The camera should be rotated around a point at its center (ideally its sensor), to minimize parallax. I believe the 4/6 numpad keys should indeed give the proper horizontal rotation in Space Engine. edit: Oops, correction: 4/6 will work properly only if the camera is level with the horizon (both elevation and roll angle) to begin with. Otherwise you have to adjust it manually. Hugin might be able to fix the anomalies with the level horizon option, depending on how severe they are.

SE's camera has no parallax, it is a mathematical point. To align camera with horizon, you may use 'land' command (Shift-G), then use R and F keys to adjust elevation, and then use 4 and 6 keys to rotate horizontally.

Tried that. 4 and 6 keys do a horizontal movement, not a rotation about the center axis of the camera.

They do rotate the camera. Did you make sure you had Num Lock on? If it's off then I think they function as left and right arrows, which is not what you want obviously. Also, try changing them in the Controls menu (from the main menu).

Rarely if ever do I have num lock on

I always have it off because the light annoys me.

Well, I don't like the concept of annoying lights on my keyboard, even ones I can't see. Plus, it'd hide the cAPS lOCK light. But it doesn't really matter anyway, since I just press Num Lock whenever I want to do an alt-code or something. And I think I changed some keys in SE.