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G'day David,
Rebar (RIO) is nice if you have it, but not totally necessary in a pad that is not likely to support heavy machinery or vehicles. It is good to have in your weight-bearing foundations and also in your pier if it is a concrete one. 90 to 100mm, (3 to 4 inches) should be plenty to make a nice floor for your observatory. Make sure it is thicker on the edges where it supports your building walls.
Edit: you might want to consider leaving the rebar out of it. If you have to move, you may be asked to break up and remove the concrete. It's a lot harder to do so with rio in it...
Once you have levelled off the soil, we commonly use a gravel base like blue-metal, crushed granite or crushed concrete to a depth of about 50mm, screed flat and level prior to pouring the concrete.
Your form work to give your edges a nice finish can be anything from 4 x 2 timber, to strips of steel or plastic, 100mm wide. As long as they are well supported so they don't move when the concrete is poured in, you can use whatever you have got.
One thing you might want to consider, is leaving a gap around the pier footing, to allow for expansion/ground shift and vibration, to isolate the pier and telescope from the rest of the floor and building.
Dig a square hole in the middle of the floor before pouring, neatly line the sides with polystyrene sheet, then pour your concrete floor and in the hole. Once it is all set, all you need to do is pour about a cupful of petrol slowly onto the polystyrene and it will disintegrate. Voila! One isolated pier foundation.
These are some ideas, I hope they help. Every observatory is different, which is what makes them fun!
Blessings,
Baz.
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