Glossary

A

Alt-az and equatorial mounts

An alt-az mount moves up and down and side to side, like a camera tripod head, and a Seestar is one. As it follows a target the field slowly turns from frame to frame, which Stack corrects, and which can leave ragged edges on a master. An equatorial mount (or a Seestar on a wedge) tilts its axis to match the Earth's, so the field does not turn. More on the Stacking page.

Arcminute and arcsecond

Small units of angle on the sky. A degree is split into 60 arcminutes (′), and each arcminute into 60 arcseconds (″). The full Moon is about 30′ across. More on the Plate Solve page.

B

Background extraction

Removing the smooth glow (a gradient) that light pollution, moonlight or haze spreads across a picture, so the sky is even from corner to corner. Nocturne's Background step does it with GraXpert, and can show you exactly what it took away.

Bayer pattern

The grid of red, green and blue filters over a colour camera's pixels, with two green for every red and every blue, so each pixel sees only one colour. Debayering fills in the two missing colours at each pixel from its neighbours, and Nocturne's Stack debayers every sub as it reads it. More on the Stacking page.

Black point

The level at and below which everything shows as pure black. Raising it darkens the sky background; raise it too far and faint detail is crushed to black for good. You set it in Levels.

Bortle scale

A nine-step scale for how dark a night sky is, from 1 at the darkest sites on Earth to 9 in a city centre. Most suburban gardens sit around 5 to 7. A darker sky means less gradient to remove and fainter detail within reach.

Broadband, narrowband and dual-band

A broadband picture takes in all visible light, as through the Seestar's clear IRCUT filter. A narrowband filter passes only the thin slice of colour that one gas glows in. Dual-band passes two such slices at once: the Seestar's LP filter is dual-band, passing Hα and OIII. More on the Narrowband page.

C

Calibration frames

Extra frames that record the camera's own faults so they can be taken out of the subs: darks (sensor glow and hot pixels), flats (dust shadows and dim corners) and bias (the sensor's read-out offset). A Seestar takes its own darks and subtracts them from every sub before saving it, so its subs need no more dark calibration. Nocturne's Stack applies no calibration of its own: it combines the subs exactly as the Seestar saved them. More on the Stacking page.

Clipping

Detail lost by pushing values past either end of the range: crushed means pushed to pure black, blown means pushed to pure white. No later step can bring it back. Nocturne counts both, per colour, under the histogram from Stretch onwards.

Colour space

The agreed meaning of the colour numbers in a file. sRGB is what screens and browsers assume, and the right choice almost always. Nocturne's Export also offers Display P3, Adobe RGB and ProPhoto RGB for 16-bit TIFF, for a program that works in that space; a wider space does not add colour.

D

Declination

The sky's north–south coordinate, like latitude on Earth: 0° on the celestial equator, +90° at the north celestial pole and −90° at the south. Plate Solve's grid labels it in degrees, such as −14°. More on the Plate Solve page.

Deconvolution

Undoing some of the blur the optics and the air add, to tighten stars and sharpen fine detail. It belongs on linear data, and too much puts dark rings round bright stars. Nocturne uses BlurXTerminator if you have RC-Astro, otherwise a free built-in sharpening fallback, which is gentler and is not true deconvolution.

Degree

Angles on the sky are measured like any other angle: 360 degrees (°) all the way round, and 90° from the horizon to straight overhead. The full Moon is about half a degree across, and a Seestar S30 Pro frame is about 4° wide. More on the Plate Solve page.

Drizzle

A way of stacking onto a finer grid than the camera's pixels, using the small drift between frames to recover detail the pixels were too coarse to hold. Nocturne's Stack offers it at ×2 only: the master has four times the pixels, and stacking takes much longer. More on the Stacking page.

F

Field of view

How much sky one frame takes in, as a width and a height in degrees. It is set by the focal length and the size of the sensor: a Seestar S30 Pro covers about 4° × 2.2° (about 4.6° corner to corner, the figure ZWO quotes), an S50 Pro about 2.5° × 1.4°, an S50 about 1.3° × 0.7°. More on the Plate Solve page.

FITS

The standard file format of astronomy, and the one the Seestar saves. Alongside the pixels it carries a header of capture details (target, exposure, filter, camera), which Nocturne reads. A FITS export keeps the data as full-precision numbers, for another astronomy program.

FWHM

Full width at half maximum: how wide a star is, measured where it is half as bright as its centre. Smaller means sharper. Stack uses it to grade frames. More on the Stacking page.

H

Hα

Deep red light from hydrogen, at 656 nm, and the brightest glow of most emission nebulae. Through the Seestar's LP filter it lands on the red pixels, which is why dual-band frames come out red. More on the Narrowband page.

Histogram

A graph of how many pixels sit at each brightness, dark on the left and bright on the right. Nocturne shows it at the top right. On an astro picture most pixels are sky background, so most of the graph is one tall peak towards the left.

HOO

A two-gas palette: Hα in red and OIII in blue, with green made from the two (classic HOO puts OIII in both green and blue). Hydrogen glows red and oxygen teal. It suits dual-band data, and is one of the palettes in Nocturne's Narrowband tool. More on the Narrowband page.

I

Integration time

The total exposure in a master: the number of subs kept times the length of each. 190 subs of 10 seconds make about 32 minutes. More integration time gives a cleaner picture. More on the Stacking page.

L

Linear and stretched

Linear data still holds values in proportion to the light that arrived, so straight out of stacking it looks almost black. Stretched data has had its faint values brightened so the eye can see them. Background extraction, colour calibration and deconvolution belong on linear data; tone, saturation and finishing on stretched.

M

Magnitude

How bright a star looks, on a scale that runs backwards: a smaller number is brighter, and each whole number higher is about 2.5 times fainter. On a dark night the eye reaches about magnitude 6. Plate Solve lists the named stars in a picture with their magnitudes. More on the Plate Solve page.

Master

The single image stacked from all the subs you kept, and the file you open in Nocturne to process. The Seestar app can save one after a session, or Nocturne's Stack builds one from the subs. More on the Stacking page.

Midtones

The tones between the black point and the white point. The Midtones slider in Levels brightens or darkens the middle of the range without moving either end.

N

Noise

The random grain in a picture, from the light itself and from the sensor. It is always there: stretching only makes it visible, and stacking more frames makes it smaller next to the signal.

O

OIII

Teal light from doubly ionised oxygen, at 501 nm. It is usually much fainter than Hα, and through the Seestar's LP filter it lands on the green and blue pixels. More on the Narrowband page.

OSC and mono

A one-shot colour (OSC) camera records colour in a single exposure through a grid of coloured filters over its pixels, and every Seestar is one. A mono camera has no such grid and builds colour from separate exposures through separate filters.

P

Pixel scale

How much sky one pixel covers, in arcseconds. It follows from the pixel size and the focal length: a Seestar S30 Pro pixel covers about 3.7″, an S50 Pro pixel about 2.3″, an S50 pixel about 2.4″. More on the Stacking page.

Plate solving

Working out exactly where in the sky a picture points, by matching its pattern of stars against a star catalogue. Nocturne does it with the free program ASTAP, and the result powers the labels, the RA/Dec grid and photometric colour calibration. More on the Plate Solve page.

Preset

Your own edit saved by name, as a starting point for the next picture of the same kind of object. Unlike a recipe it runs on the picture you have open, where you see every step and can change it. More on the Presets page.

R

Recipe

The list of steps you applied, saved as a small .json file. Batch replays a recipe over a whole folder of masters without opening them. More on the Recipes and Batch page.

Right ascension

The sky's east–west coordinate, like longitude on Earth. It is counted in hours and minutes rather than degrees: the sky turns once in 24 hours, so one hour is 15°. Plate Solve's grid labels it as, for example, 18h20m. More on the Plate Solve page.

S

SHO

The Hubble palette: SII in red, Hα in green and OIII in blue. Real SHO needs a sulfur filter, so Nocturne does not offer it. SHO-style (gold and blue) and Pseudo-SHO in the Narrowband tool borrow only the look, made from Hα and OIII. More on the Narrowband page.

Sigma clipping

A way of combining frames that, pixel by pixel, throws out values that disagree with the rest before averaging. It removes satellite trails, aeroplanes and cosmic-ray hits, and is Stack's usual choice. More on the Stacking page.

Signal-to-noise ratio

How strong the real light from your target is compared with the grain on top of it. Stacking raises it: four times as many frames roughly doubles it. More on the Stacking page.

SII

Deep red light from sulfur, at 672 nm, and the third gas of the Hubble palette. A Seestar has no filter for it, so it cannot be captured, and Nocturne never invents it. More on the Narrowband page.

SPCC (photometric colour calibration)

Setting the colour balance from the stars themselves. Nocturne plate-solves the picture, looks up each star's colour in the Gaia catalogue, and balances red, green and blue so the stars match; it is a simpler cousin of the full spectrophotometric method (SPCC) in programs such as PixInsight. When the file says it was shot through the LP filter it steps aside, because star colours there no longer follow the catalogue.

Stacking

Aligning many subs and combining them into one master, so the random noise averages away while the real signal builds up. Nocturne's Stack grades the frames, sets the poor ones aside, aligns the rest and combines them. More on the Stacking page.

Star separation

Splitting a picture into a starless image (the nebula with the stars taken out) and a stars-only image, so each can be worked on separately and then put back together. Nocturne uses StarXTerminator if you have RC-Astro, otherwise StarNet2, otherwise its own built-in separator.

Stretch

Brightening the faint parts of a linear image far more than the bright parts, so nebulosity and dust show without blowing out the stars. Nocturne's preview is stretched for display all along; the Stretch step writes a real stretch into the image.

Sub

One single short exposure from the telescope, also called a sub-frame or a light frame. A night produces hundreds of them, and stacking combines them into a master. More on the Stacking page.

T

TIFF

A common picture file that, at 16 bits, holds 65,536 levels per colour instead of the 256 of an 8-bit PNG or a JPEG. The extra levels keep smooth sky gradients free of banding when you edit further. Nocturne exports 16-bit TIFF and opens it too.

W

White point

The level at and above which everything shows as pure white. Pulling it down brightens the highlights, but whatever lies above it is blown to white and loses its colour. You set it in Levels.