[avandonk]

Greg I use the Canon 300mm F2.8L non IS version as I consider it far superior for astro use. The IS version is better for terrestial use as it can counteract movement during the exposure. This allows you to go about two stops slower in shutter speed for better results. Remember the old rule that you should use a shutter speed which is shorter than the reciprocal of the focal length if hand held. So for 50mm FL a 1/50th sec and 300mm FL 1/300th of a sec is the longest exposure.

The problem with the IS lens assembly is that it may not 'park' in exactly the orientation of the lens axis. This gives you asymmetric stars.

Another thing that makes lenses work better at astro imaging is to use an exterior aperture and extended lens shade to completely cut out all light not contributing to the image from entering the lens. This enhances contrast and gives lovely round stars rather than blobs with spikes.

My Canon 300mm F2.8L lens does vignette a full frame to some extent. The real usable circle is about 30mm. This lens has has a single crystal of Fluorite and two ED elements in the main element group. I doubt if any so called large format lenses are even usable for astro use on axis. They are all soft and suffer from CA even on axis. They are even worse off axis. These lenses produced 'good' images only because less enlargement was needed for the final print (in black and white so CA was not even a problem) not because of any inherent quality.

This is why I use a Canon 5DH for widefields as it gets really complicated with cooled astro CCD cameras and electronic lenses such as the Canon 300mm F2.8 which are far easier to focus etc when they actually connected to a camera as the focus ring is useless when not energised by the camera.

Bert