I build my own telescopes.
Go with a electroless nickel coating, polish to 6 Ra or better. Alternatively, various chromes can support a superior finish while offering excellent adherence to the substrait material. While glass is a preferential choice, I have often used other materials that capture this adherence effect with the coating of choice. In this fashion, you can machine finish the component on an NC and be absolutely sure of the radii of curvature, the equation for the geometry of the focus.
Good luck.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada