Tin sheds are absolutely fine for an observatory building!
Ken has alluded to this here and -
http://www.iceinspace.com.au/forum/s...ad.php?t=35163
You really do have to brace the inner walls with wood of course and simply modify the roof to a roll-off system.
I built one to house a spectroscopy set-up due to Ken's post. If Ken Harrison uses one, you should have no problems
, I haven't had any issues with a tin shed.