There is a very simple way to test whether a lens or a scope is well aligned or whether it has any other optical problems, or just to see what the overall quality/sharpness is. All you need is a more or less clear night and a tripod. Point the camera at the North Star, or to be more precice, at the North Celestial Pole, which is just a tad in the direction of the handle of the Big Dipper.
Usually it is more than good enough just to center on Polaris. There is so little movement in the stars around Polaris that the test is useful. On any other patch of the sky, you would have to have a good mount and guidence system to freeze the stars. Set the ISO to 800-1600 and the exposure to 20-30 seconds.
What to look for:
Naturally the first thing to check is the center of the frame. Here is the Sigma at 600mm:
full size here:
https://c1.staticflickr.com/1/615/32165605246_c53601d96b_o.jpg
and here is my triplet:
https://c1.staticflickr.com/1/354/32085393501_f92559c30e_o.jpg
What immediately jumps out here is how the scope can resolve Polaris-Ab (the tiny star right next to Polaris) where the Sigma cannot. The other stars are more clearly defined and look "smaller" than the Sigma, but the Sigma is by no means bad. The stars stay pretty round as you move away from the center. There will always be a tiny bit of elongation as you move out from the center due to the rotation of the earth. Just have to take it into consideration. Keep in mind, these were done with a 4/3 camera. FF would show more. Also, it has remarkably little CA, which is for a big zoom, impressive!
Now back to 400mm, first the Sigma
https://c1.staticflickr.com/1/399/31828703300_7de535be97_o.jpg
Here you see the deformation in the stars shapes more clearly as the lens performs better! At 600, the general loss in resolution seems to cover over the deformation.
And in comparison, the Canon 400/5.6
https://c1.staticflickr.com/1/767/31362508904_e6cb003772_o.jpg
Here the image is clearer, but at the same time, the problems with this lens become all too evident, namely CA! Notice the stars in the upper area. The elongation is clearly visible due to the sharpness of the lens, but the stars look like little Italian flags!
Seems to be a little out of alignment also.
So what does this all tell me? A lot! A lens that can't perform well on the stars is not going to resolve fine feather detail eather. There are among the manufacturers (ALL of them) sometimes big differences between one lens and another. It is a simple way to find out where you stand, and whether a newly bought lens should be sent back. *
Here is an articly by my friend Tommy, who has sent MANY a lens back because it didn't pass this (and other) test.
http://interferometrie.blogspot.co.at/
See July 5th.
The original lens block in my scope went right back to China! The second one was good.
* Oddly enough Canon seems to figured something out in this respect, and there seems to be very little variance in the 100-400 II. From what I have seen of it, it out shines both the Sigma and my 400/5.6.