are used up.

The light sensitive surface, which takes the place of the eye and of film. Three things count: its sensitivity, the size of its pixels and its noise. They decide what the instrument can reach, far more than the diameter does.

What the device sees at one go, measured in degrees. The full Moon is about half a degree. A field of 3° therefore holds six full Moons side by side.

The angle the image takes up for the eye looking into an eyepiece, as opposed to the piece of sky actually covered. The distinction comes from visual observing. A smart telescope has no eyepiece: on this site, the field quoted is always the true field.

See also Champ (de vision)

Everything that lies outside the solar system: nebulae, galaxies, clusters. These objects are faint and spread out. They are made for the long exposure, not for the eye.

The adjustment that puts the mirrors or the lenses back square on the axis. Smart telescopes are sealed and do not need it. Only large diameters with an open tube still have to be adjusted.

A ball of ice and dust: close to the Sun, its ice sublimates and releases gas and dust, which the solar wind pushes back. It moves against the stars during the exposure, so much so that the stacking has to choose: locking on the stars stretches the comet, locking on the comet stretches the stars.

Source: Jacques Gispert, Association Andromède, lexique d'astronomia.fr

See also Suivi Empilement Temps d'intégration Messier

A photograph taken in complete darkness, cap on, to record what the sensor produces on its own. It is subtracted from the real exposures: it carries away the dark current, the hot pixels and the amp glow. Same length, same gain, same temperature, because dark current roughly doubles every 6°C.

Source: Alexander Merz, « Darks en astrophotographie : le guide complet », sterngucker.de

See also Flat (image de) Offset (image d') Amp glow Empilement

The second of the two coordinates that place an object in the sky: its height above the celestial equator. It is the equivalent of latitude. It runs from minus 90° at the south pole to plus 90° at the north pole. Compared with your own latitude, it says straight away whether an object rises high enough where you are.

Source: Jacques Gispert, Association Andromède, lexique d'astronomia.fr

See also Ascension droite Latitude

The instrument is shifted by a few pixels between two exposures, at random. At stacking, the fixed flaws of the sensor no longer fall in the same place and are erased. The background comes out cleaner, without one extra second of exposure.

See also Empilement Amp glow

A filter that lets only two very precise colours through: hydrogen alpha and ionised oxygen. They are the ones emission nebulae give out. The filter stops almost all the rest, light pollution included. Those nebulae then come out in a single night.

See also Hydrogène alpha Nébuleuse en émission

The piece of sky covered by one single pixel, measured in arcseconds. It depends on the focal length and on the size of the pixels. Too large, and the image lacks detail; too small, and the same light is spread over more pixels for nothing. The seeing sets the useful limit.

See also Focale Capteur CMOS Seeing

Many short exposures are added together to make one single image. The signal of the object adds up, the noise cancels out. It is the founding principle of the smart telescope.

See also Rapport signal sur bruit Temps d'intégration

A mount one of whose axes is parallel to that of the Earth. The field no longer rotates, and the exposures can run longer. In exchange it has to be polar aligned, which means setting that axis on the pole.

See also Azimutale (monture)

A photograph of an evenly lit surface, taken to record the flaws in the optics. It shows the dust and the darkening at the edges. The real exposures are then divided by it, and the background goes flat again.

See also Dark (image de) Vignettage

The distance between the optics and the point where the image forms. A long focal length magnifies, but narrows what is seen at one go. A short focal length shows wide.

A collection of billions of stars, gas and dust, held together by gravity. Four shapes are told apart: spiral, lenticular, elliptical and irregular.

The function that brings the instrument onto the object asked for, on its own. You choose a target from a list, the device goes there and centres it. On a smart telescope, GoTo leans on star recognition and not on the mechanics alone: it lands correctly even if the device has been moved by hand.

See also Alignement Résolution astrométrique

The red light emitted by hydrogen. It is a line, which means one single and very precise colour, at 656.3 nanometres. It gives the great emission nebulae their colour. It also gets through light pollution better than the rest of the spectrum.

The Index Catalogue, the supplement to the NGC catalogue, published in 1895 and then in 1908. It holds 5,386 objects discovered after it.

The north to south position on the globe. It decides what can be seen. From 45° north, the whole of the southern deep sky stays below the horizon.

A diffuse cone shaped glow, visible after dusk or before dawn. It is sunlight returned by the dust of the solar system. It disappears from Bortle class 6 onwards: seeing it is a good test of how dark a site is.

See also Bortle (échelle de)

A scale of brightness, the wrong way round: the smaller the number, the brighter the object. With the naked eye, under a good sky, you get down to about magnitude 6.

The faintest star still seen, at a given place on a given night. The larger the number, the darker the sky: 6 in the country, 4 in town. It is the simplest measure of the quality of the sky, and the one used to set a Bortle class.

See also Bortle (échelle de) Magnitude

A catalogue drawn up by Charles Messier in the 18th century. Its last published version, in 1781, holds 103 objects. The 20th century took it to 110, adding objects that Messier and Méchain had seen without listing them. Messier was not after wonders: he was listing what must not be mistaken for a comet. It has remained the best list to start with.

The adjustment that lines the axis of the mount up with that of the Earth. Once it is done, the field no longer rotates and the exposures can run. It concerns equatorial mode only: in alt-azimuth there is nothing to align.

See also Équatoriale (monture) Rotation de champ

The assembly of several neighbouring fields into one single image. A wider area is covered that way than the instrument sees at one go. The fineness of the detail stays what it was.

A cloud of gas that shines with its own light. Nearby stars excite it, and it re-emits on a few very precise colours. It is the family that responds best to the dual-band filter.

See also Duo-bande (filtre)

A cloud of dust that does not shine and returns no light. So it is seen only in the negative, because it hides the stars behind it. The Horsehead is the textbook case.

A cloud of dust that returns the light of a nearby star, most often in blue. It does not react to the dual-band filter. It needs a broadband filter.

An envelope of gas thrown off by a star of medium mass, at the end of its life. It has nothing to do with a planet. The name comes from a confusion among the first observers.

The New General Catalogue, published in 1888 by John Dreyer. It records 7,840 deep sky objects. It is the reference catalogue, the one that comes up most.

An exposure of zero length, or nearly, which measures the level the sensor shows even with no light. It is also called a bias. It is the zero of the electronics, subtracted before everything else. With the dark and the flat, it is the third of the three classic calibration frames.

See also Dark (image de) Flat (image de)

The moment when a planet lies opposite the Sun as seen from Earth. It then passes highest around midnight, and it is at its closest to us. It is the best period to observe it, and it holds only for planets further out than Earth.

Source: Jacques Gispert, Association Andromède, lexique d'astronomia.fr

A camera body function: the sensor shifts by a fraction of a pixel between two exposures, and the software recombines the images. Each pixel then measures all three colours instead of one, and the mosaic of coloured filters, known as the Bayer array, stops holding the colour back. Dithering, for its part, shifts the instrument and not the sensor.

See also Capteur CMOS Échantillonnage Binning Dithering

The halo of towns that lightens the sky background and drowns faint objects. It does not make astrophotography impossible. It lengthens the exposure time and makes the filter more useful.

The length of one single photograph, before stacking. It often runs from 10 to 60 seconds. Two things limit it: the accuracy of the tracking and field rotation.

See also Empilement

Focal length divided by diameter, written f/. The smaller the number, the faster the image forms. An f/2.2 gathers signal several times faster than an f/5.

What decides the final quality of the image. It grows as the square root of time. Doubling the total exposure time gains a factor of only 1.4. To double the quality, the time has to be quadrupled.

See also Temps d'intégration

The software compares the stars it sees to a catalogue. That is how it recognises the piece of sky, and works out where the instrument is pointing exactly. It is what lets a smart telescope find its own place again.

On an alt-azimuth mount, the image turns slowly as the night goes on. Exposures that run too long keep stretched stars from it. It is the reason equatorial mode exists.

See also Azimutale (monture)

The churning of the air above us, which blurs fine detail. The word is English and is used as it stands. Good seeing sometimes counts for more than a dark sky, above all on the planets.

A catalogue of 313 regions of ionised hydrogen, drawn up by Stewart Sharpless in 1959. These objects are very large and very faint. They are cut out for the dual-band filter and for wide fields.

A small device that measures the brightness of the sky background. It gives a number in magnitudes per square arcsecond: about 22 under an exceptional sky, less than 18 in town. It is the Bortle scale put into figures, and it does not depend on the observer's eyesight.

See also Bortle (échelle de) Pollution lumineuse

A family of sensors made by Sony. They are back illuminated, very sensitive in the deep red and low in noise. They are found in almost every recent smart telescope.

The instrument turns at the speed of the sky to hold the object still on the sensor. Without it, the stars become streaks in a few seconds. Its accuracy sets the maximum length of a sub-exposure.

See also Pose unitaire Mise en station

The total exposure time on an object, which means the sum of every stacked exposure. I show it everywhere. It is the figure that explains the difference between two images of the same object.

See also Empilement Rapport signal sur bruit

The ability of the air to let light through. A transparent night is a night with no haze, no humidity and no dust. It decides how deep you can reach, whereas the seeing decides how fine. The two vary separately: a very transparent sky can be very turbulent.

See also Seeing Bortle (échelle de)

The gradual darkening of the edges of the image compared with the centre. It comes from the optics, not from the sky. On an even sky background it shows straight away. It is exactly what the flat corrects.

See also Flat (image de) Rapport d'ouverture

Stacking the images live, and showing them on a screen as they come. In a few seconds you see what an eyepiece would never show. It is the founding argument of the smart telescope.

Our own galaxy, seen from the inside. It forms a milky band across the summer sky. It needs a dark sky and a very wide field, not a telescope.