Short paper: Polar alignment of the equatorial mount: how accurate must it be for trail-free astrophotography?

Sooner or later, all astrophotographers have to get to grips with polar alignment of the equatorial mount. Accurate alignment of a mobile system takes patience, practice and a systematic approach. For DSLR astrophotography on a fixed tripod, there is the ‘Rule of 400’ – divide 400 by the focal length of your lens to find the maximum exposure in seconds before the stars begin to trail. As a rule of thumb, it is about right for most situations. In this paper, I develop a similar rule of thumb for the not-quite-perfectly-aligned equatorial mount.

Many factors influence the accurate tracking of an equatorial mount: calibration, alignment, balance, flexure, periodic error and so on. These are important, but are outside the scope of this paper. There are many polar alignment techniques of varying levels of accuracy (e.g. polar scope, engraved reticle, drift alignment) and equally many sources of advice on how to use them. Robert Vice’s method of drift alignment using a DSLR is perhaps one of the most accessible. This paper, however, is aimed at the user of a fully computerised GOTO mount, which uses its Star Alignment function to generate feedback about the accuracy of the mount’s polar alignment. Having displayed the polar alignment error (‘PAE’) on the handset, the system invites the user to make a series of guided adjustments before going round the alignment process again… and again… and again…

In just a couple of iterations, a mount such as the Skywatcher AZ EQ6-GT can be aligned to about one arcminute of PAE, but is that enough? How does the user know when to stop adjusting? (continued)

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