16-35mm f/2.8L II possible lens blur problem

MitchAlsup wrote:

While I agree with the person above who stated that one (or more) lensing elements are out of (what astronomers call) collimation;

There might not be a lot Canon can do for you.

The other thing is that I never use a lens wide open for astro images, try stopping it down to F/4 and F/5.6 or F/8 and see if the images clean up. Also note at 24mm, a 30 second exposure will have stars on the equator move 6 pixels or so unless you use a motorized mount.

Thanks Mitch. Wonder if you may clarify the reason you say Canon might not be able to do a lot in the case of a collimation? Not sure if it was mentioned above but the lens was previously dropped on concrete once, and I was told by Canon that the lens elements misaligned after. Their first fix apparently didn't fix the problem (would this be what you speak of? i.e. that it's very difficult to completely align the lens elements out of the original factory so there will always be collimation once it happens?).

I'm not particularly seasoned at astrophotography (I just like looking at the stars and take the occasional photos of them while on holiday), but I've also only ever heard of people suggesting to use the largest aperture when taking photos of the night sky. Wonder if your suggestion of a smaller aperture is only to see if it is a lens problem or an inherent lens characteristic?

Thanks for the tip on star trail movement. I usually go for around 15 seconds, at 16mm, that should be a lot less movement? (Though again I'm not particularly well versed and don't know how much movement that will be).

I might take a few photos with a simple patterned subject over the weekend to see if there is noticeable uneven blur.

Thanks!