Create Hdri from max scene?

NAIMA

Hi sorry for me newbie question , its my first steps in the world of rendering and I was wondering if I can do an hdri custom . 
Two questiona:
1
I was thinking to make a sort of fake sci fi scene ( almoust a matte painting ) and make an hdri out of it , is it possible ? If so are good tuts around? I see there are lots of tuts on how to use hdri in mental ray but not on how to make a custom one . Also there are many free hdri images and I wanted to work on mixing some of them in PS like adding some planets , clouds or extra stars to make a sort of sci fi scenery directly from other hdri images .
2 
for a space scene instead I. Wanted to setup a stardome , some asteroids a large planed a sun etc and was thinking to create directly from max an hdri , I read its doable with vray , but with mental ray ? Is there a way to or a know how?

thanks a lot for any answer.

AgelosAp

My knowledge with this is pretty basic but I had done something similar a while ago.
One way would be to set your FoV to 90 degrees and a rectangle render output and have 6 views (front,right,back,left,top,bottom) I think there are some PS plugins that can combine different lighting levels of the same image to HDRI. So you would have a skybox basically.
Alternatively you could maybe import your objects to Terragen for instance, but that may not suit what you need.

NAIMA

That way I would generate a cubemap? Though what about a spheremap and the radiation illumination info?