Following on from what Jase said, the various combinations for your ED80/350D are:
2x Barlow - 1200mm fl - 1.12 arcsecs/pixel
Prime focus - 600mm fl - 2.19 arcsecs/pixel
0.8 reducer - 480mm fl - 2.72 arcsecs/pixel
Given that typical seeing is around 2 arc seconds the use of the 2x barlow is limited by the seeing.
Cheers
|