I have 2 ball eyepieces for sale ($40 each) that I constructed using a 6 mm Edmund BK7 high-tolerance ball (4.4 mm focal length) and a 10 mm BK7 ball (7.4 mm focal length). The housings are in good shape (the 10 mm ball housing has a scratch on the upper black anodized surface and some dust shows in the photos). The glass is in pristine shape. Siebert Optics sells a similar design with the designation "Planisphere Series". The eyepieces come with top caps only (pictured)
If you don't know about this design do a google search on single element eyepieces for some info.
I will give first preference to buyer who is interested in purchasing both eyepieces.
You will notice that the balls are slightly recessed in the housing. I prefer this design over the protruding design to limit greasing up from my eyelashes. It also helps restrict the view to the center of the field due to off-axis aberrations.
Thanks for looking