An optical imperfection that can make an image less sharp, distorted or incorrectly coloured.
The diameter of a telescope’s main lens or mirror. A larger aperture generally collects more light and can resolve finer detail.
The angle of sky that appears visible through the eyepiece, measured from one edge of the view to the other. It is a characteristic of the eyepiece itself – for example, an eyepiece may have an AFOV of 50°, 68° or 82°.
A relatively small rocky or metallic body orbiting the Sun.
The measurement of the positions and movements of objects in the sky.
The separation of different colours of light by Earth’s atmosphere. It can produce coloured fringes around planets and stars, particularly when they are close to the horizon.
A technique for seeing faint objects by looking slightly to one side of them rather than directly at them. This places the object on a more sensitive part of the eye.
The distance between the rear optical reference point of a telescope or accessory and the position where the image comes to focus. It is particularly important when combining cameras, reducers, flatteners and other accessories.
A camera sensor designed so that light reaches the photosensitive area more efficiently, potentially improving sensitivity.
Small unwanted movement caused by mechanical play in a mount’s gears. It can affect accurate focusing, tracking and guiding.
Internal structures in a telescope that block stray light and reflections, helping to improve contrast and make faint objects easier to see or photograph.
A focusing aid placed over the front of a telescope. It produces a diffraction pattern that makes precise focusing easier.
An optical accessory that increases a telescope’s effective focal length and therefore increases magnification.
A very short exposure taken with the camera shutter closed. It records the camera’s electronic offset and read noise and can be used as a calibration frame.
The number of brightness levels an astronomical camera can record for each pixel. Higher bit depth allows finer differences in brightness to be recorded, which can be useful when processing faint astronomical images. An 8-bit camera can capture 256 brightness levels, a 12-bit camera can capture 4,096 levels and a 16-bit camera, 65,536 levels.
A filter that passes a relatively wide range of wavelengths while blocking or reducing some unwanted light.
An image used to correct unwanted characteristics in astrophotographs. Common calibration frames include dark, flat and bias frames.
A system for specifying the position of an object in the sky, similar in principle to using latitude and longitude on Earth.
Colour fringing caused by a lens focusing different wavelengths of light at slightly different points.
The process of aligning a telescope’s optical components so that they work together correctly.
An optical aberration that causes stars towards the edge of an image to appear stretched or comet-shaped.
An event in which two astronomical objects appear relatively close together in the sky.
The difference between light and dark areas in an image or view. Good contrast can make faint details easier to see.
Reducing the temperature of a camera sensor, usually with an electronic cooler, to reduce thermal noise during long exposures.
A shaped optical element used in some catadioptric telescopes to correct optical aberrations.
A weight used to balance a heavier telescope on a mount.
The process by which your eyes become more sensitive after spending time in darkness. Good dark adaptation can take 20–30 minutes or longer.
An exposure taken with the camera shutter closed or telescope open covered. It records sensor noise and other unwanted signals that can later be removed.
A cloud of dust that blocks light from objects behind it, appearing as a dark patch against the Milky Way or a bright nebula.
The celestial equivalent of latitude. It describes how far north or south an object lies from the celestial equator, measured in degrees. Declination ranges from +90° at the north celestial pole to −90° at the south celestial pole.
A general term for astronomical objects beyond our Solar System, particularly galaxies, nebulae and star clusters.
The spreading of light as it passes through an optical system. Diffraction places a fundamental limit on the finest detail a telescope can resolve.
The theoretical limit to the resolution of an optical system imposed by diffraction.
Lines extending from bright stars in some images. They can be caused by structures inside a reflecting telescope, such as the supports holding the secondary mirror.
An adjustment, usually found on binoculars, that allows the focus of one side to be adjusted independently to compensate for differences between the user’s eyes.
Small, deliberate movements of the telescope between exposures. Dithering helps reduce fixed-pattern noise and other sensor-related artefacts when images are stacked.
An optical lens made from two separate elements, usually different types of glass, designed to improve optical correction.
A method of improving polar alignment by observing the apparent drift of stars.
The range between the faintest and brightest signals that a camera or imaging system can record without losing useful information.
Extra-low-dispersion optical glass used to reduce chromatic aberration, particularly in refractor telescopes.
The stretching of a star or other point-like object into an oval or short line in an image. It can be caused by tracking or guiding errors, poor polar alignment, vibration, or optical problems.
A cloud of gas that produces its own light, usually because it has been energised by nearby hot stars.
A telescope mount with one axis designed to align with Earth’s rotational axis. This allows the mount to track the apparent movement of the sky.
One of the two times each year when the Sun crosses the celestial equator and day and night are approximately equal in length.
The small circle of light leaving an eyepiece and entering your eye.
The length of time a camera sensor collects light for a single image.
An optical effect in which the focused image surface is curved rather than flat. Stars may therefore appear sharp in the centre but softer towards the edges of a camera image.
An optical accessory designed to reduce field curvature and produce sharper stars across a flat camera sensor.
The part of an eyepiece that defines the edge of the visible field.
An optical accessory that allows some wavelengths of light to pass while blocking or reducing others.
A change in the effective wavelength passed by a narrowband filter when light enters it at an angle.
The proportion of incoming light that a filter allows to pass at a particular wavelength, usually expressed as a percentage. For example, an ND filter with 10% transmission allows 10% of the incoming light to pass through, reducing the brightness of the Moon while leaving its colour largely unchanged.
An accessory that holds several filters and allows them to be changed without manually removing them.
A small optical or electronic device used to help point a telescope at an object.
A chart showing the stars and other features around an astronomical target to help locate it.
The distance over which a telescope’s optics bring incoming light to focus. It affects both magnification and the area of sky captured by a camera.
The point where a telescope forms a focused image. An eyepiece or camera is positioned at or near this point.
The focal length of a telescope divided by its aperture, expressed as an f-number such as f/2, f/5 or f/10. A lower focal ratio produces a “faster” optical system, allowing the same amount of light per unit area of the sensor to be recorded in less time. For example, an f/2 system requires roughly 25 times less exposure time than an f/10 system to record the same signal per unit area.
Adjusting the focuser so that the image is as sharp as possible.
The maximum amount of charge a camera pixel can hold before it becomes saturated.
An electronic camera setting that controls how strongly the signal from the sensor is amplified.
A huge collection of stars, gas, dust and other material held together by gravity.
A tightly packed, roughly spherical group of hundreds of thousands to millions of stars held together by gravity. Globular clusters are generally very old and orbit the outer regions, or halo, of the Milky Way. Through a telescope they often appear as a compact, hazy ball of light that can resolve into individual stars with sufficient aperture and magnification.
A computer-controlled system that moves a telescope mount towards a selected astronomical object.
The process of aligning a computerised mount so that its GoTo system can accurately locate objects.
A camera used to monitor a reference star and provide corrections to a telescope mount during astrophotography.
A small telescope used with a guide camera to monitor a star for guiding.
Using a guide camera and software to make small corrections to a telescope mount during an exposure sequence.
A specific wavelength of red light emitted by excited hydrogen. H-alpha is particularly important when observing and photographing emission nebulae and the Sun.
A type of precision mechanical drive used in some modern telescope mounts. It allows a compact mount to carry relatively substantial loads without conventional counterweights.
A graph showing the distribution of brightness values in an image. It is commonly used when assessing and processing astrophotographs.
The boundary between the sky and the ground or landscape as seen from an observing location.
A camera pixel that produces an unusually strong signal even when it is not receiving corresponding light from the subject.
The hour angle is the angular distance of a celestial object westward from an observer’s meridian, indicating the time since it last crossed the meridian.
The amount of sky represented by each camera pixel, normally expressed in arcseconds per pixel.
The area over which a telescope or optical system produces a usable image. A camera sensor extending beyond this area may show severe edge effects.
The total exposure time accumulated from multiple individual exposures of the same object.
A fast-moving band of air high in Earth’s atmosphere. Strong or turbulent jet-stream conditions can make astronomical seeing worse.
An ordinary astrophotography exposure containing the astronomical target. Multiple light frames are usually combined during stacking.
Artificial light that brightens the night sky and makes faint astronomical objects harder to see or photograph.
The ability of a telescope to collect light. It increases as the aperture increases.
The distance light travels in one year. It is a unit of distance, not time.
The faintest magnitude that can be detected under particular observing or imaging conditions.
The total amount of energy emitted by an astronomical object per unit of time.
How much larger an object appears through a telescope compared with viewing it with the naked eye. It is determined by the telescope’s focal length and the eyepiece’s focal length.
A logarithmic scale used by astronomers to describe the brightness of stars and other celestial objects as seen from Earth. Lower or more negative magnitudes indicate brighter objects, while higher positive magnitudes indicate fainter objects. The scale is logarithmic, so a difference of 5 magnitudes represents a brightness difference of 100 times.
A calibration frame created by combining multiple individual calibration exposures.
An imaginary line running from the north horizon through the Zenith (highest point in the sky) to the south horizon.
A procedure used by many equatorial mounts in which the telescope moves to the opposite side of the mount after an object crosses the meridian.
A mount setting that determines how far a telescope can track an object as it approaches or passes the meridian.
The reflective optical element used in a reflecting telescope to collect and focus light.
The mechanical system that supports and moves a telescope. Mounts may be manual or computer-controlled.
Imaging or observing using filters that pass a relatively small range of wavelengths. Narrowband imaging is particularly useful for emission nebulae and can work well under light-polluted skies.
A cloud of gas, dust, or both in space. Nebulae can emit, reflect or block light.
A reflecting telescope design that uses a concave primary mirror and a flat secondary mirror.
Unwanted variations in a camera image or measurement that are not part of the astronomical signal.
Image processing that changes the relationship between recorded signal and displayed brightness, commonly used to reveal faint detail in an astrophotograph.
An event in which one astronomical object passes in front of another and temporarily hides it.
An accessory that diverts a small amount of light from the main telescope to a guide camera, allowing the main telescope itself to be used for guiding.
An electronic baseline added to the signal from a camera sensor so that very low signal levels can be recorded without being lost below zero.
A group of stars that formed from the same cloud of gas and remain relatively loosely bound by gravity.
Occurs when a celestial body (usually a planet, asteroid, or the Moon) appears directly opposite the Sun in Earth’s sky, so the Sun, Earth, and that body are nearly in a straight line with Earth in the middle. Opposition is typically the best time to observe an outer planet or similar body because it is usually closest to Earth in its orbit at this time, and so it looks larger and brighter, it shows a full phase (fully sunlit disk), like a full Moon and it’s above the horizon all night, giving many hours of dark-sky viewing.
An imaginary line passing through the centre of an optical system.
Describes eyepieces, filters or other accessories that require little or no refocusing when changed.
The maximum equipment weight a mount is designed to carry.
The visible surface of the Sun from which most of its visible light is emitted.
An individual light-sensitive element of a digital camera sensor.
The physical dimensions of an individual camera pixel, normally measured in micrometres.
An object that orbits a star and is massive enough to have become approximately spherical under its own gravity.
A glowing shell of gas expelled by a dying low- or medium-mass star. Despite the name, planetary nebulae have nothing to do with planets.
Using software to identify the stars in an image and determine exactly where the telescope is pointing.
A small optical scope built into or attached to some equatorial mounts to assist with polar alignment.
The main light-collecting mirror in a reflecting telescope.
Using a camera directly at the telescope’s focal plane without an eyepiece.
A large structure of relatively cool solar material extending above the Sun’s surface.
The percentage of incoming photons that a camera sensor successfully converts into measurable electronic signal. Higher quantum efficiency generally means greater sensitivity.
Noise introduced when the camera sensor’s signal is read and converted into digital data.
A telescope that uses mirrors as its main optical elements.
A telescope that uses lenses as its main optical elements.
The ability of a telescope or imaging system to distinguish fine detail or visually split two objects that appear close together.
The apparent backward movement of a planet against the background stars as seen from Earth.
The celestial equivalent of longitude. It describes an object’s position eastward around the celestial equator and is normally measured in hours, minutes and seconds, from 0 to 24 hours. RA is used together with declination to specify the position of an object in the sky.
The relationship between the detail resolved by a telescope and the size of the camera’s pixels. Appropriate sampling helps make effective use of the telescope’s resolution.
A condition in which a camera pixel has reached its maximum recordable signal and cannot record additional information.
A smaller mirror used in many reflecting telescopes to redirect light towards the eyepiece or camera.
The steadiness of Earth’s atmosphere. Good seeing produces sharper views and images; poor seeing causes stars to shimmer and fine detail to blur.
The light-sensitive electronic component inside a digital camera that converts incoming light into an electrical signal.
The rate at which a telescope mount tracks the apparent movement of the stars across the sky.
A measure of the strength of the useful astronomical signal compared with the random noise present in the measurement. It is commonly expressed as a ratio: a higher SNR means the signal is more clearly distinguished from noise.
The overall brightness of the night sky caused by natural and artificial sources.
A specially designed filter that safely reduces the Sun’s intense light to a level suitable for observation or imaging. Only filters specifically designed for solar viewing should be used for direct observation of the Sun.
Combining multiple images of the same object to improve the signal-to-noise ratio and reveal more detail.
A map of the night sky showing stars and other astronomical objects.
Finding a faint astronomical object by moving from a bright, easily identified star to nearby stars using a finder, eyepiece and chart.
A compact motorised mount designed to rotate a camera in step with the apparent movement of the stars, allowing longer exposures without star trails. Primarily used for photographing the Milky Way.
A streak produced when a camera records the apparent movement of stars during a long exposure.
Adjusting the brightness and contrast of an astrophotograph so that faint details become visible. Most astronomical images require some form of stretching before they look like a finished photograph.
The movement of a telescope mount following the apparent motion of the stars across the sky.
The passage of one astronomical object across the visible disk of another, or the passage of an object across an observer’s meridian.
How clear the atmosphere is. Good transparency allows faint objects to be seen more easily, while haze, dust and moisture reduce it.
A refractor objective made from three optical elements, designed to provide better colour correction than simpler designs.
The actual width of sky visible through the telescope when a particular eyepiece is being used. A wider TFOV allows you to see more of a large object or more of the surrounding sky.
A camera gain setting at which the conversion between collected electrons and digital signal is approximately one-to-one. It is a useful reference point, but is not necessarily the best setting for every camera or target.
A gradual darkening towards the edges or corners of an image, often caused by the optical system, accessories or an opening that is too small for the light path.
Observing astronomical objects directly through an eyepiece rather than using a camera.
An adjustment to the relative colour levels in an image, used to produce a desired or more natural colour balance.
The distance between an optical element or accessory and the point at which it is intended to work correctly. The exact meaning depends on the equipment involved.
The point in the sky directly overhead.
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