Hi Simon,
Found this in the Series 3 documentation, not very helpful but its something.
This dialog is displayed by selecting the ‘View |Exposure Settings’ menu option, or by clicking the toolbar button. Here you are able to set the exposure time for the image, the individual X and Y binning factors, and you can numerically specify a sub-frame if you wish.
The "Pre" check box, activates the preview mode, where that image download will be about twice as fast. However, quality will suffer. This mode is specially useful for focusing the camera.
Also, a delay between exposures setting is available, since it is desirable to have the auto guider stabilize before starting the next exposure. This can be set in the “Dly(s)” box, and it’s measured in seconds.
You can find the manual at
www.optcorp.com/pdf/Atik/ManualAtikSeries3.pdfDave