Star Drift Method for Polar Alignment of Equatorial Mounts





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Published on Apr 28, 2007

I explain how to polar align an equatorial mount using the star drift method. This is the spoken text:
This video illustrates the principle of the star drift method for polar alignment of an equatorial mount. A rough alignment is first performed by pointing the RA axis of the mount towards Polaris.
The up-and-down tilt of the RA axis is called the altitude adjustment and the left-right adjustment [is called] the azimuth adjustment.
We will illustrate here the azimuth adjustment for the northern hemisphere.
This black line on the skyboard stands for the actual path of a star during the night. We choose a star that is close to the southern meridian and near the celestial
equator. It will rise in the southeast and set in the southwest.
I drew the black line by pointing my meccano mount straight towards the skyboard and turning the torch around the RA axis. This is the star going across the sky
and the bright disk of light represents the view I would have in my telescope, perfectly aligned so the star stays centred.
I will now turn the mount counterclockwise in azimuth a bit and see how it tracks my artificial star. The dots follow the track my, now misaligned, mount makes in the sky. Notice that the star appears to rise relative to the track the mount makes.

So if the star appears to rise, it means we need to turn the mount in azimuth a bit clockwise. If it appears to fall, we need to turn the mount counterclockwise. Hopefully, this process will terminate(!) and we have a good alignment.
So, what about the altitude adjustment? For the altitude adjustment we pick a star in the east. Here I've drawn again a star rising in the east, by just turning the mount in the RA axis. So what would happen if my altitude was a bit lower, say, I was tracking there? Well, the star would appear to rise faster than my mount. So if the star rises I just have to turn my mount a bit up and if it's falling I have to turn my mount a bit down.


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