Diameter of the instrument D - This is the diameter (here in mm) of the opening of the telescope pointed towards the sky, called the primary objective. This is the most important measure of the size of a telescope.
Focal length of the instrumental F - The physical length (here in mm) that it takes parallel light rays to focus to a point after passing through the primary lens or reflecting from the primary mirror of a telescope. This is not the same as the physical length of the telescope! That also depends on whether the telescope uses mirrors or just lenses and how many times light is reflected. For example, telescopes with mirrors are physically shorter than telescopes with just lenses with the same focal length because the path that the light takes is “folded” over itself a few times.
Or F/D ratio - A unitless number that is the ratio of the focal length to the diameter. Note that if you change the diameter or the focal length of the instrument then the F/D ratio changes, and if you change the F/D ratio the focal length changes. For the simulation parameters below I give both for clarity but you only need to change one.
I choose my eyepieces - The focal length(s) (here in mm) of one or more eyepieces on the telescope so that an image can be formed in your eye.
Use a 2X Barlow - A checkbox (Y checked or N unchecked) of whether to include a comparison of the eyepieces with and without a Barlow lens, an optional attachment.
Celestial sights - A dropdown list of commonly observed objects in the sky.