Hello guys,
I have question about render with out geo but keep the alpha. I have a explosion effect around rocket. I only want to render explosion but keep geo light effect and shape to be alpha which can use in Nuke to put rocket render. I use constant shader, in the render rocket it change to black but in alpha chanle it still balck. How you guys slove the question during render?
Thank you!

I got answer form my classmates, in the render setting there is a matte, click it can get render effect but don;t render geo