The floor

This is a job best tackled before the external walls go on. The floor can be built out of whatever material you have handy. Don't worry if the surface finish is not the greatest. Mine looks good but I'm still going to cover it with carpet. I used tongue and groove hardwood floorboards.

First of all you need something to nail them to. I used 6" x 2" joists attached by 3/8" bolts to the upright supports. Work out where the telescope is going to be in relation to where you will stand within the dome. Measure down from the centre of the telescope as it points to the horizon (lowest point). From there you can work out at what height to begin your floor. Where the joists laid flat along the uprights, I just bolted them straight on. Where they intersected at an angle, I used a circular saw and chisel to cut some slots for them to sit in. This will make them a lot stronger and more rigid. Use a spirit level and your eye all the time to make sure they are level with the horizon and level with each other.

From there you can put on whatever flooring you like. Plywood or strong MDF (craft wood) is ok. The former is better than the latter as it lasts longer. Make sure you leave a hole in the middle for your telescope's support post to come through. Make sure that it doesn't touch the post either. The whole building should be able to shake and vibrate without moving the post a millimeter. Once the floor is complete, on goes some flexible sheeting material for internal walls. I'm using very thin plywood. This is a good time to glue or tack in some insulation if you have it.

I have also built a set of stairs from ground level to the floor out of 6' x 2" timber. Enclose the gaps if you don't want to run the risk of someone putting their foot down there. Lay them with carpet for a finishing touch. When the door to the observatory is open, there will be a trapdoor at floor level that can be swung up to gain access, then dropped back to floor level once inside. That way I won't fall down the hole when I walk around the telescope in the dark. I may even install a light as a safety feature that works like a fridge door light.