So, for starters: the two images.

Original Treeline

Moon, correctly exposed

The treeline picture has a pretty decent exposure for the nighttime, but it’s noisy, there’s a little blur, some color cast, and the moon has… funky things going on.  Not only is is pretty much over exposed, which was expected, but you can see a second sub-image.  This is due to me being lazy and it being cold outside – I shot through a window.

The correctly exposed moon is a much faster, higher speed pic.  It’s much smaller, however – there isn’t the over-exposure glow, so it’s really thinner rather than smaller.

So now comes the thought: how to combine these?  First step is to remove the moon in the treeline pic. I did this quickly with the patch tool in Photoshop CS4: select the moon, drag to a solid part of the sky, and presto – no moon! Entirely believable, too.

Now that we have a flat (albeit noisy) sky, it’s a matter of copying the moon from one to the other.  I’m interested in speed, so I’m going to cut corners. I start by duplicating the background layer, then selecting the background via the menu: Select->Color Range. If I stop here, the edges of the moon will be too sharp, so next I click on Select->Refine Edge.  Settings (after playing for less than a minute): Radius=1, Contrast=0, Smooth=3, feather=1.0.  The selection now looks like this:

Selected and smooth

Hit the “delete” key, and you’re left with only the moon on that layer. The next step: click & drag it over to the original image.  If it lands on top, you’re set: just move it around to the position of the original moon. It’ll look OK, but it’s choppy at the edges (still) – needs some smoothing.

Now, I said I was going for quick & dirty, and here’s the quick & dirty for smoothing out those edges: pull up layer styles (double-click on the new layer), and move around the “blend if” options.  Here’s what I ended up with:

"Blend If" settings

Do you see what’s going on there?  We have a black & white image, the moon.  It’s blending (i.e., hiding) from the moon layer all pixels from the upper marker and darker on the upper layer with anything darker on the lower layer.  Since the moon is brighter, a quick drag of those markers gets us right in, and looking sharp.

Moon over the Treeline

The final result, with a little noise still around the original moon: