Using setting circles on a German Equatorial mount

Question

Recently I have been working on equatorial grid system consisting of the right ascension and declination coordinates. Even after understanding the concept behind those coordinates, I am puzzled by the idea of punching these coordinates on my equatorial mounted telescope.

Declination is the angular distance of a body north or south of the celestial equator. It seems, after a few sessions of stargazing, that I am comfortable with aligning my telescope with the diurnal motion of the stars.

Right ascension is the angular distance of an object measured eastward from the First Point of Aries, also called the Vernal Equinox. Thus, this coordinate can be used to find an object on its diurnal path (or its tract around the sky). Now, how do I achieve this? If the ‘First point of Aries’ is the initial point (0 hr point) from where I can begin rotating my RA axis, then how can align with that point?

Answer 1

The 0h for Right Ascension rotates about the sky during the course of the day, at the sidereal rate. Equatorial mounts often have a clockwork mechanism (based on an electric motor in modern scopes) that rotates the RA shaft at the same rate; with this tracking, the telescope will automatically follow (non-solar system) objects in their course around the sky. The RA setting circle on an equatorial telescope must be adjustable around the shaft, so it can be moved to the position where it will mark the correct coordinates when the telescope is set up or after a period when the clock drive is off. It then is locked to the drive so it rotates at a sidereal rate.

This article in Sky and Telescope has a more thorough discussion of how setting circles work.