Astro-Physics sells two nice piggyback adapters:
P/N: PGBCBR
http://www.astro-physics.com/products/accessories/photo_acc/photo_acc.htm#10
P/N: PGBCNB
http://www.astro-physics.com/products/accessories/photo_acc/photo_acc.htm#11
Very well made, adjustable, and inexpensive.
P/N: PGBCBR
http://www.astro-physics.com/products/accessories/photo_acc/photo_acc.htm#10
P/N: PGBCNB
http://www.astro-physics.com/products/accessories/photo_acc/photo_acc.htm#11
Very well made, adjustable, and inexpensive.
Jim McSheehy