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My comparision is sort of valid since despite 6x light collecting advantage (the effective aperture is 145mm due to the large secondary), the exposure is 1/6th as long. Given that the sky will be darker with the 300D image due to the shorter exposure, you would then expect it to see slightly fainter stars if the camera performance is similar. This is what you actually see.
As far as extended detail is concerned, your optical system is only a stop slower so there is a factor of 3 against the 300D image. Hence the 300D image looks a lot noiser.
I know this is not precise and doesn't take into account a number of other factors, but it is a ballpark indication.
I disagree re 3 x 3 not being equivalent to a single 9 minute (it will be >90% as good provided it has been aligned correctly).
I am suprised with your observation re the SXV-H9, but then the newest DLSRs seem to have very uniform sensitivity and don't seem to require the same level of calibration many astroccds need. Perhaps this is the reason...
Terry
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