"sas_merge" is a simple command line tool for combining night sky
images, either to reduce noise or produce "star trails".
It can also be used to produce virtual long exposure images taken during the
Currently the following features are implemented:
- Dark frame generation.
- "Lighten" mode, for star trail image generation.
- Simple averaging without alignment, for noise reduction and virtual
- Averaging with star detection and alignment, for deep sky images.
Code has been tested and developed on Linux, although there is no reason for
it not to work on other platforms. It depends on the
FreeImage toolkit for image
loading and saving.
The current release tarball can be downloaded
here, or the latest development code can
be pulled from the Mercurial
repository with the command line
"hg clone http://www.spectral3d.co.uk/repo/sas_tools". The build
system currently uses cmake.
To build, type:
% cmake CMakeLists.txt
This should result in the compilation of several executables. Of these only
sas_merge is of interest. The others are test programs for various
Spectral Astro Stacker is intended to be used with pictures taken using either a tripod
or equatorial mount. The image alignment algorithm currently implemented is
only suitable for aligning starry skies, and is unable to handle general scenes.
If you want to use Spectral Astro Stacker to merge daytime images shot by hand then I
recommend aligning them with the align_image_stack tool, distributed
First, you need some pictures. Manual exposure with the same settings for all
frames is best. I usually start by some "dark flat" frames - full length
frames taken in the dark with the lens cap on. These will find any hot pixels,
dark noise etc.
Taking pictures for star trails
For star trail photography you ideally want low noise, long exposure images
with the minimum possible time between exposures. Exact details depend on
the brightness of sky, and how many stars you want to get. If you are well
away from sources of light pollution then a single long exposure at
ISO 100 or 200 may be acceptable, with no stacking required.
Taking pictures for sharp stars
For sharp stars you need to either use an equatorial mount, keep the exposures
short or both. A good rule of thumb for a static tripod is
max exposure time = 300/focal length for crop sensor DSLRs, or 500
for full frame sensors.
Running Spectral Astro Stacker
Once you have your images, you can try a merge. For a set of star trail images
then all you need to type is:
% sas_merge -o trails.png -t l -d dark_frame1 -d dark_frame2 -d ... image1 image2 image3 ...
The -t l parameter tells the software to combine by "lightening", i.e.
for each pixel it uses the brightest of all the input pixels. The -d
parameter specifies a dark frame, and -o specifies the output. Any
later arguments are taken to be input files.