How far can you reasonably push a focal reducer?

[Kaptain Klevtsov]

Like Barlow lenses, the effect of a focal reducer is increased as the distance from the sensor to the focal reducer is increased, so I was wondering how far this can reasonably be expected to work? I have a couple of large (ish don't start Arthur )hanging about and I want to have a bash at M42 etc. when the clouds go. Using the SC3 webcam, the FOV is miles too small so I use an 80mm cheapo refractor. The ideal thing for me, would be to have a super reducer which will drop the focal length of the 200mm Newt. to around 400mm, or f/2. This would make it massively more sucessful than the Frac. as the exposure times would be very much smaller because the 200mm mirror would 'scope up all those photons onto the chip.

Am I talking rubbish yet?

My first guess would be that vignetting could occur as the FR moves away from the sensor, as the diameter of the FR has to be less than the diameter of the light cone, or some of the light will miss the glass bit with obvious consequences, but I'm assuming that I know how these things work, hence the question.

Captain Chaos