Comparing PoleMaster and SharpCap/PhotoPolarAlign Mount Polar Alignment Routines.

[StuartJPP]

I have been using the PoleMaster software for a while now and I am quite happy with it. It certainly serves my needs for my mount and focal lengths.

I did use PhotoPolarAlign from Themos for a short while previously. It does a good job at indicating polar alignment error, but presenting the corrections and adjusting the mount was tedious to use. Lots of iterative steps were involved and no simple correlation between alignment error and mount adjustment required. Now that it is incorporated into SharpCap I thought I'd compare the two side-by-side.

At the start of the session I did a PoleMaster alignment, I then compared it to SharpCap. I am amazed at the speed SharpCap can plate solve the captured image. My netbook is a Lenovo Yoga 3 11" and uses a Core M-5Y10c processor, so no slouch but not a power beast. I was also pleasantly surprised at how few steps there are using SharpCap to polar align. Literally 3 steps and you are done, the plate solving happens automatically and you are basically ready for step 2 before you know it. It would be nice if SharpCap automatically cranked up the exposure and gain of the camera when selecting the Polar Alignment tool, but it only takes a few seconds to adjust...a future feature perhaps

Following are some screen captures I did during the night...

 

1. Starting with the mount previously aligned using PoleMaster, this image shows the first step in SharpCap. Plate solving has already been done long before I had read the dialog box as to what to do next (all the red and yellow squares).

 

2. This image shows step 2 after the mount has been rotated by 90° in RA. Again, the plate solving happens almost instantly.

 

3. This image shows step 3 before any adjustment was made, so basically comparing the PoleMaster result to SharpCap's error.

 

4. This image is a zoomed version of the above but after tweaking the mount in Alt/Azi.

 

5. I then did another PoleMaster sequence.

 

6. Aligning the template.

7. Rotating the mount.

 

8. The error before adjustment, basically SharpCap's result.

 

9. Final adjustment in PoleMaster.

 

10. Monitoring in PoleMaster.

 

11. Back to SharpCap.

 

12. Final tweak in SharpCap.

 

13. Slewed over to Vega and this was the guide graph (ignore the peaks, as it is a combination of me fiddling with the mount and dithering the subs).

 

14. A 20 minute test sub using the Canon 500mm f/4 L IS and Canon 6D guided with QHY5L-II-M and miniGuideScope on Avalon Instruments M-Zero.

 

 

PoleMaster setup (QHY5L-II-M camera with a 25mm f/1.2 CCTV lens):

 

SharpCap setup (QHY5L-II-M camera with 130mm miniGuideScope at f/4.3):

 

How they were both mounted to the OTA dovetail (shown with miniGuideScope):

 

Conclusion(s):
There is probably less then 1 arc minute of difference between my two units. At my focal lengths, either will be fine.

PoleMaster always seems a bit off when selecting Polaris/stars and then fitting the template. I am led to believe that it doesn't really matter where you click on the star as the centroid is calculated...however the template can still be off sometimes. The 25mm lens used on the PoleMaster does give a nice wide FOV in which Polaris is easily identifiable, however at 25mm tiny changes (i.e. a single pixel) has an impact on accuracy.

I knew how SharpCap worked since I had already used PhotoPolarAlign, but it was really easy to use and quick to solve. Physically adjusting the mount seems slightly easier with PoleMaster, but that may be just because I have only used SharpCap once. It may also be that with PoleMaster the update/refresh rate is a quicker due to its f/1.2 lens versus f/4.3 for the miniGuideScope.

 

Thanks for looking...

[jambouk]

Interesting stuff. Thanks for doing the experiments to save us the bother.

One of the astronomy magazines might be interested in this story.

James

 

[StuartJPP]

On 03/11/2016 at 18:53, jambouk said:

Interesting stuff. Thanks for doing the experiments to save us the bother.

One of the astronomy magazines might be interested in this story.

James

 

Well not much chance these days to do much other than test kit unfortunately...

[PeterCPC]

Good job Stuart. It gives me even more confidence in using Polemaster now.

Peter

[red dwalf]

i used sharpcap 2.9 polar alignment tool the other night and found it a breeze to use with my altair astro 60 mm guide scope and a zwo asi 120mm camera, got the guiding error down to 00.00.20" from 00.06.20" i can see a few pole masters coming up for sale soon. 

[clarkent]

Fine work indeed, Stuart. 

I had just bought the Polemaster at the IAS and, having come across the Sharpcap PA routine demonstrated by the developer, I seriously debated returning the Polemaster.

Did you ever try using the Polemaster camera with Sharpcap?

I tried this but could not see any star field with it, altho Sharpcap did recognise the camera as a Polemaster.

cheers

Andrew

[JemC]

Nice report Stuart,

i'm still waiting to try mine out, been working nights since i got it,

[PeterCPC]

Looking at your images, it looks like the axis/pole image on the magnified view does not exactly correspond with the view obtained by hovering the mouse over the axis/pole. I have been finding that to be the case too and have raised it on the QHY Forum.

Peter

[Owmuchonomy]

Thanks for posting Stuart, very informative.  I will be honest and say I have no intention of adopting either option but enjoy reading your beautifully presented thread, as always.  I have been equally as mercenary with Gina's thread on the ASI1600MM!  Thanks again. 

[Jonk]

I tried Sharpcap's polar alignment a couple of week's ago and although it plate solved using the finder guider easily, and told me I was in less than 20 arcsecs, I didn't believe it.

Reason is, I immediately slewed South and did an azimuth drift...miles off.

I suspect it was because the finder was not perfectly aligned to the mount. In fact I know it's not because it's just bolted onto a dual bar.

So, my guess is a polemaster or sharpcap method will probably only work if the camera / scope combo is perfectly aligned to the mount's  axis.

[StuartJPP]

On 16/11/2016 at 21:08, Jonk said:

I tried Sharpcap's polar alignment a couple of week's ago and although it plate solved using the finder guider easily, and told me I was in less than 20 arcsecs, I didn't believe it.

Reason is, I immediately slewed South and did an azimuth drift...miles off.

I suspect it was because the finder was not perfectly aligned to the mount. In fact I know it's not because it's just bolted onto a dual bar.

So, my guess is a polemaster or sharpcap method will probably only work if the camera / scope combo is perfectly aligned to the mount's  axis.

How do I convert "miles off" into arcseconds?

From what I have been led to believe, and from my own findings and I could be totally wrong, is that it doesn't have to be perfectly aligned with the RA axis of the mount. It obviously has to be firmly clamped and have no flexure. I don't agree with people double-sided taping a PoleMaster to their mount...however the focal length of 25mm probably might be fairly forgiving, but since we are trying to utilise every little bit of accuracy out of it (30 seconds of arc), it should be firmly clamped.

Lets disregard any imperfections in the mount for a minute...

When you rotate the mount 360° in RA, a chosen star captured on the camera should produce a perfect circle. It doesn't matter if the camera is pointing "in" our "out" or too far "up" or "down" or "left" or "right" or whatever, it will still form a perfect circle. Now if you then map the centre of that circle to the NCP/SCP, you will be rotating around the NCP/SCP regardless of where the camera is pointing. Of course the camera has to be pointing in roughly the same direction, say at a guess less than 5°.

This implies that if the centre of rotation of the mount in RA perfectly matches the NCP/SCP then the mount is perfectly aligned.

Now the question is...does the PoleMaster software (template based) and SharpCap (nova.astrometry based) determine the exact centre of NCP/SCP correctly for the given time and map it to where it needs to be? I'd certainly hope so...but I don't know.

So what happens if your mount isn't perfect? What happens if you perfectly align your mount whilst pointing to the NCP, but then slew to the south and there was some flex in the mount/tripod that affects that perfect polar alignment? Then it would no longer be polar aligned when pointing south....

There is merit in polar aligning whilst pointing the OTA in the direction of the target and not NCP, especially for long focal lengths but for the focal lengths I use I don't need to worry about that. This would imply mounting the PoleMaster on the RA axis and not the DEC axis.

Just my thoughts...

[dyfiastro]

Thanks for pointing me here Stuart.
 

Your findings would correlate with the issues that I have been having with aligning using the polemaster software.
Having only just go the mount back from being repaired and tuned I was wondering why my guide graph was worse that I thought is should be and was pointing towards an alignment issue. I have used the polemaster software about 6 or 7 times and always got mixed results but wondered just how far off
Here are my findings from tonight.

To be honest I think mine looks a little further off but did not get a chance to try it the other way round as I got clouded out.
My stars never seem to line up in the template very well either in the polemaster software and was starting the think I was doing something wrong.

Have you found any reason behind the polemaster software being so inaccurate or contributing to it? I may have a shot a focusing the polemaster a bit better and seeing if that helps at all.

thanks again

[StuartJPP]

You could try focusing the PoleMaster lens but from the images you have provided it looks pretty well focused already.

Personally I don't think that the PoleMaster is that "inaccurate", to me being within 1 arc minute is probably good enough if you are guiding (obviously depends on focal length), but if you are imaging with the 130PDS I would have thought it would be fine. If you weren't guiding then obviously it would be an issue.

It is a bit frustrating that the stars don't sit perfectly within the template, but I think this is because the PoleMaster uses a cheap(ish) 25mm CCTV lens and I am sure there will be variations from lens to lens so they are catering for that. The problem being that even 1 pixel out at 25mm is the equivalent of 30 arc seconds, only need two pixels and you are back to an arc minute.

Were you using the PoleMaster within SharpCap to do the alignment? It looks like it. I use the miniGuideScope which is 130mm focal length versus the 25mm of the PoleMaster so that give a lot higher resolution.

 

[dyfiastro]

Thanks for the reply.

Yes I was using the polemaster in sharpcap. I have tried using my 9x50 guiding setup with it but always got strange results.
I have a feeling I will be using sharpcap instead of the polemaster software from now on and will see how I get on.

[shamantanthew]

@StuartJPP Will the miniguidescope screw on to the PoleMaster camera? I already have the PoleMaster, but am wondering if I could pick up the miniguidescope like you have and use the two together--first to polar align with SharpCap, then to autoguide.

Edit: nevermind, just clicked that the PoleMaster can't send the autoguide message to the mount 

[newbie alert]

On 11/11/2016 at 12:37, clarkent said:

Fine work indeed, Stuart. 

I had just bought the Polemaster at the IAS and, having come across the Sharpcap PA routine demonstrated by the developer, I seriously debated returning the Polemaster.

Did you ever try using the Polemaster camera with Sharpcap?

I tried this but could not see any star field with it, altho Sharpcap did recognise the camera as a Polemaster.

cheers

Andrew

You have to adjust the gain and exposure settings so you can see the stars

[steppenwolf]

On 12/2/2016 at 20:19, StuartJPP said:

t is a bit frustrating that the stars don't sit perfectly within the template, but I think this is because the PoleMaster uses a cheap(ish) 25mm CCTV lens and I am sure there will be variations from lens to lens so they are catering for that.

Great report Stuart and I agree with the your reason above about the template not being a perfect fit, however, for those who haven't read it, in the manual 'wot i wrote' () for this device, I show how to allow for this on page 4. There is no allowance for being a pixel out though!

[tooth_dr]

My guidescope has a focal length of 400mm and I’ve used it with sharp cap, but it says it’s better if it’s shorter. What are the disadvantages of being longer?

[StuartJPP]

1 hour ago, tooth_dr said:

My guidescope has a focal length of 400mm and I’ve used it with sharp cap, but it says it’s better if it’s shorter. What are the disadvantages of being longer?

Depends on the arcseconds per pixel of your camera and guidescope combination but I would have thought that the main disadvantage would be that you have to be pointing pretty much accurately to begin with. I doubt you could just put the mount down and point in the general direction of the NCP and go from there. 

With the PoleMaster I can basically do that, eyeball it roughly and I will get Polaris in view and be pretty much guaranteed to fit everything on screen needed to do the alignment.

At 400mm you'd probably be adjusting quite a bit as things may not even be in the FOV....

But I may just be speaking rubbish...

On the flip side you should be able to get a more accurate polar alignment with the longer focal length...

[tooth_dr]

5 minutes ago, StuartJPP said:

Depends on the arcseconds per pixel of your camera and guidescope combination but I would have thought that the main disadvantage would be that you have to be pointing pretty much accurately to begin with. I doubt you could just put the mount down and point in the general direction of the NCP and go from there. 

With the PoleMaster I can basically do that, eyeball it roughly and I will get Polaris in view and be pretty much guaranteed to fit everything on screen needed to do the alignment.

At 400mm you'd probably be adjusting quite a bit as things may not even be in the FOV....

But I may just be speaking rubbish...

On the flip side you should be able to get a more accurate polar alignment with the longer focal length...

 

I see, no I guess I was lucky as it was like 1'24" when I set it up by eye using the reticle, was able to fine tune it at more then.   That is if it accurate I suppose.

[StuartJPP]

If you are using a polar scope to begin with then there would be no issue as you'd be pretty close to start with unless your polar scope was very poorly collimated. 

[tooth_dr]

Just now, StuartJPP said:

If you are using a polar scope to begin with then there would be no issue as you'd be pretty close to start with unless your polar scope was very poorly collimated. 

I did adjust the polar scope during day time so it’s good. Thanks for the write up on the thread. Very informative.