10 x 50 is a good size for astro. More important that they have a flat feild of view ie don't distort the stars near the edge of the views and have clean and clear optics.
My old 12 x 50 Zeniths have an excellent flat feild and beat my much newer Olympus 10 x 50 bins for astro. Daylight viewing is not a good test IMHO, astro work definitely pushes them if you are critical about it.
Try them and see what they can do, nothing to lose, they were 'free'..
