Multi-object spectrometry is a special version of imaging spectrometry, where only a relatively small number of objects or interesting image points of a scene is spectrally resolved. This allows to measure nearly all interesting points of a typical astronomical scene within one or only a few measurement steps. The essential component of a multi-object spectrometer is the field selector device, which selects multiple image points for a simultaneous measurement. Reconfigurable field selectors or reconfigurable slit masks can be MEMS, such as micro mirror or micro shutter arrays.Alternative field selectors will be based on micro-mechanical devices and can mostly be referred to as slit positioning systems, since the elements which form the slits are mechanically positioned. Novel examples for such field selectors are an individual micro-mirror element positioner and bar arrays with slit structures. Two layer devices are presented, which form transmissive and reflective slits.The concepts are focused on the near infrared multi-object-spectrometer for the Next Generation Space Telescope.