Hi Fahim,
I recently bought the telecentric 2X barlow and the 17mm Ultra e/p direct from Sieberts in the the U.S.
The reason for the telecentric is firstly and simply a better product than the cheaper one offered.
The more technical reasons are this, If you have a "fast" scope, F5 or faster the light cone is a lot "fatter" than a slower scope of the same aperture.
A barlow that is configured for anything slower than F6 will still work on a fast scope, but because of its design a lot of the light cone is wasted.
The telecentic and other high end barlows are configured for the fast light cone, subsequently giving you as bright an image as possible.
Harry Siebert is always available to help you pre & post purchase.
Del is about 10 ten days.
I hope this helps.
Cheers, John.
