old frac

[thommo10]

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.

[Telrad]

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.

[mallux]

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.