Can't centre polaris.. is this normal.

[StargeezerTim]

I tried slewing via synscan to polaris the other night (I wanted to use it test airy disc).

The centre of the crosshairs was way above the star. When I tried to use the hanset to centre the star, only the left/right worked. The other two did not move the scope.

Is this normal? I have no problems centering other stars...

[Earl]

Polaris is not easy to center as visual RA movement is minimal so close to the pole.

[Gazabone]

Polaris is not easy to center as visual RA movement is minimal so close to the pole.

Yep, totally agree, even if you slip both clutches and try to "manhandle" the scope to Polaris it's still far from easy!

[StargeezerTim]

Thanks...thought I had a problem... especially when some use polaris to test collimation.

Perhaps polaris is a bit shy and doesn't like being the centre of attention...

[opticalpath]

It can be quite awkward with a German mount.  You may have to make a very large swing on the RA axis to get Polaris lined up exactly so it's easier to do this by loosening the clutches and moving the scope by hand. as was suggested earlier. 

It's much easier to start by using the finder scope, so first make sure that your finder crosshairs are lined up with the centre of the main scope field of view then:

1. Move the scope in RA until the tube is on the east side of the mount and the Dec axis is approximately horizontal; lock the RA axis.

2. Without moving in RA, push the scope up and down in Dec only until it's pointing north and roughly at Polaris.  You should be able to see Polaris pass through the view in the finder scope as you move the mount slowly back and forth in Dec.  Notice on which side of the finder crosshairs Polaris passes.

3. Now unlock the RA axis clutch and move the scope through a full 180 degrees in RA so the tube moves all the way across past the meridian to the opposite (west) side of the mount until the Dec axis is again roughly horizontal. (You'll have to rotate the scope in Dec at the same time as you do this to keep it pointing roughly in the direction of Polaris.

4. When the scope is on the west side with the Dec axis horizontal, as before lock the RA axis and move the mount in Dec only again until you see Polaris moving through the field of the finder.  It will probably pass on the opposite side of the crosshairs this time. 

There will be a position on the RA axis, somewhere between those two east/ west extremes where Polaris will pass exactly through the crosshairs as you move the scope up and down in Dec.

5.Move the scope in RA a little at a time back towards the east side, each time locking the RA axis and scanning up and down in Dec. You should see the line that Polaris takes crossing the finder getting closer and closer to the centre of the finder crosshairs as you get nearer to the correct RA position.

6. When you find the RA setting that allows you to see Polaris moving exactly through the crosshairs as you go back and forth in Dec, lock both axes when Polaris is centred in the finder, and it should be visible then in the main scope.

Possible problems:

- If you cannot see Polaris at all even in the finder in one (or both) of the extreme east/ west positions as you scan in Dec, your polar alignment is badly out and that needs to be fixed first.

Adrian

[brantuk]

Altitude and azimuth axes will work on polaris. If you can't slew to it (using RA/Dec) then one of Alt/Az needs adjusting to get your polar alignment right - and your scope needs aligning to the mount once the mount is PA'd correctly.

[Freddie]

Polaris isn't a great test star as it is a double so can mess up some tests.

[glowjet]

A thought provoking question, I passed on this when you first started the thread, as there had been some useful suggestions, however, after thinking about this for a little while, I would ask, have you checked for any cone error? where the scope is not truly at right angles to the Dec axis and not parallel to the RA axis. Not an uncommon problem as you might think, a little shimming is often required to correct matters.  

Useful instruction on the subject, see Astronomyshed video 5 Cone Error. Hope you solve the problem

[StargeezerTim]

A thought provoking question, I passed on this when you first started the thread, as there had been some useful suggestions, however, after thinking about this for a little while, I would ask, have you checked for any cone error? where the scope is not truly at right angles to the Dec axis and not parallel to the RA axis. Not an uncommon problem as you might think, a little shimming is often required to correct matters.  

Useful instruction on the subject, see Astronomyshed video 5 Cone Error. Hope you solve the problem

Thanks,

I'll check out my mount manual and the video.