Near zero angle rotation=good polar alignment?

[BrendanC]

I've been struggling with polar alignment recently, but started using Sharpcap Pro. It's great - but totally does not agree with my polar scope.

The weather in the UK has been so bad recently that I haven't had the chance to try out any other technique to verify whether Sharpcap is right, or my scope is right, such as AstroTortilla, PHD2, NINA, drift align etc etc

However, I just stacked 270 images from my most recent session (which was seven weeks ago - yes, I really have not had a decent night since!), and DSS tells me there was not an angle of rotation about the reference frame that was greater than +/- 0.1 degree.

In that session, I used Sharpcap for polar alignment.

So, given that I'm just tracking, not guiding, does this imply that Sharpcap did indeed do the business? Does a very low angle of rotation imply good polar alignment? I think it should, I'm just wondering.

[gilesco]

3 minutes ago, BrendanC said:

I've been struggling with polar alignment recently, but started using Sharpcap Pro. It's great - but totally does not agree with my polar scope.

The weather in the UK has been so bad recently that I haven't had the chance to try out any other technique to verify whether Sharpcap is right, or my scope is right, such as AstroTortilla, PHD2, NINA, drift align etc etc

However, I just stacked 270 images from my most recent session (which was seven weeks ago - yes, I really have not had a decent night since!), and DSS tells me there was not an angle of rotation about the reference frame that was greater than +/- 0.1 degree.

In that session, I used Sharpcap for polar alignment.

So, given that I'm just tracking, not guiding, does this imply that Sharpcap did indeed do the business? Does a very low angle of rotation imply good polar alignment? I think it should, I'm just wondering.

If you are following the sharpcap process correctly, then it will give you accurate polar alignment, from what I can see it is essentially the same process as the kstars polar alignment tool and works by platesolving two rotated images. It uses data from your imaging train, so it will be accurate measured against your imaging train.

Your polar scope may need calibrating to your imaging train if it is disagreeing with sharpcap's process.

[fifeskies]

It can come down to how good the polarscope alignment is in the mount body.

I have an NEQ6 as well , and I have not bad alignment as is , but I have heard of others where the polarscope needs some serious tweaking to get it accurate (there are 3 little screws to do this).

The Sharpcap result is liable to be the best way to verify actual alignment of the mount.

 

 

 

(just beaten to the reply)

 

Edited August 28, 2020 by fifeskies

[andrew s]

You mainly get drift if your just tracking and mainly rotation if guiding if the polar alignment is out.

Regards Andrew 

 

[BrendanC]

Sure, and I agree that Sharpcap is great, and of the two I should believe Sharpcap over my polar scope.

But I was wondering whether the evidence is in the stacking stats ie does a very low angle of rotation throughout the entire stack, from first to last, imply good polar alignment? Because if it does, then it answers the question - Sharpcap worked. In which case, yes, I agree, the polar scope needs adjusting. But I'd just like people's opinions here first, based on these results!

[BrendanC]

2 minutes ago, andrew s said:

You mainly get drift if your just tracking and mainly rotation if guiding if the polar alignment is out.

Regards Andrew 

 

Ah, that appeared just after my last post. Interesting. So low angle of rotation might not necessarily suggest good polar alignment then? In which case it would be better to compare the first and last frames and see whether there's drift rather than rotation?

Edited August 28, 2020 by BrendanC

[gilesco]

To turn the question on it's head, if you came to ask why you had rotation on your stack of frames, I might suggest that perhaps your Polar Alignment wasn't too good, but in itself the reverse is not necessarily true...

[ollypenrice]

20 minutes ago, BrendanC said:

Ah, that appeared just after my last post. Interesting. So low angle of rotation might not necessarily suggest good polar alignment then? In which case it would be better to compare the first and last frames and see whether there's drift rather than rotation?

Comparing a first and last image is very informative. If your software will do it, ask it to align on stars and combine images without cropping the borders. You'll then be able to see just what has happened to the pointing during the run. 

To align your polarscope set up the mount so you can see a distant feature like a steeple or chimney through it. It should be distant, a mile or two. Put the crosshair on the feature.  Now rotate the mount in RA to rotate the polarscope. Does the crosshair stay on the distant feature? It should, but it may describe a circle, in which case you need to adjust the three radial screws which locate it.

Olly

[BrendanC]

Thanks. In my case the scope seems so far out of alignment that, after using Sharpcap, Polaris is barely within the FOV. Which is why I'm so keen to determine exactly what's going on. 

[Seelive]

Could it be that the telescope optical axis isn't correctly aligned to the mount axis (cone error)?

[BrendanC]

It could be, and that's the next thing I'm going to check - when I get any clear weather!

Nice suggestion though. I'm going to continue believing Sharpcap is right because everyone loves it (and it is so very easy to use - what a killer feature!), but I would like to get the polar scope at least kind of pointing in the right direction, just because, well, I want to. So yes, cone error probably comes into this too.

In the meantime I'm going to analyse what DSS is telling me about any drift as well as rotation.

Thanks again everyone, great suggestions that help me move forward again!