no alpha channel in shadow pass

Im trying to render a shadow pass to use in a composit (in Maya), and i been having difficulties with it. However i notice that many just turn of primary visibility and have the objects cast shadow. So i try that, but it renders out as pure black and with no alpha channel when i open it in photoshop, what is it im doing wrong?.... Im having the same scene render other passes with alpha channel, so i think the general settings for alpha in maya are ok...

Actually i could need some info in general about shadow passes and how to composit them later, if annyone would like to give some tips?

Thank you

this is how the sunlight with shadow renders out

sunlight with shadow

but this is how the alpha become

alpha

Some places wich is suppose to be covered in shadow is light. In this case i rendered with use background shader with mental ray (in maya), but any time i managed to make an alpha, its the same....i have no idea what to try next

tips??

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