The Bintel Projection lenses in 32mm and 40mm focal lengths are useful if you can't get your telescope to reach focus with a conventional camepter and T-ring.
Many Newtonian telescopes don't have enough focuser in-travel with a DSLR cam - the Bintel Projection lenses provide a solution.
The top of the eypiece housing unscrews(T2 thread) allowing a T-ring to attach directly.
With the same fine optics as Bintel's Plossl range it also makes a great low power eyepiece!