poblem with 19th c telescope

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  hi, I have a large 19th century telescope which has a damaged objective lens I am trying to calculate the focal length being a novice it baffles me I am trying to find or have a 35mm wide lens 4mm thick but need the focal length can anyone help with focal length or a lens supplier who would be sympathetic

  Monday, April 6, 2015 3:57 PM