Image Processing: Rotational Shift Differencing
Rotational shift differencing has been first described by
and Sekanina. They found out that when a comet image, which has its
mainly as "rays" from the center is rotated and then substracted from
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
The proper angle depends on the comet and the angle between the jets.
the detail will be visible with all angles between 7 and 15 degrees -
the contrast of the image will change. The image below illustrates the
Find the centre of the comets nuclues. Therefore be sure that the image
is not overexposed!
Rotate the image with an angle between 7 and 15 degrees clockwise and
Rotate the image with the same angle as before, but this time
Save this image as well.
Multiply the original image and substract both rotated images.
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
on the distance from the comets core but at the moment I personally
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.
my cometary images I wrote a MIDAS script which applies the rotation
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
image. The processing of the image has been done using a Linux version
of ESO's MIDAS...
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