The ED80 has a F/7.5. I gather that the matched reducer made by SW is a 0.85x, taking down the focal ratio to f/6.375. The SW option is pretty expensive.
There has been a popular cheaper choice in the ED80 community, specifically the Altair Astro Lightwave 0.8x reducer. It gives the scope a ratio of f/6. Unfortunately it seems to be sold out everywhere.
Why is it not possible to use a stronger reducer, like the Altair Astro Lightwave 0.6x which would give a ratio of f/4.5? That one is readily available, and on checking specs, it looks to be physically identical to the 0.8x reducer?
I must be missing something here? Will it just distort the image? Physics involved?
Is there a rule that I am not aware of, or some kind of equation that limits how much an f ratio can be lowered using a reducer?