Nikon D70s

[WaveSoarer]

I've been dabbeling with DSO astrophotography with Mrs WaveSoarer's Nikon D50 DSLR. This has proved to be reasonably successful and we've produced fairly decent images given our fairly basic setup. Unfortunately, due to the camera falling over on the tripod in Iceland, when Mrs WaveSoarer was photographing the Northern Lights, the card reader has been unreliable. Although the card reader occasionally comes to life, the camera is basically unusable. As we have a number of Nikon lenses, I decided to get a second-hand Nikon D70s DSLR, which I believe to have the same sensor as the D50 but has a number of additional functions - including a programmable shutter release. It arrived from the London Camera Exchange a couple of days ago (they've provided excellent service so far) and it appears to be in excellent condition. It produces very nice photos in daylight.

We tried it out last night, for some wide-field, with just a normal lens on the motorised EQ5. We noticed that for long exposures (from 30s up to 120s) there was a glow in the top left corner, which became brighter the longer the exposure was running. This also occurred on the dark images with just the cover in place rather than the lens. We see some lamp glow from the D50 on the top left edge of the sensor, slightly in from the corner, but this is fairly faint and doesn't significantly affect even long exposures and is dealt with quite well with the darks.

As far as I know the settings we're using are identical (800 iso and the noise reduction for long exposures turned off) and I'm wondering if their is something that needs to be done to reduce this effect with the D70s. I would have expected the performance to be near identical. Does anybody have any experience with the D70s and does anybody have any suggestions?