Polemaster - Accuracy

[Jkulin]

Nah Ray, the reason why I was looking was if there was a double star was it the cause of the triangulation and the potential for inaccuracy, not because it maybe/was a double star.

So can't you use SharpCap Pro if you use an OAG?

[RayD]

5 minutes ago, Jkulin said:

Nah Ray, the reason why I was looking was if there was a double star was it the cause of the triangulation and the potential for inaccuracy, not because it maybe/was a double star.

So can't you use SharpCap Pro if you use an OAG?

I didn't mean you John, I meant just generally.  I see Polaris as an ovoid but as noted earlier, I believe the software calculates the centroid in a similar way to PHD2, rather than relying on the user to hit dead centre.

SharpCap needs a FOV of 1 - 2.5 degrees (some have success outside this but I didn't) which is difficult with a OAG and not possible or me.

[Jkulin]

Thanks Ray, didn't realise about the FOV, makes complete sense.

[michael8554]

The PHD2 developers say about 5 arc mins is good enough for guiding (as measured by Drjft Alignment)

In fact they recommend a small drift in Dec if you have Dec Backlash, as you can then guide in one direction without having reversals.

Michael 

 

[newbie alert]

14 hours ago, RayD said:

Because it starts from a much closer position so the calculation of the centre of rotation is more accurate.  Give it a go, it works.

I shall try it but..as I see it you're trying to align to a point of rotation on the sky..so you align on that point and get it as good as you can ..sky refraction is the only thing that changes..then you do it again?? Does that point in the sky change?

It's almost like celestrons ASPA  that people do that multiple times..makes no sense..

[RayD]

Just now, newbie alert said:

I shall try it but..as I see it you're trying to align to a point of rotation on the sky..so you align on that point and get it as good as you can ..sky refraction is the only thing that changes..then you do it again?? Does that point in the sky change?

It's almost like celestrons ASPA  that people do that multiple times..makes no sense..

No problem at all, I'm not trying to convince you, I don't need to.

[souls33k3r]

I bought SharpCap pro last night. Small fee to pay so won't hurt to try it nonetheless but Polemaster is my bread and butter and will give the 2 iterations a go first.

Also it was mentioned that polaris seems oval/elongated. I noticed that too and i was a bit concerned that PM was fine at first and now showing eggy polaris. I was ready to open the lens and adjust it but then realised that the PM wasn't sitting flush on the adapter

[1DegreeN]

The resolution of the PM is 30 arc seconds per pixel so you would have to be lucky to get a more accurate PA than that. By contrast, if you were using e.g. a typical 50mm or 60mm guidescope with a QHY5 guidecam (same camera that is used in the PM), the resolution is around 3 to 4 arcsecs per pixel and so using Sharpcap with this combination is potentially 10 times more accurate. You can work out how exact your PA needs to be for your imaging camera and longest sub length. E.g. for your Esprit 100 with ASI1600, a PA error of 30 arcsecs and 10 minute subs gives a drift of around just 1 pixel. 

[fwm891]

Be worth turning on polemaster a few hours after doing initial PA and see if the bulge has rotated too. I don't think Polaris's companion is bright enough or polemaster's lens that good to resolve it. Polaris always quite round on my monitor sitting evenly within the reference circles in the later steps.

[AngryDonkey]

To be honest even at the resolution the Polemaster is working at, I sometimes find it hard to accurately align the two circles during precision alignment purely because the mechanics of the mount are not fine enough, so I don't think I would benefit much from increased accuracy. Having said that since using the Polemaster I've not had any issues with guiding whatsoever so it definitely seems to be close enough.

One great thing I like about the Polemaster is that at any time (even during imaging) I can monitor the polar alignment without having to interrupt the sequence. This way I can easily see if deteriorating guide performance could be related to polar alignment.

[souls33k3r]

If polemaster is so accurate, why does PHD2 not agree with it's alignment?

[Davey-T]

1 minute ago, souls33k3r said:

If polemaster is so accurate, why does PHD2 not agree with it's alignment?

Try running PHD2 twice and it doesn't agree with itself in my experience 

Dave

[RayD]

4 minutes ago, souls33k3r said:

If polemaster is so accurate, why does PHD2 not agree with it's alignment?

It does if you do a drift align.

[RayD]

2 minutes ago, Davey-T said:

Try running PHD2 twice and it doesn't agree with itself in my experience 

Dave

That was my experience too with the static one.

[souls33k3r]

20 minutes ago, Davey-T said:

Try running PHD2 twice and it doesn't agree with itself in my experience 

Dave

God help us all then. Relying on the software for our guiding which doesn't agree with itself other than drift aligning. Insane! 

I was hoping to see someone able to do drift align in 10 minutes but seems like it's at least an hours worth of a jobby. I start loosing patience hence PM keeps me sane. 

If PHD2 can't be relied upon for it's PA error, why do we get so fixated about it's guide graphs and numbers? Unless the guide graph only reflects the balancing/mount dec/ra behaviour and not the PA.

[Davey-T]

21 minutes ago, RayD said:

That was my experience too with the static one.

Wasn't a criticism of PHD as such, the problem is all the other variable flies in the ointment 

Dave

[souls33k3r]

3 minutes ago, Davey-T said:

Wasn't a criticism of PHD as such, the problem is all the other variable flies in the ointment 

Dave

Says the man with 10 Micron mount who doesn't need to guide for an hour long sub  i'm well jel mate! 

[Davey-T]

Considering that you're trying to aim a 6" tube at a star some four hundred odd light years away there's a lot of room for error so it's a miracle we can get as close as we do.

Manual drifting with a graduated eyepiece is pretty time consuming done properly, easier compromise is to use the green PHD cross with guiding switched off and keep bringing the star back to the cross until it goes the other way and gradually close in on it until it stays put, best to get the guide camera at right angles north up on the screen for this.

General opinion is the poor PA doesn't make eggy stars it just put's more stress on the guiding.

Dave

ps: The 10 Micron PA routine gives different results when repeated, after several iterations it can get down to nearly zero but not worth wasting the time on.

Dave

[Rob63]

If you are not on a permanent mount then you don't really need perfect PA.

I'm using sharpcap pro (guide scope ST80 + ASI120MM) , I adjust until under 10" on both axis and then get on with it! Done in under 5min, add another min if I do it twice (little benefit In my experience)

If I feel the need to check it, a PHD drift and PHD guiding assistant and log viewer usually say my PA error is around 15" - 30" and the mount (EQ6-R) will happily guide that out.

Rob.

[RichLD]

On 27/11/2018 at 00:46, michael8554 said:

The PHD2 developers say about 5 arc mins is good enough for guiding (as measured by Drjft Alignment)

In fact they recommend a small drift in Dec if you have Dec Backlash, as you can then guide in one direction without having reversals.

Michael 

 

+1