Others gave nice answers (with which personaly generaly agree), will add one more.
Questions having to do with application based on physical principles (or practical physics) should be on-topic. Even if these questions involve making things or measuring things or assessing things, etc..
This is just physics.
Now if one wants to build a telescope, the physical principles are what is relevant to build a working telescope. This involves the mathematical relations that are relevant to telescopes, the materials and their properties, the compensating factors for atmospheric effects, parallax calculations and all that.
If one wants to assess personaly the efficiency of an already made telescope (either home-made or purchased) using physical principles, this should also be on-topic.
Now if one wants the locations of the materials for making a telescope or extensive research about a specific material over another material for building a telescope. i would say this cannot be done correctly and effectively here. And this indeed involves engineering research which can be extensive.
Nevertheless the physical principles remain the same, even for assessing the various materials, but any furher investigation would be off-topic.