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Stacked Night Sky Post Processing Guide

Updated: Jan 31, 2023




Learning to process night sky images often involves taking multiple images of the night sky and then stacking these images to increase the signal-to-noise ratio and to offer enhanced image quality. Post-processing night sky stacked images involves several steps to enhance the image quality, correct color balance, and remove noise.

  1. Stacking: The first step is to stack multiple images taken of the night sky to increase the signal to noise ratio and reduce noise.

  2. Color Correction: Color correction is the process of adjusting the colors in an image to make it more visually appealing. In night sky images, the colors may appear off balance due to the limited light available.

  3. Noise Reduction: After stacking, there may still be some noise in the image. This can be reduced using software tools like noise reduction filters or noise reduction plugins.

  4. Sharpening: Night sky images may appear blurry due to long exposure times or camera shake. Sharpening the image will help to bring out the details.

  5. Adjustment Layers: Adjustment layers are non-destructive editing tools that allow you to adjust the brightness, contrast, saturation, and other image parameters.

  6. Exposure blending: If there is an overexposed or underexposed area in the image, exposure blending can be used to correct it by blending multiple images taken with different exposure times.

  7. Final Touch-ups: Finally, you can make any final touch-ups or crop the image as desired.





Stacking:

Stacking multiple night sky images is a method to increase the signal to noise ratio and produce a clearer image. The process involves combining multiple images taken with the same camera and lens to produce a single image with a higher signal to noise ratio. The following are some common software and methods for stacking night sky images:

  1. Deep Sky Stacker: Deep Sky Stacker is a free, open-source software that is commonly used for stacking night sky images. It allows you to align, register, and stack multiple images. The software uses a median method to stack the images and eliminate noise.

  2. Sequator: Sequator is a free, open-source software for the Microsoft Windows O/S that is used to stack night sky images either for noise reduction or exposure accumulation.

  3. Starry Landscape Stacker: Starry Landscape Stacker is a paid software that is specifically designed for stacking night sky images. It features a user-friendly interface and includes tools for noise reduction, color correction, and exposure blending.

  4. Adobe Photoshop: Adobe Photoshop is a powerful image editing software that can be used to stack night sky images. The process involves using the "Load Files into Stack" function to load the images and then using the "Median" blending mode to stack the images and eliminate noise.

  5. Manual Stacking Method: In manual stacking, you manually align and stack the images using image editing software like Adobe Photoshop. This method requires more manual effort but gives you greater control over the stacking process.

Regardless of the method you choose, it is important to have good quality, well-exposed images to produce the best results. Before stacking, make sure to crop, align, and adjust the brightness and contrast of each image to ensure they are consistent. Once the images are stacked, you can further edit the image to correct color balance, remove noise, and enhance image quality.






Color Correction:



Color correction is an important step in post-processing night sky images to ensure accurate representation of colors and make the image more visually appealing. Here is a tutorial on how to color correct night sky images:

  1. Initial Adjustments: Start by making any necessary initial adjustments to the image, such as cropping, adjusting brightness, and contrast, and removing noise.

  2. White Balance Correction: The first step in color correction is to adjust the white balance. This involves setting a neutral gray or white point in the image to ensure accurate color representation. In Adobe Photoshop, you can use the "White Balance" tool in the "Camera Raw" filter to make this correction.

  3. Adjusting Color Temperature: The color temperature of an image refers to the overall hue of the image, with warmer hues having more red and yellow tones, and cooler hues having more blue tones. Adjust the color temperature using the "Temperature" slider in the "Camera Raw" filter to achieve the desired look.

  4. Adjusting Tint: The tint adjustment allows you to correct the green-magenta balance in an image. Use the "Tint" slider to correct any color cast in the image.

  5. Adjusting Saturation: Saturation refers to the intensity of colors in an image. Increase saturation to make colors more vivid, or decrease saturation to make colors more muted. Use the "Saturation" slider to adjust saturation to your liking.

  6. Adjusting Vibrance: The "Vibrance" slider in Adobe Photoshop allows you to adjust saturation in a more targeted way, by increasing saturation in less saturated colors while leaving more saturated colors unchanged.

  7. Final Touch-ups: Finally, make any final touch-ups or adjustments to the image as desired.



Noise Reduction:


Reducing noise is an important step in post-processing night sky images after stacking multiple images to increase the signal to noise ratio. Here is a tutorial on how to reduce noise in night sky images:

  1. Initial Adjustments: Start by making any necessary initial adjustments to the image, such as cropping, adjusting brightness, and contrast, and color correction.

  2. Gaussian Blur: Gaussian Blur is a simple and effective method for reducing noise in images. In Adobe Photoshop, use the "Gaussian Blur" filter to apply a gentle blur to the image, removing noise and smoothing out details.

  3. Median Filter: The Median Filter is a method that replaces each pixel in an image with the median value of neighboring pixels. This helps to remove noise and preserve details in the image. In Adobe Photoshop, use the "Median" filter under the "Noise" category to apply the median filter.

  4. Selective Sharpening: Selective sharpening is a method where you only sharpen specific areas of the image, such as stars and other features, while leaving the rest of the image unsharpened. This helps to preserve the natural look of the image while reducing noise. In Adobe Photoshop, use the "Unsharp Mask" filter to selectively sharpen the image.

  5. Noise Reduction Plug-ins: There are several noise reduction plug-ins available, such as the "Noise Reduction" filter in Adobe Photoshop and the "Neat Image" plug-in, which are specifically designed for reducing noise in images. These plug-ins offer more advanced noise reduction techniques and allow for more precise adjustments.

  6. Final Touch-ups: Finally, make any final touch-ups or adjustments to the image as desired.

It is important to be mindful of over-editing and maintaining a natural look in the image. Make small adjustments and use a soft brush to target specific areas if necessary. Experiment with different techniques and find what works best for your particular image.


Sharpening:


Sharpening is an important step in post-processing night sky images to enhance details and make the image look more crisp. Here is a tutorial on how to sharpen night sky images:

  1. Initial Adjustments: Start by making any necessary initial adjustments to the image, such as cropping, adjusting brightness, and contrast, color correction, and noise reduction.

  2. Unsharp Mask: The Unsharp Mask is a popular method for sharpening images in Adobe Photoshop. It works by increasing the contrast of edges in the image, making details appear sharper. To use the Unsharp Mask, go to "Filter" > "Sharpen" > "Unsharp Mask". Adjust the "Amount", "Radius", and "Threshold" sliders to find the best balance of sharpening and preserving details.

  3. High Pass Filter: The High Pass filter is another method for sharpening images in Adobe Photoshop. It works by separating high-frequency details from the low-frequency background and increasing the contrast of the high-frequency details. To use the High Pass filter, go to "Filter" > "Other" > "High Pass". Adjust the "Radius" slider to control the amount of sharpening.

  4. Selective Sharpening: Selective sharpening is a method where you only sharpen specific areas of the image, such as stars and other features, while leaving the rest of the image unsharpened. This helps to preserve the natural look of the image while enhancing details. To perform selective sharpening in Adobe Photoshop, use a layer mask to apply sharpening only to specific areas.

  5. Sharpening Plug-ins: There are several sharpening plug-ins available, such as the "Topaz Labs Sharpen AI" plug-in, which are specifically designed for sharpening images. These plug-ins offer more advanced sharpening techniques and allow for more precise adjustments.

  6. Final Touch-ups: Finally, make any final touch-ups or adjustments to the image as desired.






Adjustment Layers:


Adjustment layers in Adobe Photoshop and Adobe Lightroom are a powerful tool for non-destructive image editing. They allow you to make color and tonal adjustments to an image without permanently altering the original image data. Here is a tutorial on how to use adjustment layers in Adobe Photoshop and Adobe Lightroom:

  1. Create an Adjustment Layer: In Adobe Photoshop, go to the "Layers" panel and click on the "Create a new adjustment layer" button at the bottom of the panel. In Adobe Lightroom, go to the "Develop" module and click on the "New Adjustment Layer" button at the bottom of the right panel.

  2. Select Adjustment Type: Choose the type of adjustment you want to make, such as "Curves", "Levels", "Hue/Saturation", etc. Each adjustment type has its own specific controls and settings.

  3. Adjust the Settings: Use the controls and settings for the selected adjustment type to make the desired changes to the image.

  4. Masking: In Adobe Photoshop, you can use a layer mask to limit the adjustment to specific areas of the image. In Adobe Lightroom, use the "Brush" tool or the "Graduated Filter" tool to mask the adjustment.

  5. Blend Modes: In Adobe Photoshop, you can use the blend mode option to change the way the adjustment layer affects the image. For example, you can use the "Soft Light" blend mode for a more subtle effect or the "Overlay" blend mode for a more intense effect.

  6. Layer Opacity: In both Adobe Photoshop and Adobe Lightroom, you can adjust the opacity of the adjustment layer to control the intensity of the effect.

  7. Repeat: Repeat the process of creating and adjusting multiple adjustment layers to build up a series of adjustments.

  8. Reorder: You can reorder adjustment layers in Adobe Photoshop by dragging and dropping them in the "Layers" panel. In Adobe Lightroom, you can reorder adjustment layers by clicking and dragging them in the right panel.

  9. Save as a Preset: In Adobe Lightroom, you can save a series of adjustment layers as a preset for easy reuse on other images.

Using adjustment layers allows you to make complex edits and adjustments to an image while preserving the original image data. Experiment with different adjustment types and blend modes to find what works best for your particular image.


Exposure Blending:


Exposure blending is a powerful technique for creating high-quality night sky images with a wider dynamic range. However, it can be time-consuming and requires a good understanding of Adobe Photoshop and layer masks. Start with a small number of images and work your way up as you become more comfortable with the technique. Here is a tutorial on how to perform exposure blending on night sky images in Adobe Photoshop:

  1. Load Images: Load the images you want to blend into Adobe Photoshop. Make sure each image captures a different exposure of the same scene.

  2. Create Layers: Convert each image into its own separate layer in the same Photoshop document.

  3. Mask Layers: Use the "Layer Mask" tool to mask out areas of the image that are not needed, such as the sky in one exposure and the foreground in another exposure.

  4. Blend Layers: Use blend modes, such as "Screen" or "Lighten", to blend the layers together. Experiment with different blend modes to find the best one for your particular image.

  5. Fine-Tune: Fine-tune the blend by using a "Brush" tool to paint in the correct exposure where needed. You can also use the "Dodge" and "Burn" tools to further adjust the exposure.

  6. Final Adjustments: Make any final adjustments to the image as desired, such as sharpening, color correction, and noise reduction.


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