Twilight Moon gradient

[Ju_Cooper]

Hi All,

I'd be grateful for any thoughts on the origin of the gradient in this (unprocessed except to resize for posting) image of the Moon taken in morning twilight 19 November 2011. The second image is of the open sky with the Moon out of the shot and only shows a little vignetting. Both 1/8s exposures, ISO100, RAW, Canon EOS450D prime focus on Sky-Watcher 150 Explorer Newtonian.

The dark part of the gradient was to the south of the Moon whatever the camera orientation, so I assume it comes from a shadow cast by the Moon inside the telescope tube. The gradient was obvious across the Moon on the 1/40s shots I took exposed for the Moon itself. It was also obvious through the viewfinder and live view on the camera. I've never seen this sort of gradient on shots of the Moon in darkness or indeed full daylight. The small image of the gradient shows some sky banding on my monitor, but the gradient looks smooth on the original full-sized image.

Thanks for any advice on the cause and avoiding this in future