Processing Images with Starry Landscape Stacker

Processing images with Starry Landscape Stacker proceeds in four main steps: 

1. opening the images and finding stars, 

2. editing the “red dots”,

3. making the sky mask, 

4. selecting the composition algorithm and saving.

Opening Images and Finding Stars

There are several ways to open image files Starry Landscape Stacker. 

• Launch Starry Landscape Stacker and it will present a file selection dialog.

• If you have Starry Landscape Stacker running, in the menu bar select File -> Open (or ⌘ O) and a file selection dialog will appear.

• Select your image files in the finder and select “Open With -> StarryLandscapeStacker.”

There are four types of files that can be supplied to Starry Landscape Stacker. Only light frames are required.

1. light frames. These are your normal exposures. Any number of light frames can be supplied. At least 2 light frames must be supplied, but at least 10 should be supplied to get good results. Increasing the number of light frames generally increases the quality of the result. I typically have at least 15, but I have processed as many as 30. 

2. dark frames. These are images captured with the lens cap on. They are used to reduce fixed pattern noise. Any number of dark frames can be supplied. For Starry Landscape Stacker 1.9 and later I recommend 10 dark frames, although some cameras with sensors that have very low noise might see good results with as few as 5. If you are using very high ISO settings or have a camera with a noisy sensor you may need more dark frames. Dark frames can be processed into master dark frames. Opening a single master dark frame instead of all the individual dark frames with a set of light frames saves some time and memory. A single master dark frame can be a convenient way of sharing a single set of dark frames with multiple sets of light frames that were taken at about the same time. For version 1.8 of Starry Landscape Stacker and early versions, I recommend against using dark frames. These versions did traditional dark frame subtraction that requires a very large number of dark frames, at least 5x the number of light frames. Also, these versions could not make a master dark frame. See Preparing Master Dark Frames.

3. master flat-field frame. At most one master flat frame is allowed. Flat-field frames primarily correct for lens vignetting but may also compensate for other lens and sensor defects. If you are happy with the vignetting correction provided by the image editing tools you use after stacking then flat-field frames may not be useful to you. See Preparing Master Flat Frames.

4. mask. A single mask image created with a photo editor (e.g., Photoshop, Affinity Photo) or saved from a previous session Starry Landscape Stacker (the “.-sky.tif” file written as the second file during the save can be used as input to Starry Landscape Stacker to supply a mask). For masks made with image editors, the alpha value (or grey for greyscale masks) should be non-zero for sky and 0 for ground. Supplying the mask is optional and is most useful when the mask needed for the sky can be more easily created with a tool other than Starry Landscape Stacker or when you wish to repeatedly process a set of light frames with the same mask.

No matter how you are selecting your files, select all the files that you want to process into a single image. Starry Landscape Stacker automatically recognizes light frames, dark frames, flat-field frames and masks. If Starry Landscape Stacker is uncertain about the frame types for the files it has read or if there is a problem with the numbers of frames of each type, it will present a table showing all the frames that it read, along with the type of the frame it thinks each frame is, and allow the user to make corrections (see Frame Classification Panel). 

Here are some suggestions on how to organize your files to make them easier to manage.

Editing the Red Dots

After Starry Landscape Stacker reads all the images, it computes the mean of the images to display to the user. If the user supplied a mask, it will jump to the last step, Aligning and Saving. Otherwise it will attempt to find stars in the image as a first step in making a mask for the sky. It will place a red dot at each star it finds.

Sometimes the red dots will thoroughly cover the sky, and nothing else. More often, some minor editing will be needed. You can both paint in more red dots in parts of the sky that are empty and erase red dots if they appear on the ground. You you can vary the opacity of the overlay that shows the red dots to make them more or less visible and vary the brightness and contrast of the image to help you see where the boundary between sky and ground is.

It is common that bright points in the foreground will be mistaken for stars. Red dots in the foreground must be erased. Similarly there may be areas in the sky with no red dots because of fog, cloud, bright horizon due to light pollution or sunset. It may be necessary to paint in some red dots in the sky near the horizon to hint to Starry Landscape Stacker that this region is sky even if there are no obvious stars. In general, the red dots are hints about where the sky is, they should be in the sky and not in the ground. 

Occasionally the automatic placement of the red dots will be of no value. This happens when the the foreground is very bright and there are few visible stars—think of a night cityscape shot, for example. When this happens, it is best to click the button that toggles between starting with “automatic dots” and “no dots”. Starting with “no dots”, loosely draw a row of dots near the horizon, up the sides and across the top. You do not have to draw any dots in the middle of the sky. You can draw the dots up the sides and across the top by clicking where you want to begin a row of dots, and then hold shift while clicking to draw a line of closely spaced of dots from the first click to the shift-click. With a click at one end of the horizong, followed by three quick shift-clicks (top corner, other top corner, other end of horizon), you can outline the entire sky. You can also shift-click while erasing to erase a straight line which is the width of the current eraser. 

Making the Sky Mask

When you are satisfied that the red dots are a good hint at where the sky is, click on “Find Sky”.

Starry Landscape Stacker starts from the red dots and tries to generate a mask for the sky. The mask is shown as a blue overlay over the image. The goal is to have all of the sky that contains stars be masked in blue, and none of the ground masked in blue. If there are dark areas that you cannot be sure if they are sky or ground, it is best to leave them not masked. Sometimes Starry Landscape Stacker will get the mask right, but usually some minor cleanup is need. There are tools to paint and erase the mask and to zoom in to work on details. Sometimes there will be small areas of sky missed along the edges, in the corners, and in dense areas of stars. These areas can be quickly filled in with a large brush.

If your foreground has trees or other objects where you can see stars through gaps in the objects try clicking the button that is alternately named “mask without islands of sky” and “mask with islands of sky”. The option “mask with islands of sky” attempts to find areas that are similar to the sky but separated from it. This can automatically fill in small areas of sky separated by branches and other things. The default is with islands of sky. You may have to select “without islands of sky” if there is something in your foreground that is being mistaken for sky. I have seen this happen with dark rock walls. Even with this option set to “with islands of sky” it may be necessary to carefully paint small areas of sky around the stars and be sure that all of the trees limbs or other objects are not masked. This can be time consuming. To save time, do not paint every bit of sky with the sky mask—it is only necessary to paint the sky mask over visible stars. Areas of sky with no visible stars do not need to be masked.

There are controls to zoom in and to vary the opacity of the mask and to hide/show the mask. You can also vary the brightness and contrast of the image. These may help when working on difficult details.

It is best not to have the mask touching tree limbs or the horizon. You want a few pixels between the ground and the mask, otherwise there is a risk of some annoying artifacts in the stacked image from atmospheric effects or small movements of tree limbs moving something that should be foreground into the edge of the sky. Having a small gap between the mask and the foreground is far better than having no gap. The generated mask may be useful for replacing the foreground with a single long exposure. You can export the mask and use it in Photoshop or Affinity Photo to composite in the foreground exposure. But the mask SLS generates is optimized for aligning and stacking. For blending in the foreground you will probably want to enlarge the mask slightly into the foreground and soften the edge slightly. Photoshop has tools for making these changes to masks. I assume all other app will have tools to make these changes as well.

When you are satisfied with the mask click on the button “Align and Composite”. 

Selecting the Composition Algorithm and Saving

The process of aligning and compositing is automatic. When it is done, a composited image will be displayed. Now you can choose from 6 composition algorithms:

Version 1.9 

Min Horizon Noise Remove outliers, then take the 40th percentile of the remaining values. At the horizon minimize noise at the risk of duplicating stars. This is the default and works very well in many cases. It might not be the best choice if there are bright stars near the horizon, as the stars may be duplicated or appear as streaks.

Min Horizon Star Dupe Remove outliers, then take the 40th percentile of the remaining values. At the horizon avoid duplication of stars at the risk of increasing noise. Use this option only if you get duplication of stars with Min Horizon Noise. Also consider saving the results of this and Min Horizon Noise and blend the two using the results of Min Horizon Star Dupe only where it is needed to avoid duplication of stars.

Mean Min Hor Noise Remove outliers, then take the mean (average) of the remaining values. At the horizon minimize noise at the risk of duplicating stars. The result for this algorithm should be similar to Min Horizon Noise, but it will be slightly brighter, and, if there are few images, it may be slightly noisier.

Mean Min Hor Star Dupe Remove outliers, then take the mean (average) of the remaining values. At the horizon avoid duplication of stars at the risk of increasing noise. The result for this algorithm should be similar to Min Horizon Star Dupe, but it will be slightly brighter, and, if there are few images, it may be slightly noisier.

Max Take the maximum value at each pixel. This is useful when you want to preserve transient lights in the sky like meteors, airplanes and satellites. See the video Meteors and Satellites in Stacked Images.

Min Take the minimum value at each pixel.

Version 1.10

Min Horizon Noise Remove outliers, then take the mean of the 50% of the remaining values centered on the 40th percentile.  At the horizon minimize noise at the risk of duplicating or supressing stars. This is the default and works very well in many cases. It might not be the best choice if there are bright stars near the horizon, as the stars may be duplicated or appear as streaks.

Max Horizon Accuracy Remove outliers, then take the mean of the 50% of the remaining values centered on the 40th percentile. At the horizon avoid duplication of stars at the risk of increasing noise. Use this option only if you get duplication or suppression of stars with Min Horizon Noise. Also consider saving the results of this and Min Horizon Noise and blend the two using the results of Min Horizon Star Dupe only where it is needed to avoid duplication or suppression of stars.

Mean Min Hor Noise Remove outliers, then take the mean (average) of the remaining values. At the horizon minimize noise at the risk of duplicating stars. The result for this algorithm should be similar to Min Horizon Noise, but it will be slightly brighter, and, if there are few images, it may be slightly noisier.

Mean Max Hor Accuracy Remove outliers, then take the mean (average) of the remaining values. At the horizon avoid duplication or suppression of stars at the risk of increasing noise. The result for this algorithm should be similar to Min Horizon Noise, but it will be slightly brighter, and, if there are few images, it may be slightly noisier.

Max Take the maximum value at each pixel. This is useful when you want to preserve transient lights in the sky like meteors, airplanes and satellites. See the video Meteors and Satellites in Stacked Images.

Min Take the minimum value at each pixel.

When you select one of the composition algorithms, you are shown the result produced with that algorithm. You can save that result by clicking on the Save button or with ⌘ S. There is a check box that you can check to also save a copy of the result with the mask if you think you will need it to process the images again, or if you think the mask will be useful in post processing. If you want, you can select a different algorithm and save the result of it as well. For example, if you are trying to show a meteor in the sky you will usually save the result from Min Horizon Noise and Max and blend the two in an image processing application like Photoshop. 

Usually you will finish processing the TIFF files generated by Starry Landscape Stacker with an image processing tool such as Lightroom, Photoshop or Affinity Photo.

Align With (Optional)

By default Starry Landscape Stacker will align all the images with the image that is closest to the mid-point in time between the first image and the last image. You can align to a specific image. You might want to do this because in one of your images a bright star or planet is lined up with some point in the foreground. Also, if one of your images has a bright meteor in it you should probably align to that image to avoid adding minor artifacts from shifting the image with the meteor to align it with some other image. 

You can open the control “Align With” by clicking on it. It tells you the name of the image that the other images will be aligned with. Use the menu of images at the bottom of the viewing controls, or the Prev and Next buttons, or the forward and back arrow keys on the keyboard to change the display to the image you want to align to. Then click the button “Align to current image”. If you align to an image near the beginning or end of the images, the time to align will increase and the quality of the alignment might be slightly worse than normal.

Exclude Images (Optional)

If you want, you can exclude individual images from the composition. You can open the “Exclude Images” control by clicking on it. To include or exclude a specific image, first make it the current image using the menu of images at the bottom of the the viewing controls, or the Prev and Next buttons, or the forward and back arrow keys on the keyboard. Then you can use the buttons to include or exclude the current image. You can hold the option (or alt) key down and the buttons change to Exclude All and Include All. This is a quick way to include or exclude all images before making changes to individual images.

Last updated Dec, 2023.