Polar Alignment - new funcionality

[carlomuccini]

Hi

I’ve noticed that the post on "Polar Alignment via Platesolving" is attracting a lot of attention and has a large following.
I normally use SharpCap's built-in function because I have a clear view of the North Celestial Pole, so I don't face the issue that those who cannot "see" Polaris do.

I was wondering at this point why not *also* implement Bigourdan's method in SharpCap.
It wouldn't be the first choice for many, but for many others, it would be a lifesaver.
With a little practice, it takes about ten minutes, but the result would be guaranteed.

I’ll briefly summarize it here for anyone who might not be familiar with it.

The Bigourdan method (or drift method) is the most precise polar alignment technique for equatorial mounts; it is ideal for long-exposure astrophotography and can be used even when the North Star is not visible.
Formalized by the French astronomer Guillaume Bigourdan, it relies on observing the "drift" (spurious movement) of a star within an eyepiece reticle, ignoring right ascension movements and correcting one axis at a time.

Perhaps it wouldn't be too difficult to implement it along the lines of the current method, which works wonderfully well.

Carlo

[Jean-Francois]

Hello Carlo,

Do you use PHD2 ? If yes, then PHD2 has already the drift (Bigourdan) alignment method.
It has some guided steps for the ~ 90° pointing direction and a graphic.
You can do the same with "a little" bit programming in a script in SharpCap ... but why, if it is already useful in PHD2 ?

Jean-Francois

[carlomuccini]

No, I don't use PHD2, and I'm not interested in using it.
As I wrote above, I use SharpCap for polar alignment.


My comment was a suggestion for SharpCap's development, not a request for instructions on how to perform polar alignment.

[Jean-Francois]

Hello,

It is already possible to use SharpCap and perform some drift measurement ... with the Live Stack. The "Drift Graph" shows the DEC (and RA) shift.

The drift (Bigourdan) method look only at the DEC drift.
So it is simple ... move the telescope to the meridian region and start the Live Stack and show the Drift Graph.
After 5 or 10 minutes ... perform an adjustment of the mount basis ... wait 5 or 10 minutes more and continue to adjust the mount basis.
Point the telescope to the East and restart the Live Stack for the adjustment of the other direction.

Regards,
Jean-Francois

[carlomuccini]

Dear Jean-François,
we clearly misunderstood each other.

This section of the forum - Feature Suggestions - is for suggestions regarding new developments.

[admin]

Hi,

so I have kind of made a policy decision to just stick with one built in polar alignment technique in SharpCap. Before I added polar alignment to SharpCap, I tried out almost all the different polar alignment techniques at one time or another and they all have their drawbacks and inaccuracies...

For instance I used the 'drift' tool in PHD2 and found that if you let it drift for say 15 minutes, the calculated correction would actually change over time (even after 10 minutes of drifting already, it would still be significantly different at the 15 minute mark compared to the 10). So, which measurement do you correct to? The one after 10 minutes of drift, or the one after 15? Or maybe wait until 20 minutes and get another different value again. Plus, of course, you have to do this separately for the EW and NS alignment, so you are probably looking at 30 minutes for even the first adjustment without then going back to check!

Another option I looked at was a '3 star' tool - you know the score, GOTO each of 3 targets in turn, then manually center the targets, and the measurement of how much adjustment was needed to center is the data that feeds the calculation. That one has its inaccuracies too, particularly if your mount movement has backlash or isn't always as accurate as it should be. On an EQ6, I found that if you tried it several times in a row without touching the polar align adjusters, you got different correction values each time (mind you, before I had the EQ6 belt modded).

Obviously SharpCap polar align has its own problems that can cause inaccuracy - if the camera has a dangling USB cable and/or isn't firmly fixed, the change in the pull of gravity as the mount rotates can change the angle of the camera relative to the mount slightly and give dodgy results...

So, why does this mean only one way to polar align in SharpCap? Well, having two or more would be a support nightmare! Astronomers, being the sort of people they are would try out all the different methods and find they didn't agree with each other (probably due to one of the causes I've mentioned above). The future would then be full of forum posts on this subject, and that's something I want to avoid...

As mentioned further up the thread, there are other (usually free) tools that have these different PA methods in them if you want to use them - PHD2 has drift alignment, Astrotortilla has (had? is it still being developed) a plate solving based drift align variant, NINA has 3 star align I think, etc.

cheers,

Robin