Vignetting is dependant on the light path to the sensor. The QHY-8 has a clear window of 40mm, and the sensor is 28.4mm diagonally, so the camera doesnt have an issue, check to see that you have nothing to prevent all the light from reaching the sensor.
BUT, the sensor has microlensing, and you may need a flat field to remove the gradient it may cause due to this effect. Any CCD with micro lensing will have possible issues with wide angle len's or mirrors.
Theo
|