I have been discussing the spacing of the sky ed80 focal reducer in another thread and it has got me thinking about my own set up.
I have a skywatcher ed80pro with an Atik 460ex and presently use the skywatcher focal reducer/flattener for this scope. This was great for when I used an Atik 314L+ with the smaller chip as it gave me a much needed wider field of view and so in my opinion the pro's outweighed the cons. However now with the bigger chip of the 460ex I am wondering whe
I don't know if you'll get a good flat field without the flattener.
To answer your question about what flatteners do, yes they do reduce the focal length and they do speed up the F ratio. However, this cannot be treated by the same F ratio rules as govern camera lenses because in camera lenses you add aperture (more light) to reduce F ratio/exposure times while keeping focal length the same. You do not do this with a reducer, you change the F ratio by changing the focal length without addin