Ron, I am no expert but looking at the sensor of the A200, it has a lot more going on around those corners, so I can only guess that the glow is coming from the way the sensor was built. That is why I am a bit hesitant to do any cooling mod as I don't think that it is going to do much to improve it. I don't know if all DSLR's have this problem. But it is probably safe to say that Nikon and Canon don't. And certainly Hasselblad has no such issues but who uses it for astrophotography??? Hehehe.
Gary, the sensor of a Sony A200 is a CCD type while the A700 is CMOS. Very different. Sony's CMOS sensor is marketed as the Exmor, said to be a very good sensor with a very good SNR. I plan to upgrade my A200 in the future with this. Maybe modify it as well.
But I will be using the modified A200 for quite a while for astrophotography. I am able to fix the problems of signal noise in post processing anyway. There is a built in noise reduction option in the camera which I think is set as default. But I turn this off because, as Andrew said, you can't take an image straight away after a long exposure if you leave the default option on. The most I want to wait is 30 seconds to cool down the sensor before I take the next one. An intervalometer is very useful in this regard.
I don't mind digressing from the topic a bit, Andrew, it makes the thread more interesting.
I have attached a close-up view of the A200 sensor, on the right corner is the place where the amp glow comes from (encircled).
Clear skies!