Understanding drift alignment
There are various methods for aligning the orientation of a telescope mount with the pole, but one of the most popular is drift alignment. It's often referred to as the 'gold standard' of polar alignment methods and has a reputation for being the most accurate. So how do you do it, and why does it work? And how accurate really is it, compared with other methods? Here we'll attempt to answer some of these questions. We'll also look at the effects of atmospheric refraction and see how understanding this is crucial to achieving the best alignment using the standard method of declination drift. But we'll also see that a better method exists for adjusting the elevation of a mount, popularised by the early twentieth century American astronomer Edward Skinner King (of King tracking rate fame), although it seems largely to have been forgotten. Finally we will decide whether to align to the true pole or the refracted pole and see how complex the apparent motion of a star across the sky really is.
The first part considers stellar drift due to polar misalignment and the second part discusses the effects of refraction and how best to choose stars for drift alignment. The third part considers King's method of polar drift. The final part looks at the effects of refraction on a star's general motion across the sky with reference to the work of E.S.King and looks at whether we should align on the true or refracted pole. Finally there is a summary of drift alignment do's and dont's.
All of the graphs are made using data from the spreadsheet. You can use it to investigate the drift of a star caused by polar misalignment and atmospheric refraction for yourself. If you have measured the rate of declination drift, you can calculate the direction in which your telescope's polar axis is actually pointing. The equations used in the spreadsheet are derived on the Equations page, but they are not needed to follow this discussion or to use the spreadsheet.