A nice start to the month, both of you.
Pete, be careful with those sun stars, shooting into the sun can be harmful to both eye and camera, even if it is an overcast day.
krag96, nice shot, but I have a feeling you might be a bit diffraction-limited here. I have had all three versions of the 16-element 80-200 and they were all slightly soft wide open and started going soft again from f/7.1, so I tried to use them between f/3.5 and f/6.3. This was in the film days, but the AGFA CT-18 I used had similar resolution to the D700 sensor.
I'll review a few past photos that are very sharp and look at the histograms. I did notice a few slightly out of focus shots when the pup was moving, I'm going to try shifting my focus setting around, I thought I had it on Matrix, but looking at it a second time I found it was on Center Weighted, (I'm used to shooting on Spot metering). I'll give your suggestion a try though, ( it's my favorite lens).
When I shoot the sun, I never shoot a full sun for the reasons Peter suggested. I shoot partial suns, screened by a cloud, tree, horizon, (rising or setting) building, or about any object. I'm not at all familiar with your Sigma lens, it's rare I see a six point sun star though. My old Nikkors produce eighteen points of light.
When my brain goes numb and I can't think of anything else to shoot, there's usually an interesting sky.
Street lights at night are also pretty good makers of stars.
All photos on this post are old and from my files.