Polar Alignment Accuracy

[dazza1639]

Making the most of a brief gap in the clouds last night. I ran the Ekos polar alignment tool on the EQ8 in the observatory. After a few iterations I have got the error down to 1 minute 32 seconds of arc. 

My question is, do you think this is good enough or do I need to get it lower, if so what should I be aiming for?

Thanks

Darren

[david_taurus83]

I haven't polar aligned now for about 6 weeks and I'm using an AZEQ6 on a tripod that I take indoors after each session! I have 3 slabs in the garden that I have drilled a small hole where each of the tripod legs sit. I used to polar align each session with Sharpcap but a while back I was doing some testing and I just set up without aligning. I decided to try a few subs at the end so I just fired up PHD and started guiding. The average RMS error was no different to what it usually is if i aligned. So the next night out i done the same and got the same result. So I've been doing it for weeks now. Set mount up on the slabs, power everything up and plate solve my way to target and image. No polar aligning and no PHD calibration. My guiding RMS is typically 0.6" to 0.8".

 

To make my long story short, I'm saying you will probably find little difference in in your guiding result whether your mount is perfectly aligned or slightly out.

[Ouroboros]

It would probably be good enough for me with guiding and using an 80ED refractor which has a focal length of 510mm. Whether it's good enough with your scope of 2000mm focal length I couldn't say.  I too for the first time used the EKOS polar alignment routine and got an error just under a minute of arc. The test of the pudding and all that is how good the stars look.  I am reasonably pleased with the results I got recently with 300s exposures. Did you try taking an image?

PS I forgot to mention. Like David  I obtained RMS guiding errors of around 0.75" when all was running smoothly. 

Edited October 10, 2019 by Ouroboros

[dazza1639]

My stars are usually pretty round before my re-alignment and my initial error was over 7 minutes. It was a very brief clear period last night just enough time to do a few iterations of the alignment routine. My last image before the alignment was the Crescent Nebula, 10X600s exposures in Ha.

 

[Ouroboros]

9 minutes ago, david_taurus83 said:

I haven't polar aligned now for about 6 weeks and I'm using an AZEQ6 on a tripod that I take indoors after each session! I have 3 slabs in the garden that I have drilled a small hole where each of the tripod legs sit. I used to polar align each session with Sharpcap but a while back I was doing some testing and I just set up without aligning. I decided to try a few subs at the end so I just fired up PHD and started guiding. The average RMS error was no different to what it usually is if i aligned. So the next night out i done the same and got the same result. So I've been doing it for weeks now. Set mount up on the slabs, power everything up and plate solve my way to target and image. No polar aligning and no PHD calibration. My guiding RMS is typically 0.6" to 0.8".

 

To make my long story short, I'm saying you will probably find little difference in in your guiding result whether your mount is perfectly aligned or slightly out.

I'm quite surprised at this and somewhat impressed - not least due to your carrying the mount and tripod in and out like that! 🙂   Nevertheless guiding seems to compensate for quite a lot of polar alignment error. 

How long were your subs? 

[david_taurus83]

^^^ 15 minutes!

 

When I do polar align with Sharpcap the starting error was always around the same. Down around 5 arc minutes and left or right by a minute or 2.

Edited October 10, 2019 by david_taurus83

[michael8554]

The PHD2 developers say up to 5 arcmins is easily guided out.

So no point spending time getting it spot-on.

Michael

[kens]

The main issue from poor polar alignment when guiding is field rotation causing elongation of stars

If you plug in the details of your specific imaging situation into the field rotation calculator at http://celestialwonders.com/tools/rotationMaxErrorCalc.html you can se if field rotation is an issue. It gets worse the closer you are to the celestial pole but for the vast majority of likely situations, 5 arcmin is indeed good enough.

 

[dazza1639]

Thanks. I will see how I get on with my current PA.

[newbie alert]

If it's in a observatory on a fixed pier just spend abit of time drift aligning in phd..once done you won't need to redo for a long time..

Depending on your image scale dictates what you should be looking for in terms of rms figures

[dazza1639]

It is in the Observatory. I have aligned using the Polar Alignment routine in Ekos. It is currently reporting an error of 1'32" so I'm less than 2 arc miniutes If we get some clear weather I will give it a go and see if it improves my guiding. I currently get an RMS of about 0.7" and that was with a mis-allignment of about 7 arc minutes.

[JamesF]

6 minutes ago, dazza1639 said:

It is in the Observatory. I have aligned using the Polar Alignment routine in Ekos. It is currently reporting an error of 1'32" so I'm less than 2 arc miniutes If we get some clear weather I will give it a go and see if it improves my guiding. I currently get an RMS of about 0.7" and that was with a mis-allignment of about 7 arc minutes.

I used Ekos and was getting errors around the same values.  I do wondeer if the mount mechanics actually allow any better than that.  I'm not unhappy with the star shapes I'm getting from my 80ED at ten minutes.  I have some 15 minute exposures I can check.

James

[JamesF]

Here you go.  A screen grab from roughly the centre of a 15 minute sub of the Crescent at 4x actual size.  I don't think I'm going to distress myself too much about the star shapes 

James

[dazza1639]

They look pretty good to me.

[JamesF]

It will be interesting to see how you get on with the C8 starting from a very similar position.  I intend to set up my C9.25 at some point soon and I have no idea what to expect there.  Assuming the weather ever gives either of us the opportunity, that is.

James

[JamesF]

Assuming you're using the C8, that is.  I missed the Equinox 80 in your list of kit

James

[dazza1639]

I haven't used the C8 in ages, I do plan on trying it out as it is still mounted next to the Equinox 80 on my EQ8.

[JamesF]

In the past I've mostly used the C9.25 for planetary and given the altitude of Saturn and Jupiter this year I've not been in a huge hurry to get it back in use.  I'm thinking I'd like to have a go at using it for imaging PNs as well now.  I probably need an OAG, but given what I've paid in the last couple of weeks for the septic tank installation, plumbing and electrics for the barn conversion my bank account is now quivering in fear in the corner.  For the  time being I think I'll have to make do with what I have.

James

[blinky]

I am also wondering how good polar alignment needs to be and also how good can it be.....  My mount is an old EQ6 Pro, if i use PHD and use the drift align feature, will it ever be really, really accurate due to the inaccuracies in the gears.  I think mine is currently around 5mins out and at 800mm focal length/0.9 arcsecs/pixel I can get round starts some of the time in 5 min exposures.

[dazza1639]

The consensus seems to be that less than 5 minutes is good enough for guiding. I can't reccomend enough the polar alignment routine in Ekos, it is really simple, I ran the process 3 times in about 1/2 an hour. I could never get drift aligning with PHD to agree with the handset when I used to use the Nexstar polar alignment routine.

[newbie alert]

Just now, dazza1639 said:

The consensus seems to be that less than 5 minutes is good enough for guiding. I can't reccomend enough the polar alignment routine in Ekos, it is really simple, I ran the process 3 times in about 1/2 an hour. I could never get drift aligning with PHD to agree with the handset when I used to use the Nexstar polar alignment routine.

It won't agree with the ASPA,  it won't agree with sharpcap or polemaster either, they all disagree with each other..just have to work out which one agrees with what you're doing..

[JamesF]

10 minutes ago, dazza1639 said:

The consensus seems to be that less than 5 minutes is good enough for guiding. I can't reccomend enough the polar alignment routine in Ekos, it is really simple, I ran the process 3 times in about 1/2 an hour. I could never get drift aligning with PHD to agree with the handset when I used to use the Nexstar polar alignment routine.

Ekos PA I did find relatively straightforward.  PHD took me somewhat longer to get the hang of.  I can't always use Ekos as my HEQ5 doesn't have a particularly good view of the area around the NCP so I had to stick at PHD until I could get that working as well.

James

[johneta]

I use Sharpcap and find it so easy to use I Polar align every time. It usually agrees with PHD quite closely

[newbie alert]

28 minutes ago, johneta said:

I use Sharpcap and find it so easy to use I Polar align every time. It usually agrees with PHD quite closely

Agrees with phd drift align?

[johneta]

Yes agrees loosely with PHD Guiding assistant. Sharpcap can report down to a few seconds off perfect polar alignment (dubiously optimistic I'm sure).

The PHD guiding assistant usually will show less than 1 minute of Polar align error. Loose agreement as far as I'm concerned.