Understand Nocturne

Getting started

Nocturne turns a stacked smart-telescope image into a finished picture, one guided step at a time. Nothing is permanent until you export, so you can experiment freely: undo, redo, or go back to any step.

1. Install it

Download Nocturne for macOS, Linux or Windows from the downloads page. Install & set up explains the first-launch step each platform needs, and the free tools worth adding. You don’t need any of them to start. What Nocturne works with lists the telescopes, files and computers.

Laptop screens

Nocturne works on any screen from 1280 × 800 up. On a Mac that means the size under System Settings → Displays marked “Looks like”, not the panel’s pixel count; on Linux it is the screen resolution. On Windows it is the resolution divided by the Scale setting under Settings → System → Display.

• 1280 × 800 and up: it works. Choose Icons only for the toolbar in Settings → General, and the tools that still do not fit sit under More. Curves opens its large editor from a button instead of squeezing the curve into the panel, and Stack starts with its help, options and chart folded, one click from open.
• About 1440 × 900: comfortable. A 13-inch MacBook Air’s default (1470 × 956) and a 16-inch MacBook Pro’s (1728 × 1117) are here: everything shows, Stack with its chart open.
• 1920 × 1080 or an external display: best. Room for the preview beside the panels, and for the Plate Solve window beside the picture.

2. Open a stack

Choose Open Image and pick a stacked master: a .fit or .fits file, or a .tif stacked in another program (see below). No stack yet? Point Stack at a folder of raw subs. It sets aside the poor frames, aligns the rest, including the field rotation of an alt-az mount, and combines them into a clean master. If your subs cover several pointings, tick Mosaic: Nocturne stacks each panel, plate-solves them and joins them into one wide image. Mosaic needs ASTAP and takes considerably longer. Press Stack in background to keep using the app while it stacks, with progress in the Jobs panel.

Opening a TIFF

A master stacked in Siril, APP or DeepSkyStacker can be finished here. A picture you have already finished can still use Share, Star Spikes, Upscale and the plate-solve annotations. Nocturne measures the pixels to tell whether the file is still unstretched or already stretched. An unstretched file goes through the whole process, just like a FITS. A stretched one opens at the finishing steps, so nothing stretches it twice. Nocturne says which it chose, and you can correct it if a starless frame or an unusually bright subject fools it.

A TIFF doesn’t record capture details such as the target, frame count, exposure or camera. The Import panel shows only what can be read from the file and its name, and Nocturne won’t guess the camera. An embedded colour profile is honoured: the picture is converted as it opens, so it looks the same as in the program that saved it.

No data of your own yet? Download a sample set: real Seestar captures, free to use and ready to open.

3. Work down the steps

Each step offers a few simple choices, often just Light, Medium and Strong, with a live before-and-after view. The panel explains what the step does. Once you know, fold the help away with “How this works” and it stays folded. When you’re happy, click Next.

Settings

Settings has three tabs. External tools is where Nocturne looks for GraXpert, RC-Astro, StarNet2 and ASTAP. It finds them in their usual places on its own; Test checks one, and Download opens its page.

General shows which version you have (Check now asks whether a newer one is out), your default folder, the handle printed on shared images, and whether the toolbar shows icons with text or icons only. Privacy lists the two things Nocturne sends on its own, the update check and an anonymous daily count, each explained beside its switch.

How the pipeline works

A quick tour of the steps, in the order Nocturne walks you through them. You can skip any you don't need.

Prepare — Stack, Crop, Background

Stack builds a master from raw subs, or you open one directly. Crop trims stacking artefacts at the edges, with composition guides. Background (GraXpert) removes gradients and light-pollution glow so the sky is even and neutral. Afterwards, tick Show what was removed to see the removed glow on its own, and check that it took sky glow and not the faint edges of your target.

BeforeBackground removed

Background extraction on IC 1396A. The difference in brightness from corner to corner falls by 39%: the glow goes and the nebula stays.

Colour — sky balance or photometric

Colour neutralises the sky and sets the white balance. Choose Sky balance, which works offline, or Photometric (SPCC), which measures catalogued stars in your picture to get the colour objectively right. If the result isn’t quite to your taste, two sliders, Green ↔ Magenta and Cool ↔ Warm, shift the overall colour with a live preview. Photometric colour steps aside on pictures shot through a dual-band filter, because star colours seen through it don’t match the catalogue; Nocturne balances on the sky instead and says so. They scale the channels rather than clipping one, so the colour differences between stars survive: an orange star stays orange next to a blue one. Seestar data tends to come out slightly magenta, because of the camera rather than the stacking, so a small nudge toward green is often all it needs. If you shot through a dual-band filter, Colourise and the Narrowband tool turn the Hα and OIII data into a two-gas colour picture in a single click.

Dual-band masterNarrowband

NGC 281 through the Narrowband tool. The oxygen core stands apart from the hydrogen shell around it, although the camera recorded both gases together.

Detail & stretch

Deconvolution sharpens tight star and structure detail while the data is still linear (BlurXTerminator, or a free fallback). Stretch is the big reveal: it lifts the faint signal into the visible range. Recover Core tames blown-out galaxy and nebula cores (HDR), and Levels and Curves fine-tune the tones.

Choosing a stretch by looking at it

The stretch is the one step with no right answer, so Nocturne lets you choose by eye. Visual stretch, on the Stretch panel, shows your own image stretched several ways, and you click the one you prefer. It only sets the slider: Apply still commits, and you can fine-tune the slider afterwards.

It asks two things, one at a time.

Colour first: linked or unlinked. A linked stretch puts one curve through all three channels and keeps the sky’s own colour. An unlinked stretch balances each channel on its own, which evens out the sky and usually brings out more variety in star and dust colour. Both are shown at the same brightness, so you judge only the colour. Neither is marked as correct, because both are fair readings of the same data.

Then brightness. Six real stretches of your image, from darkest to brightest, each labelled with the slider value it lands on and the sky level it gives. Click the picture you like. A darker sky looks cleaner, because the same grain shows much less on it. Go too dark, though, and faint nebulosity disappears, and that loss can’t be recovered later. There is no universal answer: across four of our own targets, the same photographer chose the darkest option three times and one two steps brighter on the fourth.

You can also make the colour choice earlier, at Import, where a Linked / Unlinked switch changes only how the unstretched data is displayed. Nothing is committed there; it lets you see what is in the file before you decide anything. Whichever you pick becomes the starting point at Stretch. Worth knowing: an unlinked stretch completely undoes a photometric colour calibration, because balancing each channel on its own removes exactly that kind of correction. So if you choose unlinked, Nocturne skips the Colour step rather than having you run one that would make no difference.

BeforeLevels

Levels on the same frame: setting the black point takes the sky from 42 to 27 and the picture stops looking washed.

Curves opens a large editor for finer control: drag the curve for all three colours at once or for one channel, or aim it at a single colour range such as the blues. Presets such as Add contrast and Lift faint detail give you a starting point.

Finish

Saturation (with a masked nebula boost), De-green Stars, Noise Reduction, Local Contrast and Star Reduction put on the final polish — each with a live preview and its own undo. Star reduction and the star-aware steps work even without RC-Astro, using a built-in star separator. Star Spikes can add artistic diffraction spikes if you like the look.

BeforeDenoised

Noise Reduction, shown at full pixel scale rather than shrunk to fit. Background noise falls by 93% and the stars keep their brightness.

BeforeLocal Contrast

Local Contrast adds 31% more mid-scale structure. It lifts the background grain with it, which is why it belongs after denoise.

BeforeStars reduced

Star Reduction takes 13% out of the star area. The bright cores stay sharp points; it is the faint wings that go.

Enhancements & Export

Enhancements offers eleven finishing touches: colour boosts for single hues, vibrance, star colour, sky darken and lighten, dark structure, soft glow and nebulosity sharpening. Each tap is a small step you can undo, and taps add up. Export writes the finished image as a 16-bit TIFF, a PNG or a FITS. If you plate-solved, the annotations are burned in, and an exported FITS keeps the WCS. Every export is tagged with its colour space, so other programs show it as you see it here. sRGB is the default and almost always right. A 16-bit TIFF can also be written in Display P3, Adobe RGB or ProPhoto RGB, for a program that works in that space. A wider space adds no colour; it only changes how the same colours are recorded. It isn’t offered for 8-bit PNG, where it would cause banding in smooth gradients.

Late fixes: Trim and fullscreen

Trim is a crop for flaws you couldn’t see at the start. The first crop happens on linear data that is nearly black, so ragged stacking edges and smeared corners often only appear after the stretch. Going back to Crop would throw away every step since, so Trim adds a crop at the end of your history instead, with the same aspect ratios and composition guides. Your whole edit survives, and undo still works.

To find edge problems and judge noise and star shapes, press F for fullscreen: everything but the picture steps aside, and you can pan and zoom around the frame. Esc brings the panels back.

Colour, at the end

Colour Balance, in the toolbar, is for finishing colour work you would otherwise do in another program. Pick a tonal range (shadows, midtones or highlights) and push it toward a colour. Each range keeps its own settings, so you can warm the midtones and cool the highlights in one go. Then narrow Limit to so the change only affects a band of brightness: on a galaxy, that lets you colour the arms and leave the golden core alone. Tick Show the mask and the areas that will change light up while the rest dims. Your stars are separated out and put back untouched. Colour Balance adds to your history rather than replacing work, so you can run it twice with different settings.