Hi John,
This is most likely due to vignetting, As Andrew pointed out the reducer makes any vignetting worse since it enlarges the FOV, and some reducers are under sized and additionally cut into the light cone.
The non-centered distribution is most likely caused by miscollimation, primarily in this case a non-centered secondary mirror.
However you can recover from this without messing with the OTA, you just need to take flats with the camera attached in the same orientation as you took your lights. The easiest kind, since you have a closed tube, would be Tshirt flats before sunset: Drap one or more layers of white sheet over the front of the tube to attenuate the sky, point it up and away from the setting sun, and shoot ~1-20sec exposures aiming for about 30000 ADU in the center.
A little harder because the opportune window is much shorter is sky flats. Aim the scope at ~75deg altitude and towards the east (for dusk), turn tracking off and shoot 1-20 sec exposures, again aiming for ~30k ADU. Since he brightness is changing rapidly you need to adjust the exposure more often, but the dead spot twilight sky is nearly as flat as it gets (as long as the Moon is not in the area!). Once it is too dark and you need more than 20sec to get 30K ADU background, stars will start to show in your image, you're done until the next dawn or dusk.
The camera needs only to be roughly focused for flats to work (at least for vignetting).
Use Maxim or the like to combine the flats into a master flat, then calibrate the raw object images with the master flat. The flats should have the bias removed, for this you need to gather bias frames, Maxim calibration would take care of all this automatically, other software I don't know.
You can get more general info from the online Maxim manual: see here
http://cyanogen.com/help/maximdl/Calibration.htm
Best,
EB