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Hi Stephen,
just a couple of things. There is concrete and there is cement. Concrete has stones in it and cement only has sand and concrete powder in it. Cement is used for sticking bricks together in walls. It does not have the same strength as concrete. Concrete would not fit down spaces of 10mm, only the slurry would go down and it would need to be vibrated as suggested. A concrete plug is the better solution.
Now I reckon that digging the pier footing hole just a bit further is what you want to do. Reactive soils such as here in SA can be handled with more weight. Go to 1 meter with 700mm x 700mm. That will give you around 2 tonne of material when set and rain will not move that vertically or horizontally. If it does you are having a flood.
Stability is the most important part of have a pier for imaging. The main idea is that you want to be able to walk around this without vibration. This happens at star parties and some subs are lost. An enormous mass is cheap to construct and only has to be done once. A permanent pier will allow for later imaging rigs of larger sizes without the expense of digging it up again. Over engineering is best, being under will lead to frustration.
Use this idea for when you do construct the pier.
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