This screen allows the computation of Afocal image projection from a telescope thru a secondary eyepiece lens to the camera or CCD. Enter any four of the five inputs Focal Length-F length, Aperture - Dia both in mm, Effective Focal Ratio EFR, Focal Length of Camera – Fc and or Eyepiece Focal length Fe. Press “Calculate” to figure the missing parameters. Press “Clear” to clear the variables to try new ones.