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Polar Alignment Clarification


Aeseir
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Hi guys,

I know this may have been done to death but its really starting to bug me and im hoping ro resolve it finally.

The question i have is around Drift Method of Polar Alignment as written in this article http://starizona.com/acb/basics/using_polar.aspx

That is so far the clearest and simplest article i found.

Question i have for you all is what is indicated by picking star?

I have a CGEM mount, so for me that could be use the GOTO capability to find star and track it, or do i manually get the star to appear in eyepiece (but then how does tracking work)?

Sorry for potentially obvious question but i really want to hammer this out coz so far my alignments are way off and imaging has stopped being fun, and started becoming more and more frustrating (can't even get 45 sec subs).

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Sorry, that was a bit abrupt and not very helpful !

Tis difficult to explain in text without waving arms and drawing on whiteboards !

(and I cant compete with all the texts that are out there already !)

You dont need GOTO (other than to get you to the right star if it is approx aligned) so forget goto !

You dont even need tracking, but it is a convenience.

All you need is to note in which way you need to move your controls to recover the star(s) after a period of time and then adjust the mount to minimise those needed corrections.

was that better ! :)

Edited by Ptarmigan
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Not sure what you mean by picking a star.

For adjusting your azimuth bolts just goto a star near the meridian and the celestial equator.

Depending on where you are this star will be fairly high in the sky.

For adjusting your altitude bolts goto a star thats near the east or west horizon, thats on the celestial equator.

I use a star thats about 20degrees above the horizon but still on the equator, here in the UK

that puts the star slightly SE of East.

If you have a guidescope/camera, which for long exposures you will need, just use PHD drift align.

You can align each axis in realtime, in seconds, after a rough polar align.

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I pick a star as per guide that i listed.

Question is do i manually point the telescope so that the star appears in eyepiece or do i use the automatic GOTO capability to make it go to a star, the align to the center of eyepiece.

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I think you have to manually do everything, otherwise the goto attempts to compensate for the alignment inaccuracies.

I am thinking along the lines that if goto solved the problem then as an extreme case an Alt/Az mount wouild work and it doesn't for DSO imaging.

To my thinking you need to polar align the scope as accurately as you can to minimise all the errors that may be present, then you set up and use to goto for the tracking and to do the absolute minimum of error correction.

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Polar alignment is the process of aligning your mount with the Earth's axis. 

In principle what you are doing is to point at a star with sideral tracking turned on and observing what happens.  If your polar alignment was perfect, the star would remain in the centre of the FOV for ever.  If you are not polar aligned, the star will drift away from the centre and the only way to stop this happening is to adjust the altitude and / or azimuth of your mount.  The direction and rate of drift indicating the direction and magnitude of the polar alignment error.

You are in the southern hemisphere so I'm reluctant to be too precise but if you read the instructions it will tell you to pick a star in one of the area of the sky, observe drift and adjust azimuth and when that has been done use a star in a different part of the sky to adjust altitude.  Its done that way because the rate of drift is maximised that way so you don't have to wait as long to see the drift.

How you get the stars in your FOV is irrelevent - you can use go to or slew manually.  Once you start aligning the mount and recentring the star to see whether your adjustment has had a positive effect, I'm sure you'll be adjusting manually.

HTH

Mike

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Just some thoughts based on my current workflow.

Rough Alignment

I place my mount in the same place each time and do a rough align to polaris using the mount scope.

Equator Alignment

Then I choose my star near the east or west horizon and slew to it.

I get the name of the star from my smart phone using an app call SkEye. (A star map would do the same thing.)

Once I have centred the star (using live view on my DSLR) I sync. 

Now I leave my mount tracking for about 5min, then I adjust my bolts so that the star is back in the centre of live view.

Meridian Alignement

I choose a star near the meridian on the opposite horizon to polaris. (Northern hemisphere this is the southern horizon.) 

I re-centre on this star and sync same as above. (second star aligned.)

I do the same procedure and leave mount tracking for 5 min, then adjust my bolts.

Target Alignment

Now I slew to my target and I am always bang on, but I normally need to frame it and slew about slightly.

Once complete I sync for a third time on my target. (equivalent to three star align.)

My mount is now perfectly aligned and I can image through a meridian flip without any problems.

Their are easier and quicker ways, but I find this to be bullet proof.

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Hi guys,

I know this may have been done to death but its really starting to bug me and im hoping ro resolve it finally.

The question i have is around Drift Method of Polar Alignment as written in this article http://starizona.com/acb/basics/using_polar.aspx

That is so far the clearest and simplest article i found.

Question i have for you all is what is indicated by picking star?

I have a CGEM mount, so for me that could be use the GOTO capability to find star and track it, or do i manually get the star to appear in eyepiece (but then how does tracking work)?

Sorry for potentially obvious question but i really want to hammer this out coz so far my alignments are way off and imaging has stopped being fun, and started becoming more and more frustrating (can't even get 45 sec subs).

Hi,

Polar aligning and the GoTo have nothing to do with each other. You polar align your mount, in your case using the  drift align method, so your mount will move following the rotation of the sky . The Goto is to do with mapping your sky or simply telling the mount the location of the DSO's at a given time and date so the mount can find them. If you choose to use mount's Polar Align routine then the GoTo has to pretty accurate as for SW mount's, each cycle of PA is followed a cycle of 2 or 3 Star Align. Star aligning is not required for the manual routines of PA such as drift align.

A.G

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Thanks all, the problem here is that i didn't know that when you activate the mount it will move along RA automatically.

It all makes perfect sense now.

As a result I got some great shots last night that made my jaw draw (too bad i can't upload here).

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