A mechanism is described for changing the signal detector in the Cambridge ``Stereoscan'' scanning electron microscope without breaking the vacuum. This has three main advantages. First, it makes it easier to compare the back scattered electron image, the secondary electron image, and the luminescent image, which can help the interpretation in some cases. Second, a spare secondary electron detector can be kept in reserve for high resolution work, thus avoiding the degradation of scintillator performance that occurs after prolonged use. Finally, it makes it possible to optimize detectors by comparing different versions of the same basic design.