I am leaning towards it being a focus issue and being just slightly off as I am not seeing any haze.
This is an image I took with my K2000 using my old Vivitar 200mm f/3.5 manual lens after I got my M42 to K mount adapter. It was created from a stack of 5 images that were hand held. There was some light high level clouds/haze and you can see it illuminated to the right of the moon. Shutter speed was 1/200, ISO:200, and f/11. I just spun the focus ring over to infinity so no fine focus as I just wanted to play with my adapter that I got that day.
If you think the focus is good and a single image looks better or only slightly worse it could also be an artifact of the stacking as I have seen things go goofy sometimes when you get a big uniform color like the black background. To help things out I usually will crop each image down a bunch, in this case mostly the moon. Then I double the resolution (4x image size) and use nearest neighbor as the enlargement method as this will allow you to get finer alignment. For the first round of alignment I use the hugin program align_image_stack and have it crop the output images so only the overlapping parts are in the output images. For final alignment I load the hugin output crops as a stack into photoshop and have it do an auto align on them. For combining the layers I set the opacity to 1/(position from the bottom) as a percentage and then flatten it. Finally I decrease the image resolution to 50% and use bicubic sharper for the resampling.
The reason for the initial doubling of the resolution (4x image size increase) is that it will allow the alignment to become more accurate. Having tried the various enlargement methods nearest neighbor works best when stacking images. You can optionally sharpen the image some before downsampling at the end but this may shapen away some fine detail that was revealed when you downsample. I also find that by using 2 different alignment tools, hugin and photoshop, I get better results in the final alignment.