Astro editing

Learning goal

You will be able to reduce the noise in a night sky frame by laying several frames shot from the same framing on top of each other. You will know why averaging the frames reduces random noise, and how this gain grows as the number of frames increases. Because the sky slides from frame to frame, you will use programs that align the stars to each other, process the foreground that stays fixed separately, and combine the two. By removing the uneven brightness that city lights leave in the sky, you will bring the sky to a balanced darkness, pull the sky's color to a natural place and strengthen the stars or the Milky Way without exaggerating. You will know that adding stars that aren't there, or putting in the sky of another night, is not an edit but a combination that must be told. You will keep a person in the frame out of the sky edits and leave their skin in its real tone. With the night track's project you will build a five-frame night selection.

The question

Your friend gathers all the frames of the night track in a folder and enlarges the Milky Way frames on the screen. The sky is noisy; in the dark regions there are small red, green, blue specks, and the sky near the horizon has turned orange on the city's side. On Thursday evening at the club someone opens their laptop: on the screen is the version of ten exposures taken one after another from the same place, laid on top of each other in a single image; the sky like velvet, the horizon even and balanced. How do you get a noise-free night sky?

Context

While working on laying exposures on top of each other, you tried bringing several exposures together in a single image by averaging them (Lesson 22.8). While talking about exposures lasting seconds, you learned to weed out the hot pixels born of the sensor heating up in a long exposure with a dark frame shot with the shutter closed (Lesson 22.1). In the editing section you also used masks that limit an edit to an area (Lesson 21.3).

The night sky is one of photography's most demanding subjects: there is very little light, the sensor works at high ISO and in every frame a random noise appears. Smoothing this noise in a single frame erases detail too. But if you shoot the same frame several times, the real detail is in the same place in every frame, while the random noise is in a different place in each frame. This lesson takes up building a clean night sky by using this difference, and then editing it honestly.

Simple explanation

How do several frames reduce noise?

Image stacking is reducing random noise by laying several night frames shot one after another from the same framing on top of each other and taking their average at each point. The stars and the Milky Way are in the same place, at the same brightness, in every frame, and are kept in the average. The noise, on the other hand, is random; a point that is bright in one frame is dark in the next, and they cancel each other out in the average. Roughly, when the number of frames goes up four times, the noise drops by half: four frames halve the noise, sixteen frames cut it to a quarter.

That is why shooting ten to twenty frames from the same framing on a Milky Way night gives a much cleaner sky than a single frame. Dark frames are shot with the same time and ISO with the lens cap on, and given to the stacking program to separate the hot pixels. Some programs also use frames shot of a flat, bright surface to correct the lens's darkening in the corners. All the frames should be shot with the same settings and in raw.

How is the sliding sky combined?

Star alignment is the program, because the sky slides from frame to frame in stacking, recognizing the stars and seating each frame on top of the others by turning and shifting it according to them. In ten frames shot without a tracker, the sky slides over a few minutes; if the average is taken without alignment, the stars turn into lines again. Stacking programs made for the night sky do this alignment by themselves.

The foreground, on the other hand, doesn't slide; in frames aligned to the stars the ground blurs. That is why the sky and the foreground are stacked separately: the sky aligned to the stars, the foreground without any shifting. Then the two stacks are combined with a mask. Drawing the mask carefully at the edges of trees and hills prevents thin halos from remaining at the edges. If a person in the frame moved during the stacked frames, they leave a ghost; taking their face from a single frame, for example the frame where you lit them with a light, keeps them sharp.

How is the sky balanced?

A sky gradient is the difference in brightness and color, changing unevenly from one edge to the opposite one, that city lights or the air near the horizon leave in the sky. The horizon on the city's side can be orange and bright, the sky overhead deep blue. Many editing programs have tools that find this gradient and take it out of the sky; so the sky comes to a balanced darkness and the Milky Way becomes distinct. Then pulling the sky's color not to jet black but close to a deep, neutral gray gives a natural night.

When raising contrast and saturation to strengthen the Milky Way and the stars, you need to avoid exaggerating; overstrengthening brings the noise back and gives the sky an artificial purple or green. Adding stars that aren't there, or putting in the sky of another night, is not an edit but a combination, and it is told. The color and gradient corrections you apply to the sky can also spread to the skin of the person in the frame; light skin can shift to blue, dark skin to gray. Keeping the person out of these edits with a mask leaves them in their own tone.

Visual evidence

Your friend gives the sixteen sky frames shot from the Milky Way framing with the tree, four dark frames and four foreground frames to a stacking program. The program aligns the stars; in the stacked sky the noise has clearly decreased, the colored specks have vanished, the dust lanes are sharp.

They stack the foreground frames separately without shifting and combine the two stacks with a mask carefully drawn along the tree's edge. They remove the orange gradient on the city's side with a tool; the sky is balanced, dark and neutral. They raise the Milky Way's contrast a little and don't exaggerate the stars.

They take Selin's face from the single frame they lit with a light and protect it with a mask outside the sky edits. Her olive-brown face isn't affected by the cooling applied to the sky. The sharing note reads: "A stack of sixteen sky and four foreground frames; sky and foreground from the same night, from the same place."

Common mistakes

Summary

A noise-free night sky is obtained by stacking several frames shot from the same framing: the real detail stays the same in every frame, random noise cancels out in the average, and as the number of frames goes up four times the noise drops by half. The sliding sky is stacked aligned to the stars, the fixed foreground separately, and the two are combined with a mask; the sky gradient of city lights is removed, the color is pulled to neutral, and the stars are strengthened without exaggerating. Adding stars that aren't there and putting in another sky are told; the person in the frame is kept out of the edits and their skin is left in its real tone.

Mini assignment

The night track's closing project is in this assignment: you will build a five-frame night selection.

1. With the skills you built up through the nights, produce five frames: a city at blue hour, a sky with point stars in a dark place, a Milky Way frame, a star trail or a tracked frame, and a moon frame.
2. Stack the Milky Way frame from ten to twenty frames; stack the sky and the foreground separately, combine them, remove the sky gradient and pull the color to neutral.
3. Add a two-line caption to each frame of the selection: how many exposures were used, whether a combination was made, and if someone is in the frame, what their face was lit with and whether its color shifted.

Check yourself

• How much cleaner is the stacked sky than a single frame?
• Did a halo remain at the edges where the sky and the foreground meet?
• Does each frame's note in the selection clearly say how it was made?

Glossary

Mini quiz

1. Why does image stacking reduce noise?

• A) Because it makes the frames brighter
• B) Because real detail stays the same in every frame while random noise cancels out in the average
• C) Because it stops down the aperture
• D) Because it lowers the ISO

2. When sixteen frames are stacked, roughly what happens to the noise?

• A) It doesn't change
• B) It doubles
• C) It drops by half
• D) It drops to a quarter

3. Why are the sky and the foreground stacked separately?

• A) Because the sky slides while the foreground doesn't
• B) Because the foreground is brighter
• C) Because the program wants it so
• D) Because the sky needs fewer frames

4. What is using the sky of another night in a frame?

• A) An ordinary color correction
• B) Noise reduction
• C) A combination that must be told
• D) Star alignment

Answers

1. B — The stars are in the same place in every frame, while the noise, being random, cancels itself out in the average.
2. D — As the number of frames goes up four times the noise drops by half; sixteen frames is four times four.
3. A — In frames aligned to the stars the fixed ground slides, so the two are processed separately.
4. C — The sky of another moment shows that the frame isn't the record of a single night, and it is told.

Summary card

• Ten to twenty frames from the same framing; stack them, the noise cancels out.
• Four times the frames, half the noise.
• Stack the sky by the stars, the foreground fixed; combine with a mask.
• Remove the sky gradient, pull the color to neutral, don't exaggerate.
• Say it is combined; keep the person out of the edits.

Bridge to the next lesson

The night and astrophotography track ends here: from seeing in the dark to the city's blue hour, from light pollution to planning, from point stars to the Milky Way, from star trails to tracking, from the moon to stacking. After showing their selection at the club, your friend goes back to the city by day. They photograph the stone facade of an old inn up close, from below; on the screen the building leans backward, its corners closing in toward each other at the top. Why do buildings look as if they are falling backward? The next lesson opens the Architecture and Interiors track and takes up keeping the vertical lines straight.