Polar Alignment, 2 star alignment?!?

[farunj]

Hi all,

Sorry if this question sounds kind of dumb, but what is the difference between a polar alignment, and a 2/3 star alignment? Aren't they the same? If not, how is one supposed to do a polar alignment? The mount is a go-to one which my college has, and I think it's an HEQ5 but not sure.

Cheers,

Varun.

[Cornelius Varley]

Hi all,

Sorry if this question sounds kind of dumb, but what is the difference between a polar alignment, and a 2/3 star alignment? Aren't they the same? If not, how is one supposed to do a polar alignment? The mount is a go-to one which my college has, and I think it's an HEQ5 but not sure.

Cheers,

Varun.

They are two separate things. Polar alignment aligns the equatorial mount with the north celestial pole and allows the mount to track accurately across the sky star and alignment is used to map the handset's built in model of the sky onto the real sky. 

[wuthton]

You need to polar align the mount first.

http://m.youtube.com/watch?v=oXgtGQq-3V0

Then star align using the handset and telescope.

[ronin]

Polar alignment is for setting up an EQ mount, for the mount to work best the axis of rotation has to be accurately aligned with the rotational axis of the earth.

2 or 3 star alignment is for a goto. The software will determine how well aligned and set up scope/mount actually is and then enable corrections to be applied to follow something better.

On an Alt/Az goto mount the 2/3 star alignment is absolutely necessary, on an EQ you do not necessarily need it, just a motorised EQ mount has no 2/3 star alignment. However if the goto is on an EQ then you will likely have to do it anyway, the software is simply such that you have to do it..

[Paul M]

Polar alignment is the process of aligning the RA axis (the one with the polar scope through it) as precisely with the celestial pole as possible.

For visual observing Polaris is near enough. For imaging a longer process of aligning with the true pole is required.

Polar alignment is needed so that the motion of natural motion of the stars can be tracked by driving only one axis of the the mount - the RA axis.

For GOTO purposes the mount/handset needs to know where the telescope is pointing.

So after Polar alignment you still need to "program" the handset. Locate the first alignment star and enter on the handset. That gives the handset a rough idea. The second star alignment allows the handset to calculate the scope's  (actually the mounts encoders) position more accurately. Three star alignment gives even better GOTO accuracy.

So if you only polar align and center an object in the field of view and start the mount tracking at sidereal rate  the target will stay centered. But GOTO won't work without first aligning on a few stars.

You can do a three star alignment but if your polar alignment is way out then the mount won't track objects very accurately. But you need to be somewhere near with polar alignment to have a chance with 2 or 3 star GOTO alignment.

[Coastliner]

Right. I too am a newbie, and asked a similar question a few weeks back. Let's see if the knowledge has sunk in - I fully support anyone who wants to jump in and correct and/or embellish!

When you do a 2 or 3 star alighnment you will end up accurately tracking the object, but the scope will be moving in two directions at once, i.e. both sets of motors will be running, in order to track "across" and "up/down". The problem is that for long exposures you will notice that the object rotates in the sky, so that as you take several shots the image rotates relative to the frame.

When you do a polar alignment your scope axis is parallell to the axis of the Earth's rotation, this results in the scope only having to use one motor to track the image, and that image's orientation to the frame  will always be the same, meaning you can take long exposures without rotation blurring.

Is that about right guys?

Neil.

[Coastliner]

"Making Every Photon Count" by Steve Richards explains this perfectly, at around page 17 as far as I remember.

[Thalestris24]

Hi

Just to add, I believe certain Celestron mounts have ASPA - all star polar alignment which performs alignment on several stars and additionally polar alignment via a calibration star, independent of Polaris.

Louise

[Coastliner]

Louise, do you happen to know if the Celestron Nexstar 4se has ASPA?  I can't see polaris from my house so this would be very useful.

[Paul M]

Right. I too am a newbie, and asked a similar question a few weeks back. Let's see if the knowledge has sunk in - I fully support anyone who wants to jump in and correct and/or embellish!

When you do a 2 or 3 star alighnment you will end up accurately tracking the object, but the scope will be moving in two directions at once, i.e. both sets of motors will be running, in order to track "across" and "up/down". The problem is that for long exposures you will notice that the object rotates in the sky, so that as you take several shots the image rotates relative to the frame.

When you do a polar alignment your scope axis is parallell to the axis of the Earth's rotation, this results in the scope only having to use one motor to track the image, and that image's orientation to the frame  will always be the same, meaning you can take long exposures without rotation blurring.

Is that about right guys?

Neil.

That's about right.

Normally an EQ mount will only run the RA motor for tracking. It assumes you have polar aligned and so all it needs to do to compensate for Earth's rotation is tick along in RA.

If, however, you have a guide camera fitted then that will send signals to the mount to make small corrections in RA and DEC if required. As you say though, too far out with polar alignment and you will get field rotation even when guided.

The Alt Az GOTO mounts continually drive both axes but they are only good for visual because field rotation is huge. There are often wedges available for those mounts to make them more usable for imaging.

[Cornelius Varley]

Louise, do you happen to know if the Celestron Nexstar 4se has ASPA?  I can't see polaris from my house so this would be very useful.

Only equatorial mounts need polar aligning so the SE4 mount doesn't need this function.

Right. I too am a newbie, and asked a similar question a few weeks back. Let's see if the knowledge has sunk in - I fully support anyone who wants to jump in and correct and/or embellish!

When you do a 2 or 3 star alighnment you will end up accurately tracking the object, but the scope will be moving in two directions at once, i.e. both sets of motors will be running, in order to track "across" and "up/down". The problem is that for long exposures you will notice that the object rotates in the sky, so that as you take several shots the image rotates relative to the frame.

When you do a polar alignment your scope axis is parallell to the axis of the Earth's rotation, this results in the scope only having to use one motor to track the image, and that image's orientation to the frame  will always be the same, meaning you can take long exposures without rotation blurring.

Is that about right guys?

Neil.

That is only true for alt-azimuth mounts. Once polar aligned an equatorial mount will track an object using only the ra drive.

[Thalestris24]

Louise, do you happen to know if the Celestron Nexstar 4se has ASPA?  I can't see polaris from my house so this would be very useful.

Hi

Sorry, I've no idea but you could look it up on Celestron's web page or Google it. I happen to have done research on the Advanced VX mount which does have ASPA. I recall you have to align on 2 stars and then use a calibration star on the other side of the meridian - I'm sure someone else can confirm.

Louise

[farunj]

Ok I think I have understood the concept.

So what I have to do is to first polar align the scope using the polar scope and then once I am roughly polar aligned, I have to use the handset and do the 2/3 star alignment? If I just want to observe, then is it necessary to do the PA? Or can I just setup the mount, and then do the alignment using the handset?

Also for imaging, I will have to have a near perfect PA right? Can anyone post a link to a video or some material that explains how to achieve accurate PA?

Thanks for all the replies!

Varun

[Gomtuu]

So what I have to do is to first polar align the scope using the polar scope

The polar scope is for polar aligning the mount, not the scope.

[farunj]

Right, I meant aligning the mount!

[Blazar]

Hi

Sorry, I've no idea but you could look it up on Celestron's web page or Google it. I happen to have done research on the Advanced VX mount which does have ASPA. I recall you have to align on 2 stars and then use a calibration star on the other side of the meridian - I'm sure someone else can confirm.

Louise

Best to do a 2 plus 4 calibration stars...then polar align using ASPA. Then switch the whole mount off, and do another 2 plus 4, then another ASPA.... you can then check the alignment in the handset. and do anther one if you really want to. It works quite well to be honest.

took this 60 second exposure with it the other night, focus was slightly off...