Jay_UH Loc: Tokyo, Japan

When using the same telephoto lens (ED 300mm 1:4.5) for astrophotography, I find the images (raw) generated by my D750 are sharper than the images generated by my D500 when viewed on my monitor and magnified to the same size. Put aside settings and considering my application, is this to be expected?

Wallen Loc: Middle Earth

Some possibilities:

1. If you view them at the same size, It is possible that you are viewing the D750 in its native size, thus it will look its best while the image of the D500 is being interpolated from a bigger file. Interpolation may look better at certain image pixel ratio and maybe at the viewing size you chose, it was not favorable for the crop file.
Because having different sensor size, the image of the D500 should look bigger.

2. It could be the dynamic range/low light capability difference between the D750 and the D500.
https://www.photonstophotos.net/Charts/PDR.htmd

3. It is also possible that the image missed/front/back focus on the other camera while shooting.

4. Were all variables eliminated? Are the tripods the same? Same day, no clouds roll in? equivalent Camera settings?

5. I may also be personal bias, if so try comparing multiple images in a blind test.

6. At the same image size crop, the D500 will have denser pixel count and may be showing details that were just smoothed out and sharpened in the D750 thus appear less defined.
Higher pixel density are less forgiving for mistakes & glass quality.

Either way, even if you do not find a cause, just choose what you feel is the better tool for the job.

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Try viewing this thick & delicate line zoom out. At 30% view in PS the fine line do not look ok

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Jay_UH Loc: Tokyo, Japan

I sincerely appreciate your feedback.

The lens was mounted on a capable tripod, its focus set to infinity, Aperture set to f8. I simply switched between bodies - ISO fixed at 100 and utilized various Shutter Speeds. Both bodies confirmed (green dot) that my subject (focus point at the moon's edge) was in focus.

Your advise to choose the better tool for the job is sound, and was wondering for this application whether the D750 in comparison to the D500 is indeed the better tool.