text: only objective lenses? im interesting about lenses and i need to buy a:   d = 120mm / f = 1000mm air space achromatic lens. or  d = 150mm / f = 1200mm air space achromatic lens multi coated which can used for astronomical telescope. only the objective lenses so please lead me to the sites which i can find it.thanks to all these lenses are quite similar. d refers to the diameter of the lens, and f is the focal length. both are two-element lenses, meaning that there are two physical pieces of glass, and "air spaced" means that those two pieces are not physically touching (which is common), but slightly separated (which is optically better, but more expensive).  if money is no object, go with the 150mm lens. in astronomy, aperture is everything. this is a nice six-inch diameter, which will give you fabulous views. also, since the focal length of your objective is rather long (by modern standards), it's best to start with a long focal-length eyepiece (like 25mm). this will give you lower magnification, but a wider field of view.