Hi all, I know some men, who are amater astronomers. They made themseves reflective telescopes which can see the crescent Venus and many others in the sky. They say they make the mirrors parapolic shape although the depth of the mirror is very thin (about 1 - 2 mm). The diameters are often in the ranges 150-200mm. I am not sure how they could check the profile exactly a parapola, and quite curious about that. Do anyone know about this, please explain to me. Thank you.