  Cluster categories of hereditary algebras have been introduced as orbit
categories of their derived categories. Keller has pointed out that for
non-hereditary algebras orbit categories need not be triangulated, and he
introduced the notion of triangulated hull to overcome this problem. In this
paper we study the image if the natural functor from the bounded derived
category to the cluster category, that is we investigate how far the orbit
category is from being the cluster category. We show that for wide classes of
non-piecewise hereditary algebras the orbit category is never equal to the
cluster category.
