Recently, the sky was pretty bad and the transparency has been very low. It washed out the detail and resulted in very poor contrast. The example image was taken on 2007-05-14 1614 (GMT+8).
Poor contrast will make alignment very hard to do, for the registration/alignment point is not very concrete and may even disappear or smear from frame to frame, here's a stacked image under this situation:-
You can see that the image is very smooth, for the detail are smearing and averaged out each others. Finally, wavelet seems to be able to cure it somehow, but notice that the detail are wider than they actually are:-
The only solution to this problem is to choose another more contrasty feature and re-stack again, but sometimes it could be impossible.