Image Enhancement
An important step was to enhance the visibility of the object and the trail. Fortunately two additional images were available, taken 15 seconds either side of the image in question, with the camera locked in the same position. It was therefore possible to generate a difference image by subtraction, which considerably enhanced the object and trail.
The next step was to select an area of the image containing just the tail, and to enhance this area by converting to greyscale, normalising and scaling to 'stretch' the greyscale. In the following image, note that the object itself has not been enhanced, just the trail leading up to it.The image seen here is a reduced version of the high-resolution uncompressed tif.
Click on the image or here for a higher-resolution uncompressed .tif version.
The particular significance of this image is that the trail clearly starts inside the image, which tends to eliminate the theories based on ballistic weapons, contrails, and other more esoteric ideas.
This is another version of the enhanced image shown above, with a 10:1 compressed version at the left. Also shown below is another compressed image with a slightly different rotation angle, which appears to be a slightly better fit to the trail. It's quite apparent from the compressed portion of the image that the trail is not as straight as it at first appears.
Click on the image or here for a higher-resolution uncompressed .tif version.
As above, but with slightly more rotation.
(Fairly) obviously, if the trail is not straight then shadows are eliminated, along with other high-energy explanations.
According to the idea that the trail is caused by a flying insect, the wings should have slightly darkened the area to either side of the obvious 'body' trail. To test this, I summed the rows of pixels in a direction parallel to the trail, then normalised the resultant value and replicated it along the pixel row. The expected effect would be to reveal the slight dimming caused by the wings - however, this is not evident (at least, not to my eyes). On the other hand, wings are shorter (lengthwise) than the body and mostly transparent - so the darkening caused by them would be quite difficult to detect.
Note that the centre of the trail appears to be somewhat offset from the object itself; this is because the object appears to have started to curve downwards towards the end of the trail (as can be seen in the horizontally compressed image above), while the lowpass filter reveals the average centre of the trail.
Click on the image or here for a higher-resolution uncompressed .tif version.
It was suggested (by fpj) that it was possible to choose a better rotation value (the value selected for the image above was based on the first half of the trail, which appears less curved than the latter part). Accordingly, an image was processed using a rotation based on a straight line between the start and end of the trail, ignoring the curvature.
Click on the image or here for a higher-resolution uncompressed .tif version.
The image above uses 33.6 degree rotation, and does appear a better fit based on the increased contract between the trail and the surrounding area. There are hints of wing shadows either side of the main trail, but nothing conclusive.
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