thommo10 Posted June 12, 2011 Share Posted June 12, 2011 Hi all I have a old 60mm frac and I would like to know how to work out the focal length/focal ratio as the plate (if it had one) has fallen off. Link to comment Share on other sites More sharing options...
Telrad Posted June 12, 2011 Share Posted June 12, 2011 Focal ratio is the focal length of the telescope divided by the aperture. To work out the effective focal length of a telescope multiply the aperture diameter by the focal ratio. Link to comment Share on other sites More sharing options...
mallux Posted June 12, 2011 Share Posted June 12, 2011 How about taking the eyepiece out and looking through the telescope at something distant (not the sun :-) and moving your head back until it looks in focus (or alternatively, project the moon onto a bit of paper). The focal length will be the distance from the objective lens to your eye (or the paper), which you can measure with a tape measure. Then you can work out the focal ratio as Telrad describes. Link to comment Share on other sites More sharing options...
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