polar alignement... when you don't see celestial north

[cyrille.de.brebisson]

Hello,

This question will probably show how much I don't understand the alignment algorithms, but here goes.

Is is possible to do a polar align by pointing the scope away from the north?
The "problem" is that one of my observing spots does not give me access to the north star (hidden by buildings). So I was wondering if it was possible do align pointing the scope else where. and if yes, what would be the restrictions...

Thanks,
Cyrille

[admin]

Hi,

yes, you can do the polar alignment without a view of the pole, but not using the SharpCap tool. There are about 3 possible tools that you might consider

* Drift alignment (supported in PHD2) - accurate but very slow to do
* GOTO based alignment with either plate solving or adjustment to center the star. In NINA and possibly in mount handsets. Relies on your mount doing repeatable GOTO movements without significant random or systematic errors (doesn't matter if the GOTO is off due to polar mis-alignment or cone error, but the mount movement must be accurate)
* Rotation based alignment (moving in RA) away from the pole and plate solving to detect declination changes - this is like drift align but you rotate the mount rather than waiting for it to happen at tracking rate. Astrotortilla had a basic tool to do this and I think there are others, but can't recall what software offhand.

cheers,

Robin

[cyrille.de.brebisson]

Thanks for the pointers.

[turfpit]

Cyrille

Is is possible to do a polar align by pointing the scope away from the north?

What mount do you have?

Dave

[cyrille.de.brebisson]

Hello,

I use an eq3 with a custom build controler.

why?

Cyrille

[archer1960]

I don't know about the EQ3, but several mount manufacturers have a function called "All-star polar alignment", or something similar.

[alpheratz06]

There is a method called "Bigourdan" in which you alternatively and sequentially "kill" alignment errors by observing stars on the meridian (due South) or due E or W You need a good unobstructed sight in these direction.

principle :

Pointing to the South, a star is assumed to drive in a pure horizontal way => any up or down trend is due to azimutal error ==> correct azimut till drift has no more vertical motion
Pointing E or W, drift will divert from horizontal if mount is shifted in Altitude => correct altitude to kill vertical drift motion.
a couple of iteration, and that's it.

Nina has a computer aided solution (Three points polar alignment) if I'm right.

[admin]

Hi,

interesting - I had never heard the name 'Bigourdan' applied to that technique before - I have always thought of it as 'Drift alignment'. You learn something new every day

PHD2 has an alignment routine that helps with the drift align. It can be superbly accurate, but is slow because you have to wait for the drift to build up enough to measure robustly. There is a variation that avoids that delay by moving the RA axis 10 or so degrees and plate solving before/after - the difference in Dec is the drift you would have over a 10 degree (40 minute) drift measurement. I think Astrotortilla had a variant of that technique.

cheers,

Robin

[Jean-Francois]

Hello,

The "Bigourdan" method *is* the drift alignment ... initially visually with a reticle eyepiece, today with a camera.
PHD2 or other software performs the drift alignment ... the same with 2 positions at ~ 90° apart (~meridian and ~ W or E direction).

It is similar as the Foucault-test naming ... the French people says Foucault-test ... the other knife-edge-test

Note that I'm finishing my SASPA script (SharpCap All Sky Polar Alignment)

Regards,
Jean-Francois

[cyrille.de.brebisson]

Hello,

I am interested in that script!

Can you tell me more? where can I find it, how do I use it!

Cyrille