Glossary — Bhapstar Astrophotography

A

alt-azalso altazimuth, alt-azimuth: A mount that moves up and down and left and right, like a camera tripod. Simple to use, but the view slowly rotates during a long exposure.
altitude: How high something is above the horizon, measured in degrees. Zero is the horizon and 90 is straight overhead. The higher it is, the less air you are looking through. Read more
aperture: The size of the opening the light comes through, written as an f number such as f/2.8. Confusingly, a smaller f number means a bigger opening and more light.
apochromatalso apochromatic, APO: A lens design that brings the red, green and blue parts of light to the same point of focus. Without it, bright stars pick up a purple halo. Read more
apparent fieldalso apparent fields: How wide the view looks through an eyepiece, as opposed to how much sky it actually covers. A wide apparent field feels like looking through a window rather than a straw.
apparent size: How big something looks from Earth, rather than how big it really is. Measured as an angle, so the Moon and the Sun have almost the same apparent size even though one is vastly larger.
APS-C: A sensor size smaller than full frame and larger than the ones in most astronomy cameras. It sits in the middle of the range for how much sky it covers.
arcsecondalso arcseconds: A very small angle, one 3600th of a degree. The full Moon is about 1800 arcseconds across.
ASCOMalso Alpaca: A shared language that lets astronomy software and hardware from different makers talk to each other, so a mount from one company works with software from another.
ASIAiralso ASIAIR, ASIAir Plus, ASIAIR Plus: A small controller from ZWO that runs the mount, camera and focuser from a phone or tablet. It replaces the laptop at the telescope. Read more
astronomical darknessalso astronomical twilight: The part of the night when the Sun is far enough below the horizon that it adds no light to the sky at all. It starts well after sunset and is the only time the faintest things can be photographed properly. Read more
autofocusalso autofocuser, auto focuser: A programmed routine through an Electronic Automatic Focuser that refocuses the telescope as the temperature drifts through the night, or at set timed intervals. Read more
azimuth: The compass direction of something, measured round the horizon from north. Due east is 90 degrees. Read more

B

back-illuminatedalso back illuminated: A sensor built with its wiring behind the light-sensitive layer rather than in front of it, so less of the incoming light is blocked on the way in.
backfocusalso back focus: The exact distance a flattener or reducer needs between itself and the sensor. Get it wrong and the stars at the edges of the frame stretch into little dashes, and no amount of processing will fix it. Read more
background extraction: The processing step that measures that uneven glow across the frame and removes it, leaving an even sky behind the target.
backlash: The small amount of free play in a gear or a focuser before it starts moving. Measuring it once lets the software allow for it.
Bahtinov maskalso Bahtinov: A slotted cover placed over the front of a telescope. It turns a bright star into crossed spikes that line up only when the focus is exactly right.
bias framealso bias frames: The shortest exposure the camera can take, with the lens capped. It records the small electrical signal the sensor adds to every single frame. Read more
Bortle: A scale from 1 to 9 for how dark a sky is. Class 1 is a truly dark desert sky and class 9 is the middle of a bright city. Read more
broadband: A filter that keeps most of the spectrum and blocks only a slice of it. It suits galaxies and star clusters, which shine in every colour rather than a few. Read more

C

calibration framealso calibration frames: Extra frames that are not pictures of anything. They record what the camera and the optics add to every shot, so it can be measured and taken back out. Read more
celestial equator: Earth's equator projected out onto the sky, exactly halfway between the two celestial poles.
celestial polealso celestial poles: The point the whole sky appears to turn around. In the north it sits very close to Polaris, and in the south nothing bright marks it. Read more
celestial sphere: A useful pretend picture: the sky as a giant ball with Earth at the centre and everything painted on the inside of it.
clipping: When part of the picture is so bright it records as pure white, with no detail left in it. Nothing can bring that detail back afterwards.
coma: A fault that makes stars near the corners of the frame grow small wings, so they look like tiny comets rather than points.
continuum: Light spread smoothly across every colour, which is how stars and galaxies shine. Glowing gas clouds do the opposite and emit at a few exact colours.
counterweightalso counterweights: The heavy block on the far end of a traditional mount's shaft. It balances the weight of the telescope so the motors are not fighting gravity.
Crayford: A focuser design that grips the tube against a smooth rod rather than a toothed rack. It is smooth and it does not shift the image sideways as it moves.
culminationalso culminate, culminates, culminating: The moment an object reaches its highest point in the sky before it starts back down. It is the best time to photograph something, because that is when you are looking through the least air.

D

dark current: Signal that builds up in a sensor because it is warm rather than because light arrived. It roughly doubles for every six or seven degrees, which is why cameras get cooled. Read more
dark framealso dark frames, darks: An exposure with the lens capped, the same length and temperature as your real ones. It records the heat noise and the stuck-on bright pixels, so they can be subtracted. Read more
declination: The sky's version of latitude, measured in degrees north or south of the celestial equator. Straight above the North Pole is +90 degrees. Read more
deconvolution: A sharpening step that tries to undo the blurring added by the air and the optics, using the shape of the stars to work out how much blur there was.
deep-skyalso deep sky: Anything beyond our own solar system: nebulae, galaxies and star clusters. Faint, far away, and the reason for long exposures.
dew: Water that condenses out of the air onto cold glass overnight and quietly fogs everything. A small heater strip or a cloth hood keeps it off.
dew heateralso dew heaters: A low-power heater strip wrapped round the front of the telescope. Holding the glass a degree or two above the air temperature is enough to stop dew ever forming on it.
ditheringalso dither, dithered: Shifting the aim by a few pixels between frames on purpose. Faults in the sensor then land somewhere different each time and average away in the stack.
doubletalso doublets: A lens made of two pieces of glass. Cheaper and lighter than three, at the cost of slightly less perfect colour correction.
drizzlealso drizzling, drizzled: A stacking method that uses the tiny shifts between frames to recover a little more real detail than a single frame holds.
dual gain: A sensor with two amplifier settings built in. At the point it switches, the grain drops sharply, so one particular ISO is quieter than several lower ones.
dual narrowband: One filter with two windows open at the same time, usually the red light from hydrogen and the blue-green light from oxygen. It suits a colour camera, which records both at once. Read more

E

EAFalso electronic autofocuser: ZWO's motorised focuser. A small motor takes the place of the focus knob, so focus can be set and corrected by software instead of by hand.
earthshine: The faint glow on the unlit part of a crescent Moon. It is sunlight that has bounced off Earth and lit the Moon's night side.
ecliptic: The path the Sun traces across the sky over a year. The Moon and the planets are always found close to it.
EDalso ED element, ED glass: Extra-low dispersion glass, a specific type of glass used in better lenses. It keeps the different colours of light travelling together, which is how a two-element lens gets close to the colour correction of a three-element one.
emission linealso emission lines: One exact colour that a glowing gas gives off. Hydrogen produces a strong red one, oxygen a blue-green one, and filters are built around their positions.
emission nebulaalso emission nebulae: A cloud of gas that glows with its own light, in a few exact colours, because nearby stars are energising it. This is the kind that narrowband filters are built for.
EQ modealso equatorial mode: Tilting an ordinary up-down, left-right telescope so one axis lines up with Earth's axis. The view then stops slowly rotating, which allows much longer exposures.
equatorial mountalso equatorial mounts: A mount with one axis tilted to line up with Earth's own axis. Once it is aligned, tracking the sky is one steady turn on that axis at a single constant rate. It still has a second motor for the other axis, but that one is for pointing and small guiding corrections rather than for keeping up with the stars, and the framing never rotates during an exposure. Read more
extender: The opposite of a reducer: an extra lens that lengthens the focal length, so you get a closer view of a smaller patch of sky.
eye relief: How far your eye can sit from an eyepiece and still see the whole view. Generous eye relief matters if you wear glasses.
eyepiecealso eyepieces: The small lens you put your eye to at the back of a telescope. Swapping it for another one changes how much bigger everything looks. Read more

F

false colouralso chromatic aberration: A coloured fringe, usually purple, around bright stars. It happens when a lens cannot quite bring every colour of light to the same point of focus.
fast opticsalso fast optic: A telescope or lens with a small f number, which gathers light quickly. It is more demanding: focus is more critical and narrow filters work less well.
field flatteneralso flattener, flatteners: A small lens that corrects the naturally curved focus of a telescope, so the stars in the corners are as round as the ones in the middle. Read more
field of view: How much sky fits inside one frame. A wide field shows a whole constellation, a narrow one a single small galaxy.
filamentalso filaments: A thin bright thread running through a nebula. Often it is not a thread at all but a sheet of gas seen edge-on: looking along the fold stacks the same faint material over a much longer path, so it brightens into a line while the face-on parts stay invisible. Read more
FITS: The file format astronomy cameras and software use. As well as the picture it stores a record of how the frame was taken, such as the exposure and the temperature.
flat framealso flat frames: A shot of an evenly lit blank surface, taken through the same telescope. It maps the dust shadows and darkened corners so they can be divided out. Read more
focal length: How much a lens or telescope magnifies, measured in millimetres. A small number gives a wide view of the sky, a large number a narrow, closer one. Read more
focal ratioalso f-number, f number, f/number: The focal length divided by the width of the lens, written like f/5.8 and often called the f-number. Smaller numbers gather light faster, so you need less time on the target, but focus becomes more critical.
focuseralso focusers: The part that slides the camera in and out until the stars are sharp. An electronic one does it with a small motor, on command. Read more
full-framealso full frame: A camera whose sensor is the size of an old 35mm film frame. Bigger sensors gather more light, which is why they cope better in the dark. Read more

G

gain: The same idea as ISO, but on an astronomy camera. More gain gives a brighter frame with more grain in it.
goto: A mount that will drive itself to any object you name, once it knows where it is pointing.
gradientalso gradients: An uneven brightness across the frame, usually because light pollution was worse on one side of the sky. Software measures it and takes it back out.
GraXpert: A free tool that specialises in removing uneven background glow from an image, and in reducing grain.
guide cameraalso guide cameras, guide sensor: A small second camera that watches one star and reports any drift, so the mount can be corrected while the main camera keeps working.
guidescopealso guide scope, guidescopes: The small telescope the guide camera looks through, riding alongside the main one.
guidingalso autoguiding: A second small camera watches one star and sends tiny corrections to the mount, keeping the target locked in place over long exposures.

H

harmonic drivealso harmonic mount, harmonic mounts: A very compact gearbox that can hold a heavy telescope with no counterweight. It is the reason a mount this small can carry this much. Read more
histogram: A chart of how many pixels in the picture are dark, middling and bright. It tells you at a glance whether an exposure is about right.
hot pixelalso hot pixels, stuck pixel, stuck pixels: A pixel that reads bright whether or not any light reached it. Dark frames record where they are so they can be taken out later.
hydrogen-alphaalso hydrogen alpha, H-alpha, Ha: The deep red light given off by hydrogen, at a wavelength of 656 nanometres. It is the single most common thing being photographed in a glowing gas cloud.

I

imaging train: The chain of parts between the telescope and the sensor: flattener, spacers, filter drawer, camera. Every one of them takes up length, which is why the order and the spacing matter.
integration: The total time of every frame added together. Three hours of integration might be one hundred and eighty frames of one minute.
intervalometeralso intervalometers: A timer that presses the camera's shutter over and over on its own, so you can take hundreds of frames without touching it. Many cameras have one built into the menus, and if yours does not, a small plug-in remote does the same job.
inverter: The part of a power station that turns its battery power into mains power for a normal plug. It uses some power itself just by being switched on.
ISO: How strongly the camera amplifies the light it has collected. A higher number makes a dark scene look brighter, but it also makes the picture grainier, so you use the lowest number that still shows the sky.

L

LiFePO4also lithium iron phosphate, LFP: A battery chemistry that lasts for far more charges than ordinary lithium-ion and copes much better with heat. Heavier for the same capacity, which matters less on the ground. Read more
light pollution: Artificial light from streets and buildings that scatters in the air and brightens the whole night sky, hiding everything faint. Read more
linear: How an image comes out of stacking, with the numbers still in proportion to the light that arrived. It looks almost black on screen until it is stretched.
live view: The camera's rear screen showing what the lens is pointed at right now. You can magnify it to check that a star is properly sharp.
lucky imaging: Recording hundreds or thousands of very short frames, then keeping only the sharpest ones and averaging them. It works because the air is not equally unsteady every moment.

M

magnificationalso magnifications: How much bigger a telescope makes something look. It is the telescope's focal length divided by the eyepiece's focal length.
magnitude: The brightness scale for stars, and it runs backwards. Smaller numbers are brighter, and each step of one is about two and a half times.
maria: The large dark patches on the Moon, which are ancient lava flows rather than seas. Early astronomers named them seas before anyone knew better, and the name stuck. Read more
meridian: The imaginary line running from due north, straight up over your head, and down to due south. Everything is at its highest as it crosses this line.
meridian flip: As a target crosses the meridian, a tracking mount has to swing the telescope over to the other side so the tube does not collide with the tripod. Software handles it and then finds the target again. Read more
micronalso microns: A thousandth of a millimetre. Sensor pixels and focus tolerances are both measured in them, and both are small enough that a few tens of microns matter.
Milky Way core: The bright, crowded middle of our own galaxy, seen edge on. From the northern half of the world it sits low in the south in summer.
mosaicalso mosaics: Several overlapping frames joined together into one much wider picture, for targets too big to fit in a single view.

N

nanometrealso nanometres, nanometer, nm: A billionth of a metre, the unit colours of light are measured in. A 7nm filter opens a window seven of those wide.
narrowband: A filter that only lets through the few exact colours that glowing gas clouds give off, and blocks nearly everything else. Streetlight is mostly blocked, the nebula is not. Read more
nebulosity: The faint glowing gas itself, as opposed to the stars in front of it. Fainter than it looks in a finished picture, which is why it takes hours to record.
neutral density: A filter that dims everything evenly without changing the colours. On the Moon it simply makes the view comfortable to look at.
NINA: A free Windows program that runs an imaging session from a laptop, controlling the mount, camera and focuser through the night.
NPF rule: A formula for the longest exposure you can use before stars stop being points and start being short lines. It takes account of the lens, the aperture and the size of the pixels, so it is stricter than the older 500 rule.

O

objectivealso objectives: The main lens at the front of a refractor. Its width is the telescope's aperture, and it is the part that does the real work.
OD: Optical density, the measure of how hard a filter blocks. Each step of one blocks ten times more, so OD5 stops all but about one part in a hundred thousand.
OIIIalso O III: The blue-green light given off by oxygen in a glowing gas cloud. It is one of the two colours most nebula filters are built around.

P

passbandalso passbands, bandpass: The window of colours a filter lets through. A narrow passband blocks more streetlight, but leaves less margin if anything shifts. Read more
payload: The weight a mount is built to carry. Loading one near its limit works, but everything about it behaves worse than it does at half.
periodic error: A small tracking error that repeats every turn of the mount's gears, because no gear is perfectly round. Guiding corrects it.
planetarium softwarealso planetarium app, planetarium program: A program that draws the sky as it appears from a chosen place at a chosen time, and lets you run time forwards and backwards. Used for working out what is visible before going outside.
plate solvingalso plate solve, plate solves, plate solved, plate-solve, plate-solves: The software takes a quick test frame, matches the pattern of stars in it against a catalogue, and works out exactly where the telescope is really pointing. It then nudges the mount onto the target. Read more
polar alignmentalso polar align, polar aligned, polar aligning: Lining a mount's main axis up with the point the sky turns around. Get it right and one slow, steady turn follows the stars all night. Read more
Polarisalso pole star, North Star: The bright star that sits almost exactly above the Earth's northern axis. It barely moves all night, which is why polar alignment usually starts by pointing at it. Read more

Q

quantum efficiency: How much of the light landing on a sensor actually gets recorded. Eighty per cent means four photons in five are counted and the rest are lost.

R

radiant: The point in the sky a meteor shower appears to stream out from. Meteors near it look short, and the ones further away look long. Read more
RAW: The camera's unprocessed file, holding everything the sensor recorded. A JPEG throws most of that away, and you need it when you come to brighten a very dark picture.
read noise: A small amount of unavoidable grain added each time a frame is read off the sensor. Fewer, longer frames collect it fewer times.
reduceralso reducers: An extra lens that shortens the focal length, giving a wider view and gathering light faster. The picture covers more sky and needs less time.
reflection nebulaalso reflection nebulae: A cloud of dust that has no light of its own and simply reflects the light of nearby stars, usually a soft blue. A narrowband filter blocks it almost completely.
refractoralso refractors: A telescope that uses lenses to gather light, like a very good spotting scope. The other main type uses mirrors instead. Read more
registration: Lining every frame up on the same stars before stacking, so nothing is blurred by the small drifts between them.
remote release: A cable or wireless button that fires the shutter for you. Pressing the camera itself shakes it, and at these exposure lengths that is enough to smear the stars.
right ascension: The sky's version of longitude. It is measured in hours rather than degrees, going all the way round from 0h to 24h. Read more

S

shock frontalso shock fronts, shock wave, shock waves, blast wave: The leading edge of an expanding blast wave, where it is ploughing into gas that was already sitting in its path. The gas is compressed and heated as it is swept up, which is what makes it glow. Read more
shutter: How long the camera collects light for one frame. Twenty seconds means the sensor is gathering light for twenty seconds before the frame is saved.
sidereal ratealso sidereal: The speed the sky appears to turn, one full circle in just under twenty four hours. A tracking mount turns at exactly this rate.
Siril: A free, open source program for turning a folder of raw frames into a finished astrophotograph. It does the stacking and most of the processing.
skyglowalso sky glow: The glow of that scattered artificial light coming back down at you. It is what makes a city sky look washed out rather than black.
slewalso slews, slewing, slewed: The mount moving quickly from one part of the sky to another.
smart telescopealso smart telescopes: A sealed telescope with the mount, camera and processing built into one body, run from a phone. It finds the target, tracks it and stacks the frames on its own. Read more
SPCC: Spectrophotometric colour calibration. It compares the stars in your picture against a catalogue of real measured star colours, and sets the colour balance from that rather than from taste.
spectrum: The full spread of colours light can be, from deep blue through to deep red and beyond at both ends.
SQM: Sky quality meter, a proper measurement of how bright the night sky is. Higher numbers mean a darker sky.
stackingalso stacked: Combining many frames of the same patch of sky into one. The random grain is different in every frame so it averages away, while the real detail is in the same place every time and stays. Read more
star trackeralso star trackers, tracker: A small motorised mount that turns a camera slowly to follow the stars. Much lighter than a full mount and enough for a camera lens.
star trailalso star trails: A picture where the stars are drawn as long arcs rather than points, made by leaving the shutter open, or by stacking a long run of frames end to end. Read more
StarNet: A tool that separates the stars from everything else in the picture, so the nebula and the stars can be worked on separately and put back together.
Stellariumalso Stellarium Web: Free planetarium software. You give it your location, date and time and it draws the sky as it will look, including where a target sits and how it will be framed. Read more
stop downalso stopping down, stopped down: Making the lens opening smaller, from f/2 to f/2.8 for instance. The corners of the picture get tidier and less light comes in.
strain wave: The gearing inside a harmonic drive. A flexible toothed cup is squeezed into an outer ring, which gives a huge reduction in a very small package.
stretchalso stretched, stretching: Brightening the faint parts of an image far more than the bright parts. It is the step that makes a nebula appear out of what looked like a black frame. Read more
subframealso subframes, subs, sub-exposure, sub-exposures, sub length: One single exposure out of the many that get stacked together. Sixty subs of one minute each make one hour of data.
supernovaalso supernovae, supernova remnant, supernova remnants: The explosion of a dying star, briefly bright enough to rival its whole galaxy. What is left behind is a shell of gas thrown outwards at enormous speed, still glowing thousands of years later. That wreckage is the remnant. Read more

T

terminator: The curved line on the Moon between the sunlit half and the dark half. Sunlight arrives almost sideways there, so craters and mountains throw long shadows. It is where all the surface detail is. Read more
thermoelectricalso thermoelectric cooling: Electrically driven cooling with no moving parts, used to hold a camera sensor well below the air temperature so it collects less heat noise.
timelapsealso timelapses, time-lapse: Hundreds of still frames played back one after another as a short video, so hours of sky movement pass in seconds.
tracking: The mount turning slowly to cancel out Earth's rotation, so a star stays in the same spot on the sensor instead of drifting into a line.
transmissionalso transmittance: How much of the light a filter is meant to pass actually gets through. Ninety per cent transmission means a tenth is lost even in the window it is designed for.
tripletalso triplets: A lens made of three pieces of glass. More expensive, but it brings the colours together more completely than a doublet.

U

UV/IR: Ultraviolet and infrared, the light just outside what our eyes can see. Camera sensors pick it up, lenses cannot focus it properly, so a filter blocks it.

V

V-curve: Plot how big the stars look against where the focuser is, and you get a V shape. The bottom of the V is perfect focus.
vignettingalso vignette: The corners of the frame coming out darker than the middle, because less light reaches the edges of the sensor.

W

wavelengthalso wavelengths: What colour a piece of light is, written as a number in nanometres. Around 400 is deep blue, around 700 is deep red.
white balance: The setting that decides whether the picture looks warm and orange or cool and blue. Setting it by hand keeps hundreds of night frames matching each other.

Z

zenith: The point straight up above your head.