What's that you ask? That is the sun. The small black dot at around 1 o'clock is Venus making it's last transit across the suns face for 105 years. Other dots on the sun are sunspots, and the cloud-like things at the bottom, are, well, clouds.
Now, I practiced this yesterday with some heavy ND filters. 2 stacked 8ND and a circular polarizer, for you photo geeks. And it worked, but on my 200mm lens. I wanted to use the 300, but the front element is huge.
I forgot the drop-in filter thing at the back, so
with some gaffer's tape and one eyepiece of the cardboard glasses people were using, Max Cooper and I made the filter which worked for the photo above.
With a little trimming of excess tape, it slipped right down in snugly. After chimping, I came up with ISO 400, f4 at 250th. With DX turned on, this gave me an effective focal length of 450mm, and acceptable (to me) resolution.
You can see more pictures of the Asheville event here.