Hall's Harbour Observatory,   Hall's Harbour, NS

How NOT to Check Your Alignment

21-Nov-2017, 18:57 AST 21-Nov-2017, 22:57 UTC

How NOT to check your alignment

I have had more than one person tell me that they knew that their polar alignment was accurate because, when they rotated the mount in RA, Polaris stayed on the large circle. What is wrong with that picture?

Well, the centre of rotation of your RA axis is the X in the middle of the reticle. You know that because, when the mount was new, you spent at least 30 minutes making sure that it was, right? And the X is also the centre of the large circle. When you rotate a circle around its centre, NOTHING HAPPENS!

No matter where the X is placed, whether on the NCP or on the Celestial Equator, that X is the centre of rotation. Any star under the large circle will remain under the large circle, no matter where the scope is pointing. If it doesn't, it is telling you that your polar scope is misaligned with the RA axis. It tells you nothing at all about your polar alignment.

Rotating the mount in RA does not demonstrate that it is polar aligned. This mount clearly is not polar aligned, yet Polaris stays on the circle as it is rotated.


The rotation that would confirm a polar alignment is to wait an hour or two without moving the mount. If polaris remains on the large circle, it is telling you that the centre of rotation of the sky coincides with the centre of the circle (i.e. the RA axis).