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It's basic optics that the image in a flat mirror is visually the same distance behind the mirror that the object is in front of the mirror. You can't escape that. An astronomical object and its reflection in a flat mirror both behave optically as if they are at infinite distance, which specifically implies that rays of light from the source are parallel to each other and rays of light from the reflection are also parallel to each other. And if your mirror were not flat to an extent great enough to change this directly, you would see it. The mirror would bulge (there is a small possibility that your mirror is not sufficiently flat, but I doubt this is an issue).

There is a phenomenon that can influence the apparent focus. Your eye itself has an optical focussing capability. When you are working close to your mirror assembly, your eyes may be adjusting to a nearby focus. When you then look through your sextant, your eyes will not necessarily relax to infinite focus. This may create the impression that you have to focus differently depending how close you are to the mirror. Try this: just before you look into your mirror, look at some distant hills. Allow your eyes to relax to infinite focus. Then look through the sextant scope. The image in the mirror should remain at apparent infinite distance.

Frank Reed