Four image buttons with arrows (GIF) would control a telescope in all four directions. It will for instance slew left as long as you keep the Left arrow button pressed (mouse down) and stop as you release the image button (mouse up). The mouse up event would send a "stop" command.
A standard button will do alike, however it can't take up images. Using a form's mouse events requires capture of screen coordinates, thus more source code and... error-prone.
Mouse events on an image button would be great for any application that controls external devices via a serial port or otherwise.
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