bluring rounded bitmaps

[Quixad�]

hi, imaging lingo gurus out there
i�m trying to simulate the effect of an image that goes in and out of focus according to the lens position on a telescope. you can see it at http://planeta.terra.com.br/arte/quixada/mast/tele.htm. the problem is that the image must be rounded, and when the blur effect takes place, it spreads in order to take the whole rectangular area of the image, disregarding the alpha channel of the original bitmap.
i tried to put a mask (the background with a hole in the middle) above the blured sprite, but it seems that the effect is kinda "direct to stage". is there a way to constrain the blur effect on a circular area, or prevent it to happen on an alpha channel area?
thanks a lot,
q


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