The focal length is half the radius of curvature (a paraboloidal mirror is nearly spherical, so I refer to the curve of the sphere). I haven't been following your progress, so I don't know what equipment you have. The formulas given by the previous writer is good. If by experimenting you can move a pinpoint light source so that your mirror focuses an image of the pinpoint exactly back on the pinpoint (or, more practically, on a white card that moves with the pinpoint at the same distance from the mirror), then you are at the radius of curvature, since a sphere focuses a point source at its center exactly back to the center. Divide by two, and that's your focal length. Or use the formula given by the previous correspondent.
--Dan Johnson

• Focal length - johns - October 29, 2002 - 01:51 UTC
  • Order of assy. of tube. - bobmay - October 29, 2002 - 20:13 UTC
  • Can you borrow a scope? - danieldgj - October 30, 2002 - 05:13 UTC