The ProScanTM III and OptiScanTM III systems are both high quality, versatile systems allowing fine control and automation of various components of your microscopy set-up. Which one is best suited for your needs depends entirely on what you need your system to do. The OptiScan III represents a balance between precision and economy, whils the ProScan is far more flexible and designed for very demanding optical microscopy applications.
The most important difference is that the OptiScan III can only control the X, Y and Z axes - therefore, if you need to include illumination, slideloaders, or other accessories then you will need a ProScan III. The ProScan is a far more versatile system, and additionally can be upgraded as your needs develop. This is not an option if you have an OptiScan. In addition, a larger variety of stages can be controlled via the ProScan III.
The ProScan III also includes TTL, which is ideal if high speeds are needed in your system. It also allows greater accuracy when positioning stages - sub micron accuracy is achievable with the ProScan III system. By contrast, if speed is not important and above micron accuracy is needed, and you only wish to control the stage and focussing, the OptiScan III system may be more than sufficient for your needs.
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