What I find out was that in order that the alpha channel to work properly U have to have your background in maya BLACK, but if it is black CANT iluminate whit fg, because is black. SO what i do was make a tipical sphere like DOME object, and reverse the normals and make this object iluminate by changing the incandescense color, kind of blue to simulate the sky light, and then in the object properties (CTRL A) GO to render stats and uncheck primary Visibility, i also uncheck cast shadows, recieve shadows. I also in the metal ray section uncheck visible in transparency. Then I render, and get the ilumination of the dome thank to FG and the background color BLACK. then in after effects When i open my TIFF secuence, the program ask, if you want to ignore the alpha chanel, Straight - unmatted or Premultiplied - Matted with Color, and in this last option U get to chose a color, you HAVE to chose BLACK, and it is in this Premultiplied - Matted with color that the alpha works PERFECT, TIPICALY the Straight - Unmatted works, or so I think but dosent work to good, even with the background black the Straight Unmatted leave a BLACK STROKE in this case. SO With this I can make some nice COMPOSITIONS.. ENJOY, hope U find it usefull and hope my english was good enough