If you are building a refractor, these achromatic doublet sets are the standard choice for hobbyists:
When buying a convex lens for a telescope, your choice depends entirely on whether you're looking for a primary (the big one at the front) or an eyepiece lens (the one you look through) . For a functional astronomical telescope, a simple single convex lens will cause "chromatic aberration"—blurry rainbows around bright objects like stars.
Instead, look for . These combine a convex lens with a concave one to correct color distortion, providing the sharp, high-contrast images needed for space. Recommended DIY Objective Lenses