In fact I've just found out that this is described in page 78 of "365 days" the book of John knoll, which the nice hatch set behind the scene picture Ady posted is from, I just didn't remembered:

"Build on stage 2, the underside of Cloud City existed in two forms: a full-sized set of the antenna and hatch (Ady's picture), and a forced-perspective-arc model used for vista views. A number of the shots of the underside were taken with moving cameras, so a miniature was chosen over a matte painting to enable true perspective shifting."

Sometimes we just reinvent the wheel :/

BTW this book is a must have, lots of great behind the scene pictures…