Dark frames are used to correct for some forms of fixed-pattern noise, that is, noise that does not change from frame to frame or changes slowly from frame to frame. Master dark frames combine the information from a set of dark frames into a single master dark frame. Using master dark frames can save time and effort if a single set of dark frames will be used multiple times, for example with several sets of light frames taken at about the same time, or if a single set of light frames will be processed several times.
The versions of Starry Landscape Stacker, Starry Sky Stacker and Long Exposure Stacker released in the summer of 2020 all support the creation and use of master dark frames.
The dark frames to be combined into a master dark frame must have been exposed with identical settings, and these settings must match the settings for the light frames that they will be applied to. (But some lens parameters--aperture and focal length--are ignored for dark frames). For some cameras with really low noise, 5 dark frames might be enough. For most users, plan to capture 10 dark frames. Capture your dark frames as close in time to the light frames that will be used with. It is best to capture the dark frames immediately before or after the light frames. This is typically done by putting the lens cap on and capturing the required number of images. If your lens cap does not fit tightly or if there will be lights in use while you are capturing dark frames, it may be necessary to cover the camera in addition to putting on the lens cap. If you are supplying RAW file to SLS, SSS or LES, you can capture your dark frames while moving the camera to a new location--the app will automatically compensate for any rotation of the camera. You can save time by shooting light frames for one composition, then collect dark frames while you are setting up for a second composition and then collect light frames for the second composition. You can use the dark frames with both sets of light frames, assuming the exposure was the same.
If you make master dark frames from RAW files (instead of tiff files) then color profile and camera orientation information will be added to the generated master dark. This information will be used to match the white balance and orientation of the light frames the master dark is applied to with the master dark. The master dark will be rotated to match the light frames and the white balance from the master dark will be applied to the light frames. (It would be nice to change the white balance of the master dark to that of the light frames, but it is too late for that. In practice, the light frames determine orientation. The dark frames control white balance. So take care when setting the white balance of your master darks).
Processing dark frames into a Master Dark
You can process RAW files directly, or first convert your RAW files to TIFF files. Using RAW files directly is probably a better approach for most users as it allows the apps to track white balance and orientation information to automatically synchronize the dark frames and light frames. TIFF files do not contain enough information to support this synchronization. If you are going to convert your RAW files to TIFFs with a RAW converter, import your RAW dark frames into your RAW converter (e.g., Lightroom, ACR) and process them exactly as you process the light frames you will apply them to. To minimize the chance of error it is best to processes the lights and darks at the same time and synchronize or copy the settings across all the lights and darks.
Start Starry Landscape Stacker, Starry Sky Stacker or Long Exposure Stacker and open either your RAW files or the 16-bit tif representations you made. All of the apps will automatically detect that you have supplied only dark frames and will make a master dark. When it is done you will be prompted to save the master dark. You might want to change the name of the master dark to something that tells you which light frames the master dark goes with .
It is possible that some dark frames will have so many hot pixels that they might be classified as light frames because the hot pixels look like stars. I have two samples from users that have this property. If this happens to you, a master dark frame will not be automatically made. It will be necessary to repeat the opening process (cmd-O or file->open in the menu bar) and then check the box near the bottom requesting that the image classification table be displayed. When the classification table appears, click on the label "Dark" at the top of the columns to reclassify all of the images as dark frames and continue.