Determination of the Screw Constant

This is a rather more difficult task since it is first necessary to determine what the angular equivalent of the linear motion of the micrometer screw is. In the example above we saw that the 20-cm f/10 Schmidt-Cassegrain has a linear scale of 1 mm = 103'' at the principal focus so that if the micrometer has a screw pitch of 0.5 mm per revolution then each rotation of the screw moves the wire 56.5''. It is necessary to subdivide the screw into usually 100 smaller intervals with visual estimates of perhaps one-tenth of each division giving values to 0.001 revolution or 0.06'' in this case. It is necessary to determine this screw value and not to take the manufacturer's data for the focal length of the telescope and Barlow lens. Note however that in those telescopes were the primary mirror is moved to adjust focus then this alters the scale constant and it is therefore important that the scale calibration is checked regularly.

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