and you thought your tarantula image was green... you should see me right now
Pier looks like it goes fairly deep, should be very sturdy for any future equipment that you may end up hanging off it... with this kind of thing, making sure its future proof is the key... A few observatories I've seen were built with the original equipment in mind, and this can limit your options when expanding.
Looks like its progressing well Matty, will the walls be Al, bricks, wooden?