[marki]

On a OSC chip it is best to think in groups of 4 pixels in a square. What the manufactures do is use dyes (RGB) which they carefully place over the individual pixels in a set pattern (eg RGGB) to act as a filter for each colour similar to the way we do on mono cameras. When you download an image in 1 x 1 bin the ADU count for each pixel is stored in the file. With astro cameras these are often 16 bit images in the FITS format. These images are grey scale and need to be debayered in a suitable program (eg MaximDL, DSS or simlar) to retrieve the colour information. When you bin 2 x 2 that information is lost and the image cannot be debayered hence Dougs comment about grey scale images.

In your original post you did not mention you were using a camera lens hence the confusion. The KAF8300 would give you higher resolution at both focal lengths as the pixels are smaller but there would be little difference between the two chips you have suggested. I would go for the one that gives you the greatest FOV as you obviously want to do widefield AP. The KAF8300 is used by many at focal lengths much greater then 400mm so I doubt you would have any problems with either chip in the range you are contemplating. The major consideration would be well depth and sensitivity as you may saturate the stars before getting the fine detail.

Mark