Why polar align?

[autonm]

 I read about polar alignment - but if you can use plate solving, or via using Eqmod and say cartes du ciel - so that it knows what object we are pointing at ?

(ok a little advanced) but what accuracy do you lose without polar aligning ?

 

thanks.

[kens]

Without polar alignment your target will drift in decliination. This is a problem for unguided imaging. Even with guided imaging, a poor polar alignment will cause field rotation leading to traling stars.

[Luna-tic]

In order to track, the EQ mount must be oriented so the declination and right ascension axes are properly aligned. A polar alignment assures the axes are in the correct configuration for the telescope to move properly when tracking an object. The computer can only do so much, it has to have a basis to operate from, which is basically something to tell it which way north is. Of course, an "all sky" alignment can do this, but it would force the telescope drives to work both the declination and right ascension axis, inducing error along the way that would prevent much, if any, chance of decent photography. With proper alignment, only the right ascension axis need operate to track an object.

[newbie alert]

9 hours ago, autonm said:

 I read about polar alignment - but if you can use plate solving, or via using Eqmod and say cartes du ciel - so that it knows what object we are pointing at ?

(ok a little advanced) but what accuracy do you lose without polar aligning ?

 

thanks.

You need to get polar and star alignment separated.. accuracy the star alignment is what helps find the targets..polar alignment accuracy stops field rotation and has been rightly said above allows the mount to track only on the RA axis..

[barkis]

Unless you have a good Polar Alignment,
Only stars centred on, or around a guide star will not be misshapen, the others, progressively outward to
the edge of the target field, will start to look like comets, the longer the exposures, the worse they will look.
The star field will tend to rotate around the central portion of your Image.

[newbie alert]

3 hours ago, barkis said:

Unless you have a good Polar Alignment,
Only stars centred on, or around a guide star will not be misshapen, the others, progressively outward to
the edge of the target field, will start to look like comets, the longer the exposures, the worse they will look.
The star field will tend to rotate around the central portion of your Image.

I'd of thought that was to do with spacing..

If it's not aligned properly then all stars will drift

[RayD]

As @newbie alert notes it appears from your OP that you may be crossing star alignment with polar alignment.

Polar alignment is effectively pointing the centre axis of your mount directly at the celestial pole, so that as we rotate, thus the stars move, the mount rotates at exactly the same angle, preventing star trailing in longer exposure imaging.  If visual only, or very short exposure imaging it isn't as critical as long exposure DSO's.

Star alignment is a different thing, and it is what tells your mount how far out your telescope is from where it thinks it's pointing, to where it actually is (cone error).  This is where plate solving comes in as you can solve and synch to let EQMOD know what this offset is. 

[barkis]

8 hours ago, newbie alert said:

I'd of thought that was to do with spacing..

If it's not aligned properly then all stars will drift

Only by accurately aligning your mounts RA axis precisely on the North Celestial Pole, (Not Polaris). will your scope track
true, misalignment of that setting, will cause star tails on edge of the field especially with   long exposures. 

[newbie alert]

58 minutes ago, barkis said:

Only by accurately aligning your mounts RA axis precisely on the North Celestial Pole, (Not Polaris). will your scope track
true, misalignment of that setting, will cause star tails on edge of the field especially with   long exposures. 

Long exposure will differ from one person to another..

As I understand it once aligned the mount will track in RA  ..and odd shaped stars in different parts of the image can be lots of things such as flexture, coma and spacing...mis alignment in PA the whole image will drift showing the errors..Sara posted this previously in another thread describing spacing 

[barkis]

Of course there are other effects at work in earths atmosphere that can adversely affect an imaging
run.  Also if the imaging system has not reach ambient temperature can disrupt things too.   
Atmospheric diffraction can be disruptive also. These are elements out of control of the imager.         
I'm not aware of the significance your spot grids are supposed to convey in respect of Polar Alignment.     

This thread is mainly a discussion on the merits or otherwise of good polar alignment of an equatorial mount. 
It seems to be wandering away from that topic.   Flexure, insecure objective, a loose focuser, insecure camera,
and many other anomalies can be a source of image shift during exposures. However we try to assume 
All those anomalies are absent in our example, and the mount is perfectly aligned on the NCP, that being the case, 
the result can be expected to be as perfect as possible.

 

[newbie alert]

13 hours ago, barkis said:

Unless you have a good Polar Alignment,
Only stars centred on, or around a guide star will not be misshapen, the others, progressively outward to
the edge of the target field, will start to look like comets, the longer the exposures, the worse they will look.
The star field will tend to rotate around the central portion of your Image.

Not wondering..i suggesting that your response could be from spacing as suggested in the last paragraph of your response(see second image)

[RayD]

I think the OP has been answered in that it may be that he is confusing PA with star alignment, so the potential effects are pretty irrelevant.

Very good PA is really only absolutely essential when doing long exposure imaging, not so much otherwise.  It certainly can't be mitigated by plate solving, which is what makes me think he is mistaking it for star alignment.

[barkis]

Perhaps the importance I attach to it, is because in my early years of imaging with my Olympus OM 1 film camera,
long focal length guide scope and  illuminated cross hair  eyepiece,  accurate PA was essential. No digital wizardry was available
to counter any errors.  An arduous trial of patience and determination. No walking away to leave the  system to get on with it.   
Perhaps I've  not integrated fully yet, into the modern methods of AP.  I  will shortly be embarking on the road to Digital AP, I've certainly invested enough 
money on the necessary equipment to do so.   I'm also very aware that spending big money, and having the necessary gear, is no guarantee of success. 
The human ingredient can sometimes be the bug bear in the endeavour  .   

 

[Olsin]

On 10/2/2017 at 00:49, autonm said:

 I read about polar alignment - but if you can use plate solving, or via using Eqmod and say cartes du ciel - so that it knows what object we are pointing at ?

(ok a little advanced) but what accuracy do you lose without polar aligning ?

 

thanks.

 

You just lose tracking accuracy which makes photography a no go. Using your telescope for visual observations (eye instead of camera) though doesn't really require polar alignment.

[cletrac1922]

Southern Hemisphere is totally different

We do not have a Polaris as a guide for polar alignment

Southern Cross points towards South Celestial Pole

Use a compass allowing for magnetic variation when setting up EQ5 mount to find true south

The leg with N also has to face South.

Then do a 2 star alignment of mount

Cheers

John