Brunstein Observatory

Kreiensen, Germany


Image Processing: Rotational Shift Differencing

Rotational shift differencing has been first described by Larson and Sekanina. They found out that when a comet image, which has its structures mainly as "rays" from the center is rotated and then substracted from the raw image, will reveal the structures with much more contrast.

 The way to do this processing is rather simple:

The result will be an image with the comets core region in high contrast. The proper angle depends on the comet and the angle between the jets. Normally the detail will be visible with all angles between 7 and 15 degrees - only the contrast of the image will change. The image below illustrates the processing:


 As you can see from the image above this algorithm only works for radial structures. It also gets less accurate with growing distance from the nucleus of the comet. It would make sense to use an angle depending on the distance from the comets core but at the moment I personally have no idea how to program it using midas.

 If you want a more smoothed image you can calculate more than one rotated image with different angles and add the resulting images. For my cometary images I wrote a MIDAS script which applies the rotation algorithm for angles from 7 to 16 degrees with steps of one degree. All these 10 images are then added.

 The following image shows an example of the comet Hale Bopp.:


 Left part: logarithmically scaled image, right part: the rotated image. The processing of the image has been done using a Linux version of ESO's MIDAS...


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