I went to post this as a reply to the comment by Arthur (nightsky) in the "Little Prefab Dome" thread but realised it may be of interest to others who may not see this info there so I've set it up as a new thread instead.
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I've been investigating the polywater tank solution for ages now and it's too easy to NOT go ahead with using a poly water tank as the basis for an observatory if you really want or need one, and can't be bothered building it all yourself.
Thee's a wide variety of tank diameters, heights, and colours readily available, they are solidly built, UV protected and available all over the place from numerous, often local, suppliers!
This is one company that can have tanks delivered over a very wide area for instance.
http://www.polyworld.com.au/rainwater.htm
(scroll down to "medium tanks" for some interesting models)
Here's a few other interesting links with sizes, colours, etc:
http://www.rainkeepertanks.com.au/po...s_bushmans.php
http://www.irrigationwarehouse.com.au/category284_1.htm
http://www.teampoly.com.au/Pages/mai...ucts_tank.html
Apparently I can get a tank ith 2.55 metre diameter & 2.4 m height delivered to my location, inc GST, for approx $2000. I haven't seen a price on a proper dome that compares to that elsewhere and I think the mods to convert it to astro use per my requirements will be pretty darn inexpensive.
The ONLY reasons I don't have a tank observatory already is that my back yard doesn't have enough free space, and I don't really need it presently with my dob! When the day comes, I think this IS the way I'll be going.
Cheers!