[Carl]

I have just managed to upload a larger image showing full frame.
Please remember this is not a focus test just a test to work out what is happening to the corners.
Richardo re your earlier comments
1. Collimation was correct last month, will check again Thursday night
2. Tightness of fittings. Will check again Thursday night
3. Distance from chip to reducer is aprox 50mm using radial guider, aprox 100mm using modified extension tube. I get wider field of view with extension tube but same problem with stars.
4. Stars have exact same shape and length of ellongation regardless of exposure time.
5. Focuser and mirror alignment could be an issue. After all it is a Celestron C8, I may be asking too much of it.
6. Last polar alignment 2 months ago star remained centre of illuminated cross hair for over 10min. My mount is on a permanent pier, rediculously rigid. Will check again Thursday night
7. I have an 80ed with a Hotech field flatener, I will do exactly the same tests on it.

Any assistance greatfully accepted