Astrophotography utilities.


"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 day.


Currently the following features are implemented:


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. is 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". The build system currently uses cmake.

To build, type:

% cmake CMakeLists.txt
% make

This should result in the compilation of several executables. Of these only sas_merge is of interest. The others are test programs for various components.

Basic use:

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 with hugin. 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.