Shooting the moon: contrast

For a long time, I’ve wondered why it’s so hard to get the moon in focus all the way across the image. But here’s proof that it is hard (click the image for a larger version):

Moon, almost full. December 1, 2017.

In this photo, the edges of the moon (at least top and bottom left) appear to be relatively sharp, while the inner portions appear much more blurry. Part of this is certainly due to the much higher contrast visible near the terminator (the sunrise/sunset line), where the shallow angle of the sun provides crisp shadows compared to the low, washed-out, shadowless regions farther away from it. But it seems that even taking that into account, there are regions of poor focus and good focus, not just high and low contrast.

Here’s what I mean:

Moon, December 1 (detail).

I’m going to try again tonight, weather permitting, on the theory that maybe it’s my lightweight mount that’s to blame. So I’m going to mount my tiny scope on the big scope rig, just to be sure there’s no shake in the system.

We’ll see!