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StuartJPP

Stupid question about 1, 2 and 3 star alignment...

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...what does it actually align? Is it purely the scope to get GOTO working properly so that the mount slews to the correct object(s)?

Though I assume that the result is also used to correct for polar alignment errors by tracking in both DEC and RA when tracking an object?

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No, I don't think it affects tracking at all. It just gives you more accurate slews. I hate the way it's called "alignment" as it confuses it with polar alignment.

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If we're talking about Autostar (Meade's system), then on mine, 1 star IS polar alignment, 2 star is GoTo alignment and 3 star is more precise GoTo alignment with cone error compensation. They are just very badly named - I didn't realise 1 star was polar alignment for about 4 weeks, I just thought it was a quick and coarse GoTo alignment. I did read the manual which didn't mention it, the handset had updated software on it with the new routine.

I don't have a clue about Synscan though.

Edited by Brent

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From previous discussions on this topic I have thought of it like this. You have two coordinate systems. One for the sky and one for your telescope. You can imagine them as spheres with stars. What you have to do is align the two spheres by rotating the telescopes coordinate system so that the two stars in the telescopes coordinate system matches the real two stars.

In theory if you can match two stars then it is job done but in reality it is not that easy so three stars helps eradicate errors.

One star alignment measures the declination distance between the pole and the target but suggesting that you get "polar alignment" is bit ambitious as things like axis tilt will not be taken into account.

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In a nutshell there is no polar alignment unless you have the telecope mounted on an equitorial mount or a wedge. THe two and three star aligment is for alt-az operation.

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In a computerized alt-az application the GoTo star alignment does affect tracking but in a polar aligned scope it doesn't. Alt-Az tracking uses a model of the sky from the mount's local time and location to move both in altitude and azimuth so as to follow the object. It does, therefore, need its internal model to be aligned with the reality up in the sky.

In a polar aligned mount all that the 1,2,3 star alignment does is synch the computer's model of the sky with the real sky in order to provide GoTo. To track the sky it simply rotates in RA at the siderreal rate of just less than a revolution per day in the opposite direction to the earth's spin. The sky model isn't needed for this.

In principle a very accurately polar aligned mount without cone error needs only one star alignment because the polar alignment provides a second means of triangulation. Cone error, though, is very hard to eliminate. (It means that the scope is not truly parallel with the axes of the mount.) Once the system is given three alignment stars cone error can be factored into the mount's sky model and accurately corrected.

Olly

Edited by ollypenrice
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Lesson learned: no stupid questions (but I gave a stupid answer, please disregard).

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In a nutshell there is no polar alignment unless you have the telecope mounted on an equitorial mount or a wedge. THe two and three star aligment is for alt-az operation.

Hi Matt,

1 & 2 star alignment is for Alt-Az mounts only. 1,2 or 3 star alignment is for Eq. mounts.. It tells one this in the hand controller of those mounts..

I have 3, Meade LX200 14's in Alt-Az mode, an iOptron Mini Tower Pro, a Paramount GT1100, a EQ6 Pro and soon the new ALT-Az -Eq . mount from Synta.

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It is for tracking and errors and who knows what else.

You enter a time say 22:00 when the pips go, but live in say Oxford, so 22:00 is actually incorrect.

Your "time" is 21:55, the stars ara not at the 22:00 position they are at the 21:55 position.

Oxford is 1.25 degrees West not at 0.00 which is where the time is taken from.

If you are down in Truro or Penzance then even more out.

The base may be close to level but it will not be perfectly level and there is therefore a tilt over 2 axis that also have to be determined. These have to be worked out by the scope, or you are not going to slew to the right place.

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