Since the Moon is almost full and last night was clear I decided to test focus with the Bahtinov mask.
Once all temperatures had equilibrated (i.e. fridge at -11.0C and 300mm lens at 16.0C) I focussed as accurately as possible by inspecting test shots with the Bahtinov mask on the lens. See first image below. Even two hundred steps of the stepper motor focusing the lens was visible on the diffraction image. This is 24 micron of focus ring movement!
Slewed over to Omega Centauri and took the following data.
Canon 5DH, Canon 300mm F2.8L at f/3.5 with exterior aperture and lens hood extension. Hutech LPR filter.
Exposures dithered 40 X ( 15s, 30s and 60s ) at 1600 ISO. Stacked upsized X1.6. Usual HDR method.
Gradients due to the Moon eliminated with GradientXterminator.
Here is the full frame FoV 6.9 X 4.6 degrees. 5MB
http://d1355990.i49.quadrahosting.co...5/OC_bfull.jpg
Here is a 100% tiff crop 9MB. Save this and open in your favourite viewer or PS.
http://d1355990.i49.quadrahosting.co...5/OC_bcrop.tif
As you can see the resolution is stunning for a FL of 290mm. There is no hint of CA except on brighter stars near the corners.
Focus is more critical at short focal lengths due to the greater convergence angle and the Bahtinov mask is the best and easiest method I have ever used.
I used to take test images and inspect at high magnification and it was impossible to see any obvious difference between images taken with focus by as much as a 1000 steps apart.
Bert
You could get lost in that field. It's so sharp. You've captured a faint object. I have no idea what it is. Not sure if it's a background galaxy or something else. Definitely not a trail or satellite as it's curved. I highlighted the area. Check it out. Might be worth investigating at long FL.
So you're picking up background galaxies with a 300mm lens at F/2.8 under a full moon? As my boy would day "fully sick mate!" 
