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The modification takes place at the company JTW Astronomy, Netherlands. Here is what they are doing:
The standard JTW modification has 3 features. First of all the colour correcting filter is removed. This is used to make the camera less responsive to red light for regular photography. By removing this we greatly increases the sensitivity of the camera to h-alpha - typically by close to 400%! In the Sony A7s the filter stack also contains the anti-aliasing filter, so the image is sharpened also. The filter stack is also very thick (1.6 mm!), the removal of such a thick filter stack further increases sharpness in the corners of images, particularly with fast optical systems as there is a significant loss of image quality caused by this filter stack.
The next step is to reposition the sensor. This is done with a precision of +/- 0.02 mm. The reason for this is to correct the change to backfocus distance that is caused by removing the thick filter stack. By repositioning the sensor it is possible to retain the autofocus ability with lenses and also infinity focus is retained. The repositioning also keeps the sensor surface at the correct distance from the rear of a lens for the corrected field to be retained.
The combined results of this work results in a camera that is 4x more sensitive than the stock Sony A7s and also yields significantly sharper images than a stock A7s.