Polar scope question

[Gomtuu]

I hope this isn't a silly question.

I was trying to check the alignment of the reticle in my polar scope. When I rotate the scope around the RA axis, the crosshair is moving in a circle, so it's obviously off centre.

However, the target object I'm aiming at also rotates with the crosshair, so it remains under it.

Do I still need to try to adjust it, as it's staying on the target?

[Ian123]

Have never used a polarscope myself but i think this might help >Youtube<

[themos]

Yes, everything can seem to rotate! The reason is that the polarscope's optical axis is not aligned with the mount's mechanical axis. But it doesn't have to be! That's why we have a moveable reticle, so that we can position the + at the centre of the reticle right over the mechanical axis.

[FrankieValley]

I'd agree with themos. If polaris stays under the + when you rotate the RA axis, then your polarscope is aligned with the mechanical RA axis.

Aligning the polarscope is relatively easy and straight forward, so you could always align if you are in doubt. I think that it will be the same on an EQ5, but these are instructions for the EQ6 http://ca.skywatcher.com/upfiles/en_download_caty01316546640.pdf

[Gomtuu]

Okay, I've just "played around" with the reticle and the best that I can get is that the + is aligned on a single point as the RA axis is rotated, with that point appearing to move in a small circular motion with the + , but retaining its horizontal alignment.

Does that make sense? If so, is that correctly aligned?

[Zakalwe]

Point the polarscope at a distant point during the day (I raised the tripod legs to a comfortable angle, got the RA axis as near to horizontal as I could, and used a distant pylon top). Align the centre cross with the distant point. Then adjust the tiny grub screws so that the cross stays central when the mount is turned in RA.

The centre cross in the reticule shouldn't move in a circle...if it does, then it is not along the mount's RA axis. Be careful with the grubscrews...its easy (apparently) to damage the reticule).

[Gomtuu]

I'm seeing something like this (exaggerated somewhat), where the blue dot appears to be the centre of rotation and the green line is the target.

I don't seem able to get it any better than this, no matter how I try to adjust it.

[themos]

The centre cross in the reticule shouldn't move in a circle...if it does, then it is not along the mount's RA axis.

I disagree. I don't care where the + ends up, as long as the same distant landmark is indicated under it as you turn in RA.

If it moves in a circle, it just means that the mount's RA axis and the polarscope optical axis are misaligned.

[JamesF]

AstroBaby has some detailed information on aligning the polar scope. It's even possible to do it indoors. Basically it involves adjusting the alt/az bolts on the mount to get the polar scope pointing at something specific -- say a distant telegraph pole. Rotate the mount 180 degrees on the RA axis. If the polar scope still points to exactly the same place, you're done. Otherwise, use the grub screws to move the cross on the polar scope reticle *halfway* back to where it should be. Rotate the mount back to the original position. Adjust the position of the polar scope again using the alt/az bolts. Repeat until the cross is always in the same place.

Here's the start of AstroBaby's pages:

Astro Babys HEQ5 Polar Alignment

It's listed as for the HEQ5, but it works for everything with a similar arrangement. I use it for my EQ3-2, though I use an easier method for setting the position of the reticle for date and time. Details on how to do that are here:

http://stargazerslounge.com/beginners-help-advice/103827-polaris-transit.html#post1432597

For clarity when the mount is rotated about the RA axis the cross in the reticle should always be pointing at the same position. That means that the polar scope is aligned with the RA axis and therefore when you line the indicator up with Polaris, the RA axis will be aligned with the axis of rotation of the Earth. The indicator position for Polaris *will* change position as the RA axis is rotated, because the cross indicates the north celestial pole and Polaris is actually offset slightly from the NCP and appears to rotate around it.

James

[JamesF]

I'd agree with themos. If polaris stays under the + when you rotate the RA axis, then your polarscope is aligned with the mechanical RA axis.

That's certainly true, but to avoid confusion it's not where Polaris should be when you're aligning the mount with Polaris, just where Polaris would be if you were using it to align the polar scope to the mount (which isn't the way I'd chose to do it).

James

[JamesF]

I was trying to check the alignment of the reticle in my polar scope. When I rotate the scope around the RA axis, the crosshair is moving in a circle, so it's obviously off centre.

However, the target object I'm aiming at also rotates with the crosshair, so it remains under it.

I've just re-read this and I'm not sure I can make sense of it.

When you rotate the mount about the RA axis, the crosshairs should rotate with it, but they shouldn't actually move around. They just rotate about the centre of the cross.

I really don't understand how the object you're aiming at can rotate though. If you have the centre of the crosshairs pointed at, say, the top of a telegraph pole, then the telegraph pole should appear vertical at all times.

James

[Gomtuu]

I really don't understand how the object you're aiming at can rotate though.

The object doesn't actually rotate. The image remains in the same orientation, it just moves slightly eccentrically around a central point. It's a bit like pointing towards something and then use your finger to describe a circle in the air, if that makes any sense.

[themos]

It makes perfect sense to me and it's what I see every time! It's a sign that the reticle is not on the OPTICAL axis of the polarscope and it doesn't have to be (besides, there's nothing you can do to make it so, it's down to the mechanical fit of your polarscope into your mount)!

[Gomtuu]

Thanks. I'll assume for now that it's aligned and see how I go when clear skies return.