OK, so it is a GEM. Get yourself a dovetail that has a hole through which you can use a 1/4" x 20 hex head screw. This will match the standard tripod socket. For short lenses, < 100mm, you simply use the tripod bushing of the camera. However, framing your composition gets restricted, so you can use any medium-sized ballhead to aid in framing. For lenses 100mm or longer, ballheads are, generally, not sturdy enough for astro. A far better solution is to use a rotating tripod collar for your lens. Canon sells these at exhorbitant prices but often you can find generic ones on ebay at a quarter of the price. The longer Canon lenses come with a rotating collar included. Future lens purchases. After 50 years of photography I never use zooms for any serious work, daytime or astro, and I simply purchase my primes at 2x steps; 14mm, 28mm, 50mm, 100mm, 200mm, 400mm. This gives sufficient flexibility, combined with the highest possible optical quality at a price no more expensive than trying to do it all with much slower L-zooms. Even L-zooms (and many primes even) are out-resolved by the latest mucho megapixel sensors. Why deliberately handicap your print sizes, daytime or astro?