How NOT to check your polar alignment

[KathyNS]

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.

rotate0.jpg (33.35 KiB) Viewed 3261 times

rotate90.jpg (33.36 KiB) Viewed 3261 times

rotate180.jpg (33.57 KiB) Viewed 3261 times

rotate270.jpg (33.59 KiB) Viewed 3261 times

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).

[bladekeeper]

Great tip, Kathy!

[Voyageur]

Very clear explanation; thank you, Kathy.

[Altocumulus]

Very clear. Now, when in daylight

[Don Quixote]

Thanks Kathy...
I needed that ! :-)

[StarBru]

Thanks Kathy...
I needed that ! :-)

I, too, needed that!

[bobharmony]

I used to think that rotating the RA axis and seeing Polaris stay in place was Nirvana itself. Then my back and knees said "Get up, fool, before you freeze in that position", and I never looked through that polar scope again! Great tip, Kathy, and thanks for disabusing us simple-minded folks about one of our wildly bad assumptions!

Bob

[Jennifer Christine]

I run my pier quite tall to clear the top of my obsy wall so I don't have to hunker down to use it. However, scp is starless and octans is quite dim unless the humidity is low at twilight so its use is moot anyway.

[mcolbert]

needs to be a sticky!

[Polecat]

I always had trouble getting down low and looking first through the RA bore hole and then the PA scope. I recently disassembled an old weather damaged PA scope and inserted a green laser pointer into the remaining bushing. Now I turn on the laser, adjust the dec and alt to point the beam between Polaris and Cas. Typically less than 30 seconds (and no bending).

[localhost]

Speaking of polar scope last few times I setup my AVX mount I never even noticed that the polar scope needed to be rotated to match the sky, So the last time I setup my AVX mount I happen to have a app called; Synscaninit 2.1 That app has a area for Polaris View and I noticed when I saw the the image rotate on the app and looked at my polar scope and noticed my polar scope didn't match so thanks to the app I was able to realize that.

I was told about this a couple of years ago on AF but forgot all about that!