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FWHM may be a little misleading as I would expect a shorter exposure to always have lower FWHM if only because tracking errors, PE, seeing have less chance to do their damage to FWHM.
Also this test is more for the low noise Sony sensors. For Kodak sensors with higher read noise this may not work as well. There is also a sub exposure calculator on the CCDware site. As I recall from using it in the past the results were more like 7 minutes for many popular Kodak CCDs.
Also it goes against the advice of the very top imagers who recommend long subexposures (again depends on the sensor). Sony sensors have pretty small wells so you are getting an advantage there of not filling up the wells and not making the outer airy disk halos really bright which can also make stars look fatter.
This is a massive advantage of the 16803 chip with its 100K+ well depth. Its hard to overexpose a star unless its one of the super bright ones.
Your example though is quite compelling and opens the door for a nice exposure from a mount with high PE. Not useful though for narrowband where the noise factor is more difficult to overcome.
Greg.
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