WalterT I imaged the same target at the same gain, offset, temperature, same night,
Walter, the Dawes Limit of your OTA is 0.9 arc seconds.
In the regular mode of an ASI294, your plate scale (without reducer) is just over 1 arc second per pixel, and at oversampled mode, your plate scale is just over 0.5 arc second. The former is (from Shannon-Nyquist) under-sampled, while the latter should be able to perfectly recover all information from the OTA. With a reducer, the normal mode is even more under sampled.
However, with that said, on an average good night ("seeing" of 2 to 3 arc seconds), the point spread function would be bloated to beyond the Dawes limit of your OTA, and you would not expect the higher resolution mode to be better unless you use very short "lucky imaging" exposures (or use visual) to freeze the seeing, like for planetary work. A pixel scale of 1 arc second per pixel should be perfectly adequate unless you have an exceptional "seeing" night, or you are on top of Mauna Kea.
Are you sure you didn't change focus position between the two plates? If you did, the next time you have a chance, perhaps you can try focusing first with the higher resolution (easier to focus better), then take two images without changing focus in between (assuming temperature has not changed by more than a degree between the two plates). Changing pixel scales should not affect focus.
Chen
