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It took me a while to figure it out. It is obvious when you think about it. In a typical digital image the image is sampled at the pixel spacing p vertically and horizontally. At 45 degrees the image is sampled at 1.414p or root two times p. So round stars will tend to look square if the sampling per star is low. Especially after stacking to enhance signal to noise.
How to fix this? My Canon 5DH images are 4358x2912 pixels and each pixel is 8.2 micron which is larger than most DSLR's. Rotate the image by 45 degrees CW in Photo Shop. Then REDUCE the size of the image from about 5k pixels wide to 3k pixels wide this has the effect of resampling the original horizontal and vertical to about 1.4p. Then increase the image to 7k across which now resamples the entire image with more pixels per star than the original. Then rotate 45 degrees CCW and again increase the size to 7k across. You now have an image that is sampled the same in any direction. A tiny bit of star size reduction and RL enhancement in Images Plus. At this 7k size the image is a 190MB tiff. When you are happy with levels etc reducing to about the original size gives an image with nice round stars.
I did this to the four images that made up the mosaic. And then put them together with Registar.
It is obvious when you think about it. The result proves it. I am sure a better protocol could be worked out. I would be interested if any others have had this problem.
Bert
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