Shellscripts for Astrophotography
These are my Linux/Unix shellscripts for postprocessing astro images. The image processing is done by calling ImageMagick.
wfstack
wfstack (WideField-imageSTACKing) is a script for aligning and adding a series of starfield images. The idea behind this script is to achieve the effect of a guided time exposure by taking many ~10sec shots with a stationary camera and adding up the frames. Due to field rotation, both translational and rotational alignment of the captured frames is necessary.
While there is probably better software around for image superposition (such as Registax or AstroStack), wfstack does a quite good job at aligning the images and removing CCD hotpixels. It is also capable of doing the complete processing by simply adding and/or averaging the aligned images.
To my own surprise, a brief comparison between wfstack and AstroStack produced results that were almost identical (the images had to be brightened to the final intensity before processing with AstroStack). Maybe the usual superresolution algorithms are not well suited for this task, but I still suspect I simply did something wrong. If you get different (or even similar) results, please email me.
The wfstack script is run from the commandline in the directory containing the image series to process. The image manipulation is done by calling ImageMagick, preferrably version 5.5.8 or greater (you need version 6.x to make use of all features). The bc commandline calculator is also necessary.
Features/Changelog
v. 0.1
- initial release
- rotational/translational image alignment
- average of all images for noise reduction
- sum of all images for extended effective exposure
v. 0.2
- combination of adding and averaging to fine-tune the brightness
- reads and writes a parameter file to ease re-runs with different settings
- allows to commit subsequent operations with existing aligned images
- hotpixel suppression option
- option to disable scaling for postprocessing with professional software
- optional brightness/gamma adjustment before adding/averaging the images
- minor bugfixes
- more comments and better error handling
- speed improvement by factor 3...4. Wo-hooo!
darksky
darksky is a short script for subtracting CCD noise or the brightness of the sky background from the image to increase contrast. When running this script (darksky <imagename>), you are asked for the coordinates of a rectangle that does not contain visible stars. This region is then averaged and the average subtracted from the image. In the second step, a darker duplicate of the image is added, so that the result spans the full brightness range again.
darksky needs the ImageMagick Package (tested with v. 5.4.4.; probably works with older versions) and the bc commandline calculator for doing the image processing. For use with ImageMagick older than v. 5.5.8, a change to the script is necessary (explained in the script).
Features/Changelog
v. 0.1
- initial release
- subtraction of a brightness/noise average
- brightness adjustment to the full range
- reads and writes a parameter file to ease re-runs with different versions of the same image
Example
This is a portion of an image also shown on my astro-images page at different stages of processing.
Disclaimer
Although I tried my best to ensure proper function of these scripts, they may probably still contain bugs. I take no responsibility whatsoever for file deletion, harddisk failures, power outages or airplane crashs directly or indirectly caused by use of these scripts.
Clear, dark, steady skies!
this page last updated 2004-10-19
