In the telescope tube length equation, what do fobj and foc denote?

• A. Focal point of objective lens; focal point of ocular lens
• B. Focal length of objective lens; focal length of ocular lens
• C. Object distance; image distance
• D. Aperture; focal length

Answer: (A)

The tube length of a telescope is determined by the focal properties of the two lenses. fobj and foc represent the focal lengths—how far the objective lens is from its focal point and how far the eyepiece (ocular) lens is from its focal point. The objective forms an image at its focal plane inside the tube, and the eyepiece is placed so its focal plane coincides with that image, so distant light after the eyepiece emerges parallel to the eye. That arrangement makes the overall tube length roughly equal to the sum of the two focal lengths: fobj + foc. In other words, these symbols specify focal lengths, not simply any focal points or other distances.