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There is a focal reducer by Celestron that is specific to the edge HD scopes (Unlike the generic 0.63 reducer/corrector for their other SCT scopes)
What I have found with my SCT and reducer/corrector is that the spacing between the corrector and sensor is as critical as any other, get it wrong and in the case of the standard SCT at least you will have distortions and as the spacing increases, increasingly severe vignetting. I don't know if it applied to the Edge HD specific reducer but most corrector reducers, if the stars in the corners show comet tails pointed away from the center of the field then the spacing to the sensor is too small, and if it shows a fishbowl effect with the stars displaying arcs in the corners then it is too large.
And yes, add or remove the reducer to the train and your focus point will change significantly.
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