It is not properly "coma" ... that is usually an element centering error. The more I consider this image, the more I am convinced that is is the result of a lens stretched past its design criteria for coverage - and beyond the area of distortion correction - mainly as as result of uncorrected "flatness of field". The "pincushion" effect is rather easy to see in the ~ even pattern at the lower left and right corners - it is far more difficult in the "sky" area where there are no natural patterns/ distibutions. "fhovie" says he is going to continue with another, longer, enlarging lens. I'd be interested to see what the results look like.