In my younger
days,I did full testing of all my lenses at all apertures with tech pan film under a microscope with the Patterson test cards for lines per millimeter results. My Pentax 50mm f/4 macro hit the highest resolution of all my lenses (88 lpmm) which was only a bit above the Pentax 50 f/1.4 . The macro lens started out with high resolution (56 lpmm from memory) and climbed very quickly to its peak. The 50 f/1.4 started out soft and was near its peak stopped down 2 stops(f/2.8) peak at f/6.3. Since then, I have have a Pentax 50 f/2.8 macro and a 100 f/2.8 1:1 macro, neither of which are as sharp. Macro lenses are better corrected for close up, but are better in terms of flatness of field. My sharpest lens was probably the 100 f/4 manual bellows lens which has no focus mechanism! For piggyback photography, I reckon the slowness of the macro lens would drive you nuts, given that you would still have to stop down a bit for peak optics. Another thing to check is the number of elements - the more elements, the more the light dropoff and potential for internal reflection , all the more reason to avoid zooms for astro work if possible.