True Field of View: The circle of sky that you see when you look through a telescope or binoculars.

As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters). It’s equal to the telescope’s focal length divided by the eyepiece’s focal length.

Magnification (power): The amount that a telescope enlarges its subject.

Some telescopes use extra lenses and/or mirrors to create a long effective focal length in a short tube. In most telescopes the focal length is roughly equal to the length of the tube.

Focal Length: The distance (usually expressed in millimeters) from a mirror or lens to the image that it forms.Focal Reducer: A device that shrinks the effective focal length of the telescope.Barlow Lens: A lens that's placed into the focusing tube to effectively increase a telescope's focal length and, in turn, the magnification of any eyepiece used with it.For specialty oculars, check the barrel of the eyepiece first. Eyepiece Apparent Field: For most eyepieces (Kellners, Orthoscopics, or Plössls), this value is about 50 degrees.A telescope’s magnification can be changed by using eyepieces with different focal lengths shorter focal lengths yield higher magnifications. Eyepiece Focal Length: Eyepiece focal lengths are nearly always printed on the eyepiece itself and are labeled in millimeters.For instance, a telescope with an 80-mm-wide lens and a 400-mm focal length has a focal ratio of f/5. Scope Focal Ratio (f/number): A lens or mirror’s focal length divided by its aperture.Scope Aperture: The diameter of a telescope’s main lens or mirror - and the scope’s most important attribute.Failing that, you can always visit the website of your telescope or eyepiece manufacturer. Most of these numbers can be found on your eyepiece barrel, on your telescope's tube or mount, or in the equipment's instruction manual. Rayleigh defended this criteria on sources of equal strength.To use this calculator properly, you'll need to know a few basic details about your telescope and eyepieces.

Two point sources are regarded as just resolved when the principal diffraction maximum of one image coincides with the first minimum of the other.If the distance is greater, the two points are well resolved and if it is smaller, they are regarded as not resolved. In that case, the angular resolution of an optical system can be estimated (from the diameter of the aperture and the wavelength of the light) by the Rayleigh criterion defined by Lord Rayleigh: The narrower the aperture of a lens the more likely the PSF is dominated by diffraction. The interplay between diffraction and aberration can be characterised by the point spread function ( PSF). Light passing through the lens interferes with itself creating a ring-shape diffraction pattern, known as the Airy pattern, if the wavefront of the transmitted light is taken to be spherical or plane over the exit aperture. The lens’ circular aperture is analogous to a two-dimensional version of the single-slit experiment. On the other hand, diffraction comes from the wave nature of light and is determined by the finite aperture of the optical elements. Aberrations can be explained by geometrical optics and can in principle be solved by increasing the optical quality - and consequently the cost - of the system. These two phenomena have different origins and are unrelated. The imaging system’s resolution can be limited either by aberration or by diffraction causing blurring of the image. Angular resolution or spatial resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, thereby making it a major determinant of image resolution.

Angular resolution calculator