Posted By Oliver Pettenpaul
I'm not too familiar with the SAC7B but I suppose it's a USB1.X long exposure ToUcam family device ? If so, shooting at 25fps lowers image quality noticably. USB1.X has a maximum bandwith of 12MBit/s, transfering 640*480*8Bit*3Pixels at 25fps would in theory require 175.78 MBit/s. To achieve the framerate over the slow bus a lossy online compression is performed by cam and driver - the higher the framerate the worse image quality. Personally I don't use my ToUcams faster than 10fps.
This subject is the reason why todays planetary imagers long for cameras with USB2 or FireWire interfaces with 400MBit/s or more allowing much higher framerates without quality degradation by compression.
Best regards & clear skies,