Why does FLI offer a 2 inch nosepiece?
A T-mount thread is 1.654 in diameter and has 33.866 threads per inch. The inside diameter is typically 1.615 . That is further reduced by the tube wall thickness making the actual clear aperture about 1.45 If you use a large CCD array and a fast telescope system, vignetting becomes unavoidable. A large CCD array (such as the SITe TK1024) is 1 inch square and has a diagonal dimension of 1.41 . If you could place your eye at the very corner of the CCD and look at the primary mirror, you would see the edge of the T-thread cutting off the edge of the mirror (75% or more of the mirror would remain visible). The vignetting would not be a sharp shadow, but a gradual intensity falloff. In many applications it’s effect is insignificant. For the FLI IMG series cameras using the 1024 x 1024 SITe array, our color filter wheel, and a 2 adapter tube, we ve determined that vignetting isn t an issue with f/8 and slower scopes. For faster scopes we ve introduced a 2 inch nosepiece to avoid any vignett
