Objectives are from 50 to 80 mm in diameter. They are usually monocular, but can be binocular and are quite expensive. The most useful magnification is from 20 to 40 times. Terrestrial telescopes have an optical system to erect the image. Erecting systems are made with prisms or with erecting lenses .
Prisms are for the best quality and erecting lenses for cheaper instruments.
One can have achromatic or apochromatic lenses, with or without fluorite glass. Tripod quality is very important for image stabilization. Looking through the telescope can be in line or at 45x. It is easier to find objects when looking in line but looking at 45x is more convenient. The advantage is an increase of contrast and resolution and a significant reduction of color fringes.