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How to measure a telescopes focal length?

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Hi Dean,

I too am refurbishing an amatuer built truss tube newt. This is how I done mine. You will need a tape measure, a piece of white card and a torch. Remove the primary mirror, prop it up safely indoors (you will need plenty of room for this). Holding the torch in one hand and the piece of card in the other and standing well back from the mirror and (with the card and torch in the same plane as the mirror), shine the torch into the mirror so the reflected light hits the card. Now move forward (or back) until you get a focussed image of the torchlight on the card, place a marker on the floor at exactly that spot. You then measure the distance between the mirror and marker and then double that figure. That should be your true focal lenght.

Good luck


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Isn't this method measuring the radius of curvature, which is twice the focal length ? (not half).

I use a similar procedure, but use the sun as the source, reflect the sun's image onto a piece of card, and when in focus, the distance from the card to the mirror is the focal length.


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What sort of scope? If it's a reflector & the mirror is out of the tube, the focal length is half the radius of curvature.

Otherwise the best way of measuring focal length is to measure the diameter of the field stop of an eyepiece very carefully (the ones to use are those with an accessible field stop i.e. Plossl or Ortho, not exotic types where the stop is embedded between the elements). Now measure the length of time it takes for a star to drift exactly across the diameter of the field, with the drive switched off & no manual corrections. The true field width W is then one minute of arc for every (4/cos d) seconds (where d is the declination of the star) and the focal length of the objective can be obtained (near enough) from F = D/tan W.

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