Acceptable polar alignment error for NEQ6

[JamesF]

I spent quite a while last night playing with Kstars/Ekos to align my NEQ6 using an ED80 and ASI174MM.  It uses plate-solving rather than drift alignment.  Once I got the hang of it, which took most of the time, it seemed to work ok, though I ran through a few iterations and got down to 3 arcminutes alignment error, adjusted again and re-ran it to be told that I now had 4 arcminutes error, did the same again and was then told I had 5 arcminutes error.

Mostly what comes out of this I think is that I'm really not happy with the alt adjusters on the mount and I might have to consider some sort of upgrade.  A camera with a wider field of view might also help if the alignment is too far out.  However, does small numbers of arcminutes error seem reasonable for a pier-mounted NEQ6, or could I perhaps do better?  Previously I was drift aligning using PHD2.  Would that be likely to give better results, perhaps?  Should I consider a different method altogether?

James

Edited August 6, 2019 by JamesF

[Davey-T]

A few of the PA / drift aligning apps seem to give different results even though the mount hasn't moved, don't know how accurate they claim to be, if I fiddle with the 10Micron dedicated version that uses plate solving I can get it down to zero PA error but another run gives different results.

I don't think it's worth worrying about too much, it's the results that count and if the guiding gets round stars it's not worth wasting imaging time obsessing over perfect PA IMHO.

Dave

[JamesF]

Yes, I think perfect PA probably isn't possible given the build quality of the mount.  As long as I can get to within "good enough to guide out" then I think that's acceptable.  I just don't have a feel for how good that should be at the moment.

James

[Davey-T]

Given that we're trying to aim a 6 inch tube accurately at a star over 400ly away getting as close as we do is a miracle 😂

Probably / possibly the longer the exposures you're aiming for the more accurate you need to be ? although the 10Micron can do 30 minutes unguided without particularly good PA.

Dave

[andrew s]

Perfect PA is a myth. Depending on where you are imaging the effective pole moves due to refraction. The best you can do is aim at the mean refracted pole.

As long as your field of view is not too large so that field rotation is an issue, then either guiding or model based "guiding" will compensate for the error in PA.

Some professional wide field telescope raise or lower the RA axis depending on where they are imaging.

Regards Andrew 

[stash_old]

5 hours ago, JamesF said:

Mostly what comes out of this I think is that I'm really not happy with the alt adjusters on the mount

There you have to heart of the problem - and the Kstars/Ekos blurb says something about making "fine adjustments" - no chance with the prehistoric adjusters on SW mounts 🙂. Plus using a DSLR doesn't help in my case on Ekos!

IMO Sharpcap is still better than Ekos option for Polar alignment which doesn't (well I couldn't find a way) allow you to zoom in/out the area very much. But the Ekos one does at least use the mounts movement in RA. 

 

[michael8554]

IMO a combination of two things. 

A measurement so tiny you won't ever get the same result twice. 

Wedge and mount adjustments too coarse for the adjustment scales involved. 

PHD2 will happily guide a 5 arcmins PA error. 

Michael 

[kirkster501]

Anything within 30 minutes of arc is acceptable I have found.  Indeed, with guiding in place, sometimes I have just banged Polaris anywhere in the middle of the NEQ6 polarscope circle with no precision fiddling at all with the bolts and that has been good enough for 10 minute subs.

[Jonk]

What I found works well is pick a star in the guidescope on the E horizon at 0 degrees dec, then watch it track across the sky using PHD's closeup of the star.

As the star drifts up or down, adjust E-W, if it drifts left or right, adjust N-S to keep it in the middle of the crosshairs. This assumes you have the camera orthogonal to the mount of course.

This is drift alignment without using any software to tell you what to do.

I found that once the star didn't move in 1 hour, then field rotation wouldn't be visible in most people's subs (assuming most max at 30 minutes).

I learnt not to rely on software at all for PA as none of it appears to be close enough / accurate enough.

Or if you really want to do it old school, use the same method using an eyepiece with a reticle. Just be prepared to sit there for an hour plus to adjust when necessary.

I would ignore errors flagged up by software, they're never going to be accurate, there are too many variables, some of which have been mentioned above.

[newbie alert]

Hi James, what errors you have will be highlighted if it's above your image scale.. lots say to guide at half your image scale , so with your setup it's  sampling at 2.37 with a .85 reducer and 2.01 without.. should be quite easily done with phd drift tool.. I was going to go over to Kstars etc but if it's not as accurate I stick to how it is at the moment..or at least PA and guide with phd2..

[alan potts]

Exactly the same as I was getting James, following the instruction. Ran it, settled and adjust, then it was twice as bad as before.   I moved it in PHD 2 a little at a time along the line towards the circle and got it to 1 arc min fairly quickly. As said in another thread, checked it on 3 different stars last night and got the same result 1 arc min, I am sure that's good enough.

Alan

[Datalord]

Anything within 10 arcmin will give you reasonably small pointing errors, which can be platesolved away. It's easily guidable and 3 arcmin is totally fine. Let it be, don't touch it until it is a problem in your image and enjoy not fiddling with your setup.