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Because of its particularity, the photos of stars and galaxies are also complicated in post-processing, such as star alignment, stack noise reduction, star extraction, star shrinkage, soft focus, enhancement of details, color adjustment, removal of light pollution and so on. In addition to the commonly used Photoshop software, you may also need to use some professional astrophotography software.

Some people find the post-processing of Star Galaxy photography particularly troublesome and confusing, often because they do not understand the causal relationship and do not know the purpose of each processing. Therefore, before learning post-processing, we should first understand our purpose.

The photographic works of Hoshino are roughly divided into the following parts in post-processing:

1. Reduce noise

two。 Color blending treatment

3. Starry details

01. Noise reduction photographing stars and galaxies must be in dark places at night, and a high ISO will be used, which will cause a lot of noise in the photos, so "noise reduction" is the key point.

Noise is divided into "random noise" and "fixed noise". The position of random noise is random, and the position of fixed noise is the same (mostly due to long shutter exposure).

Black field noise reduction for fixed noise, we can maintain the same shooting parameters, cover the lens cap to take a picture, so that we get a pure black and noisy photo, and the noise is exactly the same as that in the starry sky photo.

Then drag the black field photo to the top of the ps layer, change the blending mode to "difference" or "subtract", and the noise reduction is complete.

In fact, the number of fixed noise is relatively small, even if the noise is not reduced, it can not be seen. Well, it depends on your personal requirements.

Stack noise reduction for random noise, the number and position of each photo are different, so it becomes difficult to deal with. Using conventional noise reduction methods, the effect is very limited, and will also reduce the details of the stars, so we have to use a "lossless" noise reduction method: photo lossless noise reduction: stack steps and principles!

① takes multiple photos

Keep the parameters fixed, do not move the position, take 4-9 photos. If you are worried, you can also take 20 shots. No more is recommended. Too much noise reduction effect is not obvious, but waste processing time, but also test the performance of the computer.

② aligns star points / landscape

The stars rise in the east and set in the west, and the position of the stars has changed in a few minutes of your photo, so align the stars first.

However, the scenery of the ground has not moved, ah, after aligning the stars, the ground has changed. Therefore, we have to use constituencies to separate the stars from the landscape.

Magnification and comparison of 1 picture (left) and 10 noise reduction pictures (right)

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Photoshop

Open Photoshop, File-script-load the file into the stack, browse, select the star photo, and make sure.

Hold down the shift key, click on the first layer, and then click on the last layer, and all the layers are selected.

Ctrl+G, group them, and this group is called Landscape. Then Ctrl+J, copy a group, which is called Star Sky.

Let's deal with the landscape group first. Select all layers, right-click, and convert to Smart objects. Layer, Smart object, Stack Mode, average / Intermediate. Noise reduction complete.

And then deal with the Star Group. Use the selection tool to select the sky part (no matter how you use it, just select it). Add a mask to each layer, and you won't see the landscape after adding it.

Select all layers, edit, and automatically align layers.

Still select all layers, right-click, and convert to smart objects. Layer, Smart object, Stack Mode, average / Intermediate. Noise reduction complete.

Right-click the layer group and ungroup it. At this time, there are only two layers left, splicing the landscape and the sky together, and Ctrl+E merging the layers.

In this way, you get a picture of the starry sky with very little noise.

A big problem: ps's auto-alignment function may not be able to fully align the starry sky. So the photo behind the stack, the original star will appear drag shadow.

Therefore, when the above methods fail, we need to use other image processing software. Like sequator,Kandao RAW. The star alignment function of these software is very powerful, and the stack noise reduction will be carried out automatically after alignment.

Kandao RAW

The operation of this software is very simple, but it can only handle photos in RAW format.

Click add photos and select all starry photos. Change the export path and select the location where you want to save the picture.

Click render, wait a moment, and the noise reduction is complete.

Sequator

Double-click Star images to open all the photos.

Double-click Output to set the folder and photo name for the output.

Choose the stack form: Composition, align the stars (Align stars), and align only the pictures (Align only).

Select the sky area: Sky Region. Irregular area (Irregular mask), draw the sky yourself. Linear gradient (Gradient), make a gradient to distinguish the sky from the ground. Boundary line (Boundary line), automatically identify the boundary of heaven and earth.

All other options can be kept off, then click the Start below, wait a moment, then turn off the software, go to the location where you saved it in step 2, and find the reduced noise photo.

In the next article, we will continue to explain the knowledge points of starry sky detail enhancement and color blending.

To be continued.

This article is from the official account of Wechat: photography Research Society (ID:shoujiPhotography), by Zhang Xiaozhang.

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