can i use a concave aluminum mirror as an objective mirror of a dobsonian telescope??
Yes - but
It has a high thermal expansion so it will only be correct at one temperature, this also makes it difficult to polish since the heat from the polishing changes the shape while you are polishing it - unless you have access to a diamond turning machine.
You will still have to either aluminise it or find a way of removing a thin top layer of the aluminium as it oxidises
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