Okay, I think I'm with you now.
Sounds to me like the exit pupil of the image is too small when it hits the CCD (as you suspected). Since you're using a 10mm eyepiece with a f/12 scope, exit pupil will be only 0.83mm, and therefore only illuminating part of the CCD.
ie. Image Exit Pupil = 10mm / 12 = 0.83mm
Suggest you first try using a longer focal length eyepiece to increase the field of view and Exit Pupil size.
I image without an eyepiece (prime focus), and just use a focal reducer when I want to get a wider field of view such as the moon or a large nebula. There are quite a few availabe ... I'm using a $49 GSO budget version while I learn the ropes.
Bintel Focal Reducer 0.5x & 25mm extension (1.25")
Stock code: 7458
New Bintel 0.5x focal reducer is threaded to screw onto any 1.25¨ eyepiece barrel and provides a nominal reduction in magnification of 0.5x.
Double the amount of sky on your DSI chip!
This reducer will be of particular interest to anyone with DSI or Starshoot type imagers where it is difficult to fit much sky on the chip of the imager.
Halving the effective focal ratio of the telescope means shorter exposures will show the same or even more detail than at the original f/number
The actual focal length reduction will vary a little over or under 0.5x depending on where the focal reducer is placed in the optical path.
For years people have purchased Barlow lenses to double the magnification of their eyepieces. Now you can halve the power of any eyepiece you own.
The reducer features 2-element fully multi-coated optical design and blackened lens edges in a black anodize aluminum filter cell. It also has additional filter threads, so you can screw/stack additional filters on to this reducer. Included is a 25mm extension tube to allow variation in the effective reduction.
Without 25mm extension tube reduction is around 0.7x
With 25mm extension tube reduction should be close to 0.5x original image scale.
Some Newtonian and refractor telescopes may have difficulty reaching focus with the reducer attached to an eyepiece. For example:
Focal reducer without extension tube requires ~10mm in travel to reach focus.
Focal reducer with 25mm extension tube requires ~35mm in travel to reach focus.