hi guys! after quite a bit of hunting around for reviews i settled on getting either 1 of the abv mentioned lenses for my supertelephoto setup. I need help in deciding which is better and more value for money. on a side note, i am considering between 1000mm rubinar or a 1000mm f11 celestron (telescope lens) shld i be comparing nikon 500mm vs sigma 600mm and 1000mm rubinar vs 1000mm celestron or compare all 4 lenses together? would appreciate any help and reviews on the abv mentioned lenses.