Rendering clean alpha with physical sun and sky

[SonyBoy]

What's the correct workflow for rendering a good alpha with a physical sun and sky? It seems to intrinsically result in fringing, and even using cryptomatte elements to generate a matte doesn't get rid of it.

Edit: And in fact, what does the "invisible" toggle on the sun node actually do? I'd expect this to have the same effect as the toggle on a dome light, i.e. stop the sun from rendering into the RGB (in other words primary visibility off), but it seems to have no effect whatsoever. That's basically what I'm looking for -- to have the sun light the scene but not appear in the RGB or the alpha. How is this done?

[hermit.crab]

What's the correct workflow for rendering a good alpha with a physical sun and sky? It seems to intrinsically result in fringing, and even using cryptomatte elements to generate a matte doesn't get rid of it.

Could you share a screencap (and a simple scene) illustrating this? Is the alpha fringed if say the scene is lit by a VRayDome?

And in fact, what does the "invisible" toggle on the sun node actually do? I'd expect this to have the same effect as the toggle on a dome light, i.e. stop the sun from rendering into the RGB (in other words primary visibility off), but it seems to have no effect whatsoever. That's basically what I'm looking for -- to have the sun light the scene but not appear in the RGB or the alpha. How is this done?

The invisible option makes the Sun disc invisible to the camera and reflections. It works similarly to the VRayDome light option, the difference being that the reflections toggle is a separate option. What you're seeing is probably the VRaySky, the visibility of which you can control through the Environment Overrides (Render Settings> Overrides> Environment>Background Texture).

[SonyBoy]

Sorry, I was mixing up the sun with the sky... You are correct -- unplugging the background texture on the sky and setting it to black was what I was looking for.