I've done that here:
http://www.iceinspace.com.au/forum/s...ad.php?t=99072
And also discussed a bit in this thread:
http://www.iceinspace.com.au/forum/s...ad.php?t=99073
Also regarding vignetting, I'm sure you would get some if you had a 1200mm lens but with my setup, a 7D + 200mm = 320mm, the sun is quite small so I'm never using the entire area of the filter.