It may just be inherent in the sensor.
On really long exposures you can get a kind of glow around the amplifiers.
I assume that's heat, because the sensor responts to Infra Red.

http://www.astrosurf.org/buil/20d/20dvs10d.htm


See the 10d has marks remarkably like yours


Yet the 20D does not suffer as badly.



Another article says the 1DmkII turns off the amplifiers during log exposures and has no glow in astrophotography.

Also a good reply on an astrophotography thread "You're right, it is amplifier noise. This is a known problem on CCD sensors, but not on CMOS sensors (as Canon uses). However; it can be solved by turning the internal long shutter noise reduction on. I'm not planning on doing astrophotography anyway, I just wondered if it was my camera or a genereal problem."

That seems not to be true, as I also found a 300D image showing amplifier glow.

This article http://cobalt.golden.net/~kwastro/im...ummer_2005.pdf talks about fixing problems with software. But that light frame/darl frame calibration is really only suitable for astronomers


SO it may just be a built-in limitation of the camera.
Has it always done it?
Or do you have old photos that don't show this?