That lens was very much a surprise. 1) it's a single coated zoom. 2) it's supposed to be a little unsharp at 500 mm. 3) it's a 1979 design, and lacks the refinements of modern computerized engineering, and 4) it's only 500 mm –max.

I have an Olympus 1.4X tele-converter specifically designed for a couple of their Micro-Four-Thirds lenses, one being a 40-150mm zoom; here's a (non-astronomy, sorry) crop from a picture taken at 210 mm with the converter from a distance of 105 m; that's the kind of sharpness I'm looking for.
https://www.flickr.com/photos/101461001@N06/30937811736/in/album-72157656755135932/

Here's the 500mm zoom again, with a Full Moon:
https://www.flickr.com/photos/101461001@N06/32394324703/in/album-72157672817966410/

Cheers,

Cortland

Cortland (KA5S) said:

that's the kind of sharpness I'm looking for.

I can see why. The water in that shot is amazing.

This is a great photo.

Lovely full moon last night.  It wasn't pink, but WTF.