Imaging train focal length implications?

[glend]

Just wanted to ask about focal length implications for CCD cameras and such on refractors. I am building a 127mm tube frame refractor (very much along the lines of the iStar TCR). At present I have conducted focal length testing and am ready to fix the backplate in position (and thus the focuser location). Before I do that I thought it wise to ask the experts about any allowance I should make in focal length and focuser placement for future imaging use of this scope. So any thing to consider? Thanks

[cometcatcher]

Some accessories, field reducers, flatteners, filter wheels etc require a certain amount of focus inwards. Extra focus outwards is easy to do with extension tubes but if there isn't enough inwards focus, the only cure is a saw! Just something to keep in mind.

[LightningNZ]

Best to have in mind all the possible accessories you might want to use ahead of time - filter wheel, off-axis guider, active-optics, camera, etc.

Also make sure that you aren't going to vignette the field with too narrow focuser tubing.