[shoebox9 (Graham)]

As a fellow photographer I'm surprised at the suggestions the 300 prime is shifting vs the sensor. Is everyone aware these lenses come with their own mounting ring/foot and are never used 'hanging' off the camera. Either they are hand held (rarely) with one hand under the lens, or they are on a monopod. Occasionally someone may use a tripod, but always the camera is simply attached to the end of the mounted lens (supporting only their weight), not the other way around. I'm sure Peter uses the lens this way, like everyone I've ever met who owns one.

What can be an issue however, is the lens receiving a bump causing one of the elements to de-centre. This is a common problem. resulting in lenses with a tilted plane of focus. When it's only slight, it can be difficult to notice (especially with Canons when DPAF is being used- the part of the frame you focus on is always still in focus). Peter still may not have seen it before with his Nikon. A smaller aperture will cover it.

The truth is Astrophotography is a brutal sport.

In terms of advice, once you have a way if guiding your camera/lens, you can bracket your exposures so you capture both darker background stars, as well as exposures that don't blow out the very bright stuff. This will also give you more options with noise and allow you to dramatically improve the chunky blacks.

Others may disagree, but personally, if I were you I'd use the expensive glass you already own and spend money on stabalisation/guiding, before rushing out to buy a new scope.