Moon! (to TC or not to TC)

Full Moon
(Pentax K-5, Sigma 400mm F5.6 APO, f/8, 1/1000s, 400mm, ISO 800)

So last night, I went out to snap some quick shots of the full moon. I was lucky as we haven't seen clear skies the past 2 weeks here and probably won't for another week at least.

As I still have to receive my M42 to Pentax adapter, this test was only about the newly acquired Sigma lens. Nonetheless, I was curious to see how the results at 400mm and at 800mm, with the teleconverter (TC), compare.

It did turn out to be rather hazy outside with dew already condensing on car windows. Luckily it didn't seem to affect the outcome as much as I'd thought; the results are rather interesting. The image above is the moon at 400mm; no TC used.

The one below is the same moon (it would've been awesome if it were a different moon, not?) with the same framing, but shot with the 2x teleconverter mounted. In post-processing, I've tried to make these two look as similar in color/light as possible.

Full Moon with TC
(Pentax K-5, Sigma 400mm F5.6 APO + 2x TC, f/16, 1/500s, 800mm, ISO 800)

Notice the latter image seems quite a bit softer. If we a bit closer look at images with the same crop (rather than the same framing) we see the same thing:
 

100% crop of the 400mm image

100% crop of the 800mm image
(with teleconverter)

There is more moon in the shot with the TC as it covers much more of the frame, but it definitely looks less crisp. I have three theories about why the TC image is softer; not to be confused with less contrast in the image, which can be corrected in post-processing.

1. Dirty TC: the teleconverter needs cleaning, though I find this a little unlikely as it looked clear to me in daylight. I will give the lens element a little rub with alcohol though, just to be sure.
2. Shutter speed: I noticed that because of the reduced light by using the TC, I set the shutter speed to 1/500 rather than 1/1000; this makes sense as the light amount is halved by the TC. However, I forgot a golden rule of using a shutter speed that is 1/<focal length> to reduce the chances of shake blur (yes, shake reduction was disabled). Bad Krispy! 
3. Focus: I've also noticed that the setup with TC is a good deal harder to focus than the lens on it's own; setting it to infinity won't work as you'll focus past the moon (think about that: awesome notion right there)! So that might be another technical error on my side.

So looking at these possible causes I guess I will just have to practice more!

On an upbeat note: I am very pleased about how this lens handles high contrast situations where chromatic aberrations (CA) live. There is a bit of that going on at the edges of the moon, but it's well controlled and easily corrected with software.

To sum up my pointers of photographing the moon:

1. Adjust metering method to center only or go full manual.
2. Mind the shutter speed in relation to the focal length.
3. Check focus; check it again.. and again.
4. Try to underexpose a little. The moon is a sun mirror: images tend to overexpose quickly, especially during a full moon.