Polar Alignment: Alternative to Kochab Clock ?

[skysurfer]

For accurate polar alignment (for astrophotography exposures longer than a few minutes per frame) there are a lot of methods, one is the time consuming drift method. Another should be the Kochab Clock method which makes use of the fact that the celestial north pole situated 45' from the true north pole is in line with the line Polaris-Kochab (beta UMi) the second brightest star in UMi. However this method has limitations:
* northern hemisphere only
* can be aligned when Kochab is exactly above, below, left or right of the North Pole any event occurs only once in 6 hours.
I think that a variation on this is using *any* star not too far (< 20º) from the pole (can be south as well) as long as that star has the same altitude (or azimuth) of the true north / south point. This only requires the user to horizontally OR vertically move the RA axis of the mount. Of course the mount should be level. Such a star can be found in a live planetarium app.
Any indeas on this ?

[bojan]

The good alternative to drift method is polarscope. Even here on Southern hemisphere.. with limitations of course (visibility of relevant asterism, Octans or Polaris).

[tlgerdes (Trevor)]

The only problem I see with both of these, is the assumption of orthogonality of the scope (OTA or polarscope) to the mount. And we know that Assumption makes an Ass of You and Umption (sorry, bad movie quote )

[bojan]

This is not the problem if polar scope is co-axial (or parallel) with hour axis.. which can be safely assumed to be the case if adjustment was properly done.

[Poita (Peter)]

Originally Posted by tlgerdes

The only problem I see with both of these, is the assumption of orthogonality of the scope (OTA or polarscope) to the mount. And we know that Assumption makes an Ass of You and Umption (sorry, bad movie quote )

I love that film. What ever happened to Geena Davis?

[Barrykgerdes]

Polar alignment with a polar scope

The use of a polar scope is not a beginners way to polar align a telescope. I have tried it and identifying sigma octans in a small polar scope is not very easy unless you are very familiar with it. You need to get the hour angle correct and the elevation and azimuth within 1 degree to have a reasonable chance of making initial identification. The precision that can be obtained is not good enough for long exposures. Another problem with polar scopes that is that in a period of 20 years the South celestial pole has moved 6 arc minutes. How old is your mask.

Orthogonality and Stability

Orthogonality is the biggest problem with polar alignment. Using a standard GEM you need to rely on maching accuracy as there is no direct adjustment. Next is the stability (rigidity) of the mount.

Drift alignment

Drift alignment is the way to get fine and accurate polar alignment but it must be complete with alignment at the horizon as well as at the zenith. This was once very time consuming but now with auto guiding programs this can be reduced to a few mniutes.

Barry

[Wavytone]

Originally Posted by skysurfer

Any indeas on this ?

Yes, several comments.

There's no pole star down here so that's out. What Barry said is also correct; you have to be able to align the mount to a degree or so and do this reliably every time, or you'll waste a lot of time hunting for the right asterism in Octans - I used to have an 80mm finder on my old C8 for this. It's one way, but not the only way.

If you are at a low latitude, atmospheric refraction will cause an offset in where Octans appears to be, vs actual axis of rotation of the earth really is. Not huge, but not helpful either.

With respect to the orthogonality, there are two problems: the angle between RA and dec axes, and the angle between the polar axis and the dec axis, which on commercial mounts may be off by a degree or so. If the mount has bolted assemblies, you can back off the bolts and use thin metal shims to adjust these angles, but you do need to be really sure about what you are doing or you'll make matters worse rather than better. You can also put shims in or under the dovetail to adjust the angle between scope and dec axis.

As for the drift method, too slow and tedious, there is a far quicker way if you have an accurate declination circle, and have also made sure that the scope and the dec circle are aligned (this you can do in daylight, too long for here).

Basically it is a 2-star method using two bright stars whose declinations are known, and the accuracy you will achieve is about half the graduations on he dec circle (0.5 degree if it has 1 degree divisions). if you have digital circles (encoders) great, you will get this spot on in a few minutes.

First, set the altitude of the polar axis:

The first star to pick should be right over head, at the zenith. Set the scope so your dec axis reads the dec of this star and lock the dec axis. You now must centre this star by adjusting the altitude of the olar axis (north-south movement) and sweeping east-west in RA. Once centred this means the altitude of the polar axis is correct.

Next, the azimuth of the polar axis:

The second star to pick should be low near the celestial equator, low on the horizon due east or due west. Set the scope so the dec axis reads its declination, and lock the dec axis. Turn the base of the mount (azimuth) and/or sweep in RA to centre this star in the scope.

You will need to repeat these two steps once more, after that you will be aligned as close as your circle permit together with whatever errors there are in the orthogonality of the mount.

Then swing to -90 degrees and see if you can find the asterism round the pole. I also rotate the scope through 12h in RA while looking through the eyepiece, you will see the field of view turn. The apparent centre of this rotate is where your polar axis is pointing. You can strap on a laser pointer to the polar axis, this can be a help if you can align it.

This is exactly the same as what the GOTO mounts are doing in software when you use the 2-star alignment method.

After this, you can use the drift method if you like, however my own experience was that if the mount is properly orthogonal that won't be necessary.