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The need for speed...
The camera is connected to the computer with a USB cable that has an upper limit on the speed with which it can carry the image frames to the computer.
When imaging the planets, even at extended focal length, even 6MP can become overkill, since the objects are still relatively small and don’t always require such “large” images. The USB limit, when divided by 6MP images, is not particularly high.
So we chop the size of the image down using a ROI (region of interest), sometimes as small as 320x240, at which point the USB cable is capable of carrying many more images per second. We use this to our advantage, in the hope that we can capture moments of good seeing and obtain a (small) percentage of frames within a video sequence that contain good detail.
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