特殊星由四段组成，每段具有不同的周期数。因此，可以在聚焦点的良好调整中将广角和远摄位置聚焦。这对于在没有特殊准直器的情况下定位法兰焦距非常必要。

此外，西门子星可以是场景的一部分，使得清晰度可以集中在固定点。

The special star consists of four segments each with a different number of cycles. So it is possible to focus the wide angle and the tele position in an optimal adjustment of the focusing point. This is essential for positioning the flange focal distance without a special collimator.

Furthermore the Siemens star can be part of the scene so that the sharpness can be focussed definitely to a fixed point.