Short answer
Most refracting telescopes use convex (converging) lenses for both the main lens and the eyepiece. Concave elements appear in other places: reflecting telescopes use a concave mirror as the main light collector, Barlow lenses use a concave (diverging) lens, and the early Galilean telescope used a concave eyepiece.
Convex vs concave lenses
- Convex (converging). Thicker in the middle. Parallel rays of light bend toward a focal point. A convex lens can form a real image, which is what makes magnifying and focusing possible.
- Concave (diverging). Thinner in the middle. Parallel rays spread apart as if coming from a point behind the lens. On its own it forms only a virtual, upright, smaller image.
So yes, convex lenses are used to see distant objects: the objective lens is convex, bringing distant light to a focus that the eyepiece then magnifies.
Refracting telescopes
A simple refractor uses two convex lenses. The large objective gathers light and forms an image at its focal point. The eyepiece, also convex and with a much shorter focal length, magnifies that image for your eye. Magnification is the ratio of the two focal lengths, as discussed in telescope magnification power. The size of the objective is the aperture, and its distance to the focus is the focal length. The image in this simple design is inverted, which does not matter for astronomy.
Real objectives often combine a convex and a concave element to reduce colour fringing (chromatic aberration), but the combined lens still converges light overall.
The Galilean telescope
Galileo’s design pairs a convex objective with a concave eyepiece placed before the focus. It gives an upright image with a narrow field, and is the same idea used in some opera glasses. Modern astronomical telescopes rarely use it.
Reflectors use concave mirrors
A reflecting telescope does not use a lens to collect light. Its primary mirror is concave, which focuses light much as a convex lens does. A concave mirror converges light, whereas a convex mirror spreads it. A smaller secondary mirror, flat or convex depending on design, redirects or lengthens the light path to the eyepiece.
Barlow lenses, reducers and eyepieces
- A Barlow lens is a diverging (concave) lens group that increases effective focal length and so magnification.
- A focal reducer is a converging group that shortens effective focal length, widening the field.
- Eyepieces are made of several lens elements, but the assembly as a whole acts as a converging magnifier. The eye lens, the one nearest your eye, is typically convex.
Frequently asked questions
What do convex lenses do to light? They bend it inward toward a focus.
What is the main difference between concave and convex mirrors? A concave mirror converges parallel light, so telescopes use it; a convex mirror diverges light.
For how these optics determine speed and image brightness, see fast vs slow telescopes.