The sensor is sampling the image at the pixel spacing along the sensor edges. At 45 degrees it is sampling at 1.414 X pixel spacing. Dithering randomly then gives you sampling along the diagonals at the pixel spacing. With more frames the image is sampled uniformly at the pixel spacing and then with stacking the resolution actually exceeds the pixel spacing. Note real attainable resolution is twice pixel spacing due to Niquist Theorem. In reality it is a bit worse than this.
In the case of the 300mm lens the stars are undersampled so any enhancement without dithering will result in lovely square stars. Upsizing does not help as the squares just get bigger.
Here is an animated gif showing the difference with dithered and not. 160k
http://d1355990.i49.quadrahosting.co...10_04/card.gif
The effect is more dramatic than reality as the conditions are not identical. I have looked for data that was not dithered to compare to. The only definitive way to do it is to collect a set of exposures without dithering and with on the same night so all other conditions are the same.
Below is a crop of a single frame at native pixel size from the dithered set of corrected tiffs. Note how the stars are square or very blocky. The second image is what it looks like upsized X1.6 by cubic interpolation.
The third is twenty stacked upsized dithered frames. I think the improvement is obvious. The stars are far rounder in the stacked image especially the small dim ones.
These frames were all screen captures so that the pixels in the images did not change from what was actually there.
Bert