Confusion over Polar Alignment

[jACK101]

I have bought a Celestron wedge for my granddaughter's Celestron 6SE and I am planning to mount it on a permanent base rather than a tripod. I have read screeds of information about Polar Alignment and it is very, very confusing with some contradictions. I have gone back to first principles in order to get my head around this issue and have come up with the following: Please be patient, I may be totally wrong, but if so, please tell me why.

 

If we assume that we are standing on an ice flow directly above the earth's axis of rotation and the ice floe  is itself perfectly flat and horizontal and using a tripod or permanent mount we mount our telescope.

 

If we identify an object which we want to photograph,  raise the telescope to the required declination , focus and set it to sidereal tracking speed WE WILL IMMEDIATELY BE IN PERFECT POLAR ALIGNMENT. This is because everything in the sky will be moving absolutely horizontally and there will be no image rotation.  It is worth remembering that at this very special location everywhere we look is South.

 

If now we move to similarly flat and horizontal surface at latitude 80 degrees north, that happy situation above no longer applies because the objects in the sky will me following a curved path. We therefore need to put a wedge in place..

Set up the mount to look due South.

The telescope mounting plate will need to be raised by 10 degrees, the difference in latitude between the 2 positions we have used. The wedge will therefore need to be adjusted to 10 degrees so that when standing behind the telescope at the hinge end of the wedge the mounting plate will be rising by 10 degrees as it is further away from us. Just imagine the previous position of the telescope mounting surface with no wedge and now 10 degrees further South we need to make the new mounting surface parallel to the original one when we were at 90 degrees . 

 

As we continue to move South e.g. at 70 degrees the wedge will need to be set at 20 degrees and so on.

 

This seems to me to contradict the advice given to set the angle on the wedge to your local latitude, that advice would only apply at 45 degrees. The setting to use would appear to be 90 degrees minus your local latitude.

 

In fact, I would suggest that using Polaris is a complete red herring because although it is close to the earth's axis of rotation it rotates around it. Setting the scale on the wedge appropriate to your local latitude should get you close to polar alignment. It is highly unlikely to be perfect because of the likely inaccuracies in the degree scales on the wedge.

 

I have clearly not tried it, but I would hope that the previous outline would get you close and drift alignment could then be used to improve it.

 

Am I talking garbage, or would this be a simple practical procedure?

 

I would appreciate any comments.

 

Thank you.

 

Jack

[Elp]

Tbh I didn't read through all that but the whole point of PA is to align the main axis of rotation of a mount to that of the earth's pole axis whether that be in the north or south depending on which hemisphere you're in. If your RA axis is aligned, that's it, the mounts sidereal rate takes care of the rest counteracting the rotation of the earth, for better guiding to account for mount errors most imagers autoguide.

[saac]

You are right in that Polaris does not fall exactly on the Earth's axis of rotation. It is however close enough to be used in what becomes an iterative process in setting an accurate polar alignment. Most polar scopes will have an engraved reticle showing the offset of Polaris from NCP.  The exact orientation will be determined by your date/time of observation and lookup aids are available in the form of various apps. Suffice to say that if you are observing visually only then a rough alignment on Polaris itself will suffice. If you are using a controller such as AsiAir then there is a very straightforward polar alignment routine which works using plate solving and simplifies the whole process. 

You may find this article helpful:  https://www.astro-baby.com/astrobaby/help/polar-aligning-the-skywatcher-heq5orion-sirius-mount/

Jim

 

 

 

 

Edited July 9 by saac

[symmetal]

The elevation scale on the wedge is not marked in latitude degrees but in 90 - latitude degrees. So if your latitude is say 60 degrees you set the wedge to 60 on the scale and you will see that the plate is then at 30 degrees from the horizontal. I believe this solves your dilemma Jack. 🙂

Alan

[vlaiv]

1 hour ago, jACK101 said:

This seems to me to contradict the advice given to set the angle on the wedge to your local latitude, that advice would only apply at 45 degrees. The setting to use would appear to be 90 degrees minus your local latitude.

 

Depends on what is marked for wedge altitude.  Is it angle from horizontal or angle that will RA axis be angled at.

If you have alt-az mount that is normally moving in altitude as being RA axis than wedge at 0 degrees actually puts axis of rotation at 90 degrees. This other angle - angle of RA axis is the one you should match to your latitude.

1 hour ago, jACK101 said:

In fact, I would suggest that using Polaris is a complete red herring because although it is close to the earth's axis of rotation it rotates around it. Setting the scale on the wedge appropriate to your local latitude should get you close to polar alignment. It is highly unlikely to be perfect because of the likely inaccuracies in the degree scales on the wedge

You also need to move in azimuth in order to align RA to axis of rotation of earth. Adjusting altitude of wedge is not enough to bring two axis in alignment.

This is why is Polaris so handy (in northern hemisphere).

1 hour ago, jACK101 said:

In fact, I would suggest that using Polaris is a complete red herring because although it is close to the earth's axis of rotation it rotates around it. Setting the scale on the wedge appropriate to your local latitude should get you close to polar alignment. It is highly unlikely to be perfect because of the likely inaccuracies in the degree scales on the wedge.

 

I have clearly not tried it, but I would hope that the previous outline would get you close and drift alignment could then be used to improve it.

One of the way to do Polar alignment is to use:

1) Latitude

2) Compass

to adjust your polar alignment and then refine it with drift alignment

[vlaiv]

Just now, symmetal said:

The elevation scale on the wedge is not marked in latitude degrees but in 90 - latitude degrees. So if your latitude is say 60 degrees you set the wedge to 60 on the scale and you will see that the plate is then at 30 degrees from the horizontal. I believe this solves your dilemma Jack. 🙂

Alan

Depends on wedge - some are marked the other way around:

This SW wedge for Star Adventurer and other trackers is marked as degrees from horizontal and not 90-angle.

[vlaiv]

4 minutes ago, symmetal said:

The elevation scale on the wedge is not marked in latitude degrees but in 90 - latitude degrees. So if your latitude is say 60 degrees you set the wedge to 60 on the scale and you will see that the plate is then at 30 degrees from the horizontal. I believe this solves your dilemma Jack. 🙂

Alan

Or rather my mistake - it seems that all wedges are marked from 0-90 like one above - it is just the question how is RA axis mounted to wedge

Is it "up" when wedge is horizontal or is it horizontal as well.

[symmetal]

6 minutes ago, vlaiv said:

Or rather my mistake - it seems that all wedges are marked from 0-90 like one above - it is just the question how is RA axis mounted to wedge

Is it "up" when wedge is horizontal or is it horizontal as well.

Yes, the OP was referring to wedges for fork mounted SCTs which are marked in 90 - latitude degrees relative to the SCT mounting plate orientation. I should have clarified that. 🙂

Alan

Edited July 9 by symmetal