I finally realised a dream and bought myself a 600L. I could only run to the original non-IS model. My first chance to use it was on the moon a couple of nights ago. Here's the best I got, using the 600L, 1.4x Extender II, and 7D; 1/200 f/8 ISO250, using mirror lock-up. This is still cropped a bit.
I experimented with and without the extender at different apertures and thought people might be interested to see the results. The bare lens definitely sharpened up from f/4 to f/5.6, but going to f/8 made no further improvement. I made the composite below by taking the same crop from each unprocessed image and resizing so that the image scale is the same. That means the 600mm images are at 100% and the 840mm images are about 70%.
You can see from the top images the improvement from f/4 to f/5.6 in both resolution and contrast. Adding the 1.4x extender with the lens wide open (now f/5.6) does increase detail a tiny bit, but one stop down, to f/8, increases detail quite a lot, improving on the bare lens at f/5.6. That shows the extender is well worth using.
Having messed with the moon I thought I'd try turning the lens to the stars. I didn't have my Astrotrac out, so I just did tripod shots of 1 second, first at the Orion Nebula (1s, f/4, ISO6400, not cropped or processed except for LR defaults, and the camera is not modified).
A bit of processing would improve things further, but I thought you'd be interested to see the straight image from just 1 second exposure when you have a 150mm aperture. I was amazed how much it picked up.
Finally I pointed it at the Pleiades for a single 1s exposure - same settings.
In just 1 second I've got stars down to at least magnitude 12. Even at this image scale you can just see a slight trailing. At pixel-scale it's clearly visible, even at 1 second, though this is at a FF equivalent of 960mm, so maybe not surprising.
My tripod and Astrotrac and the heads are up to the weight of the big lens, so next time I get a clear night that is not too windy (could be a while yet, looking at the UK forecast) I'll try getting it up there and do some stacked driven shots to see what I can get. The high magnification might be a challenge for the Astrotrac's accuracy, but we'll see.
