Stacking solar eclipse photos with Fitswork

Deutsche Version

Fitswork is a fully featured astronomical image processing software package written by Jens Dierks. It can be used for stacking solar eclipse photographs, getting rid of the strong radial contrast inherent in these images and enhancing the delicate coronal details. Fitswork is freeware! Meanwhile there are even english menus. The example above has been prepared from 10 digital images stacked and filtered using the procedure described below.  Others may use other software offering similar options and use this page as a guideline.

Step 1: Get ready

Material: DSLR-photographs taken with exposure times rangeing from 1/500s to several seconds. The more images are available the better.  RAW images are much better than JPGs.

Software: Fitswork, other graphics packages for a final brushup (e.g. Photoshop), Canon DDP for RAW conversion. Be sure to get the English language files for Fitswork to follow this tutorial. 

Preparations: Although Fitswork can load RAWs I would strongly suggest to convert them to 16 bit TIFs using RAW-shooter. I wasn't particularly successful with RawDrop and dcraw. One can also use the Canon software.


Step 2: Stack the images

  1. Load the two images with the shortest exposure time File / Open. Display both images conveniently using  Window/Tile vertically
  2. Mark a detail visible in both images by clicking into the image. You can A) click on a detail (a yellow cross will be displayed marking the position). B) you can drag a frame around a star (like delta Cancri on the elcipse 2008 photos and use Settings/Image Correlation Method/Lightness Center. You can also C)  drag a small frame around a consipicious detail visible on both images. Fitswork will automatically match the structures, but you have to change a setting of the program by clicking  Settings/Image Correlation Method/cross correlation).  D) Another method works as follows: Fitswork allows to align and stack images manually, even at sub-pixel accuracy. Load two images and click Bearbeiten/ combine images with manual offset).  In the menu popping up select "subtract"; the difference image makes it quite easy to fit the images precisely. You can move the images pixelwise (integer) or subpixel-wise (subpixel) horizontally and vertically. I find this new option vary nice for stacking shorter exposures on prominces and other detail. 


  1. Add the images: Process/Add images (with offset rotation).  A sum image is created. Fitswork uses 3 x 32 bit for RGB, so the images will never overflow. 


  1. Close the two original images and keep the sum image open
  2. Open the next images and add it to the sum image. Repeat this until all images are added. In the end you will have a sum image containing all information of your eclipse photos. If you want to save the image, use Datei/Speichern unter and ALWAYS use FITS (32 bit floating point per color)

Step 3: Appy the Larsen-Sekanina-Filter

 Larsen-Sekanina filters have been developed to enhance jets in cometary comas,  but they will remove the radial contrast and enhance the structures of the corona, too. Basically a rotational unsharp mask will be used. You can control the filter by changing the diameter and the rotational angle. A Larsen-Sekanina-filter is implemented in Fitswork and can be easily and rapidly used.  

  1. Click the center of the sun, a yellow cross will mark the position.
  2. Click Process/Special Filters/Larsen-Sekanina-Filter 
    A copy of the sum image will be generated and displayed on the screen


  1. Select the radius in pixels and the rotation angle in degrees (Rotation in Degrees). I normally use 2 pixels. As a starting point use 1.31 degrees for the angle. The smaller you choose the angle, the smaller will be the detail enhanced by the filter.

  1. Click Calculate for a preview of the result, and confrm the result with  OK. A Larsen-Sekanina  mask (LS mask) will be computed. That takes a while. You can see the progress on a little blue bar. 


  1. The LS-mask is the dark image at the left. You now need to multiply both images (Process/More Image Combining/Image Multiplication). This will enhance dramatically all detail in the data. If you would prefer a more gentle effect you can also add the mask and the sum image: (Process/Add images (with offset rotation)). In this case you can contol the effect by multiplying factors to the mask (Process/Pixel arithmetic/Muliply value)

  1. Some final scaling on the histogram will be necessary before you save the image in 3X8 bit format (e.g. BMP). Saving is done through Datei/Speichern unter. Saving in 16 bit TIF sometimes yields onion ring effects when the images are loaded in Photohop and should be avoided. It is also wise to keep a copy  of the 32 bit FITS.



Play around with the rotational angle and prepare several versions. If the rotational angle is too small artifacts will appear, if it is too large small detail will be washed out. Since any rotational angle enhances details of different sizes, prepare a variation of several images with different rotational angles (e.g. 0.7 degrees, 1 degree, 1.3 degrees, 1.5 degrees) and stack them. This will remove artifacts and give a more realistic appearance.




More on our eclipse expedition to Turkey

Fred Espenak's homepage shows a method using Photoshop only 

Another good photoshop-based tutorial is given by Jerry Lodriguss

Fred Bruenjes has written an excellent camera control software (freeware), which I also used to capture the images for my composite experiments. 


Hartwig Lüthen, fb4a042 (ätt)