Processing with Starry Sky Stacker

There are several ways to open image files with Starry Sky Stacker.

• Launch Starry Sky Stacker and it will present a file selection dialog.
• If you have Starry Sky 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 -> Starry Sky Stacker.”

There are three types of files that can be supplied to Starry Sky 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 50 should be supplied to get good results. Increasing the number of light frames generally increases the quality of the result.
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 Sky Stacker 1.4 and later I recommend at least 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. The single master dark frame can be supplied with the light frames saving 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. See Preparing Master Dark Frames. For version 1.3 of Starry Sky 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.
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.

No matter how you are selecting your files, select all the files that you want to process into a single image. Starry Sky Stacker automatically recognizes light frames, dark frames and flat-field frames. If Starry Sky 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 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.

After the files have been read and classified, processing with Starry Sky Stacker is typically automatic up to the point where you must select which images to include in the final composite. Starry Sky Stacker will first apply the dark frame and flat-field corrections if these frames are present. Then it will align the images and sort them by estimated quality. The quality estimate is a number between 1 (best) and 0 (horrible). The quality estimate is based on the pixel-by-pixel difference between each frame and the frame with the highest contrast. The quality estimates are displayed with the image names in the menu of images at the bottom of the viewing controls.

Choosing Images to Composite

After the images have been sorted by estimated image quality, Starry Sky Stacker stops and lets the user choose which images to composite. Typically, all images with a quality measure better than 0.75 will be included. If less than 15 images have quality estimates better than 0.75, the best 15 images will be included by default. If less than 15 images were opened, all will be included by default. Starry Landscape Stacker will display the best image that is not included in the composition, or the last image if all the images have been included.

At this point you can adjust the images that will be included in the composition. There are two ways you can do this:

1. You can move the slider to select the number of images to include. This will display the quality estimate of the worst image that will be included in the composition.
2. You can select specific images to include or exclude by first making the image you want to include or exclude the current image. You do this with the menu of images names, the Prev and Next buttons, or the forward and back arrow keys on the keyboard. You can then click on the Include button or the Exclude button to include or exclude the current image. You can also hold the option key down and click the Include or Exclude button to include or exclude all images.

Before including or excluding a specific image, you might want to inspect the image closely. To do this, make it the current image and zoom in to some important part of the image. The fastest way to do this is to put the cursor in this area of the image you want to inspect and press the ‘z’ key to toggle to actual pixels, or press ⌘+ to zoom in and ⌘- to zoom out. Pay attention to how round and sharp the stars are. I think it is best to find the worst image you think you want to include in the composition and then reject it and also reject the next one or two better images just to be safe.

Compositing

When you have selected the images to include in the composite, click on the “Composite” button. Starry Sky Stacker will compute 5 composites with 5 different algorithms:

Dark Median Remove outliers, then take the 40th percentile of the remaining values. Similar to median, but darker. Because the noise in the sky usually makes the image brighter, this often produces a better result than median. This is the initial default algorithm, but Starry Sky Stacker will remember the last algorithm you selected as the default.

Median Remove outliers, then take the 50th percentile (median) of the remaining values.

Mean Remove outliers, then take the mean (average) of the remaining values. If you are compositing large numbers of images this should be very similar to Median.

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. If you want, you can select a different algorithm and save the result of it as well. For example, it you are trying to show a meteor in the sky, you will usually save the result from Median or Dark Median and Max and blend the two in an image processing application like Photoshop.

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