Polar alignment and pointing issues with my PME

[gary]

Originally Posted by rally

I think Gary meant to say polar refraction is around 90 arc secs not arc minutes !

Thanks Rally! In Sydney the refracted pole is approximately 90 arc seconds above
the true pole.

[Paul Haese]

Originally Posted by rally

Guys this is all in the manual.

Yes but it seemed to imply that Tpoint points you at the refracted pole. It does not say it will get you close and then you just turn on Protrack. The manual is not all that concise in this matter. I personally have read the manual on Tpoint at least 10 times now and never got the impression that it points you at the true pole and you just need to add for the refracted pole.

Originally Posted by rally

To get a good PA you really don't need many points at all, 10-20 is plenty, the extra mapped points are needed for Tpoint to correct for other terms of mechanical error and for Protrack.

I did several runs at 30 and the runs at 50 and 100 gave me better results. I startted doing runs of 100 once I got the PA right with the smaller runs, however the slop in the Dec axis meant that Tpoint reported the PA was way off. Yes the extra points are for pointing only and not for PA. That is the main aim here. I can drift align to get good PA but I want good pointing Rally. There is no confusion here in my mind.

Originally Posted by rally

PE has nothing to do with the polar alignment and pointing corrections - PE is usually much less of an error than the refracted pole to true pole error ie an arc second or two for PE verses an arc minute or two for atmospheric refraction shifting the optical polar alignment.

Good Guiding will take care of all of this in any event without Protrack

Yes PE is worm gear related and not a pointing related problem. PE affects guiding and tracking and nothing more. What makes you think we were meaning this?


Thanks Gary for your explanation.

[rally]

Paul,

Protrack will not improve your polar alignment any further - that is not what it is intended to do.

If the mount is setup fairly accurately and level (use an inclinometer on the PME base and on the OTA) then with Tpoint you should be able to get a very reliable polar alignment in 2 rounds of 6-20 points

The first run is the rough run - just do 6, 7 or 8 points depending on how close to the SCP you are and how good the sigma is
Make an adjustment and repeat maybe going to between 10 and 20 points.
This usually provides sub 10 to sub 20 arc secs accuracy and this is in the field on a portable pier.
With just 20 points your pointing accuracy should be within a few arc secs across the sky.

This will be good enough for almost any level of work.

If you have been getting large Sigmas ? then maybe that was an indication of the additional mechanical problems that your particular mount has and maybe that is why you needed 100 points to get some averaging within the mechanical errors - because there is no normal reason to need 100 points purely for polar alignment.


Read this
http://www.bisque.com/tom/Paramount/...djustments.asp

The Tpoint manual tells how to set your mount to the refracted pole (instead of True pole), there is a few pages covering this topic in two sections.
So you must have skipped over those 2 sections 10 times - try searching for "refract"

To get better results and start removing any extra mechanical errors then you do of course need more points for Tpoints statistical analysis engine and Protrack wants lots of extra points too.

My comment re PE was in relation to Greg's question where he asked . . "Or is it not worth it as the differences are so fine that it is lost in PE anyway?"

Why on earth would you even attempt to drift align a P-ME ?
Surely that defeats the whole purpose.

Rally

[Paul Haese]

Originally Posted by rally

Paul,

Protrack will not improve your polar alignment any further - that is not what it is intended to do.

When did I say it would. Please re read my post on this. I said,"It does not say it will get you close and then you just turn on Protrack." I did not say it would improve my polar alignment???

Originally Posted by rally

If the mount is setup fairly accurately and level (use an inclinometer on the PME base and on the OTA) then with Tpoint you should be able to get a very reliable polar alignment in 2 rounds of 6-20 points

This usually provides sub 10 to sub 20 arc secs accuracy and this is in the field on a portable pier.
With just 20 points your pointing accuracy should be within a few arc secs across the sky.

Once again read posts further back when I explained I had found I was getting good pointing on the east side of the meridian and not moving the dec axis too much, but when I started doing bigger runs I found the pointing error was far too large. That prompted me to look for more mechanical errors.

I accept what your saying is ok and Tpoint manual suggests this, but some people have suggested I use a 20-50 point models to get better Tpoint suggestions for PA adjustment.

Originally Posted by rally

If you have been getting large Sigmas ? then maybe that was an indication of the additional mechanical problems that your particular mount has and maybe that is why you needed 100 points to get some averaging within the mechanical errors - because there is no normal reason to need 100 points purely for polar alignment.

Yes that is right, I was not doing 100 points for PA but once PA was within a few seconds doing short runs I was looking to do the pointing model. Check on the PA after the 100 point run as a matter of course disclosed not only large PA error but large pointing errors and hence it led me to look for the mechanical error I found in the Dec axis.

Originally Posted by rally

The Tpoint manual tells how to set your mount to the refracted pole (instead of True pole), there is a few pages covering this topic in two sections.
So you must have skipped over those 2 sections 10 times - try searching for "refract"

No it does not. I have just done the refract search and it does not categorically state how to set for the refracted pole. Neither the PDF version nor the hard copy version I have here states exactly how to do this!

No need to be rude either by suggesting I skipped over something. I have very strong English literacy skills (Law degree and post graduate)and I can well argue that this manual does not tell you how to set for the refracted pole. I have read it all the way through that amount of times and I can attest what your saying is not true. You must have another copy that says otherwise. Even doing the search of refract. It does state that Tpoint makes refraction calculations (ergo one would think it has this in mind for the suggested adjustments to correct PA on the mount), It also states it is best to shoot for the refracted pole, it does state that southern hemisphere mounts will have a positive ME when aligned on the refracted pole, it also shows the figure that Gary has put up. However, it does not state how one goes about aiming for the refracted pole as you suggest. One can infer that you need to do the calculations and move the altitude axis until it reaches 72 for my location. At any rate, inference is not telling one how to do something. Not being argumentative here Rally but you were incorrect in what you said.

Originally Posted by rally

To get better results and start removing any extra mechanical errors then you do of course need more points for Tpoints statistical analysis engine and Protrack wants lots of extra points too.

Ah yes and had you read further back you would be seeing that is what I have been doing. The larger models have been showing me that I have had large mechanical errors. The smaller models were not showing any large errors and that is what was confusing. Not only with the RA issue but also with the Dec having slop in it too. The north south pointing on larger models as you pointed out early on meant there was something amiss. Finding all this out whilst trying to sort out software (I now have that sorted by using Maxim, Tpoint mapper and pinpoint) which was giving me incorrect plate solves has been part of the examination process to sort this out.

Originally Posted by rally

Why on earth would you even attempt to drift align a P-ME ?
Surely that defeats the whole purpose.

Rally

When I was trying to determine what the problem was initially with the software (CCDsoft5 and automapper II) I did a polar alignment and it took me 2 hours to get it to 20 minutes each side. Quite frankly that was quicker than all the stuffing around I have had to go through to get software that actually works (not to mention the expense of doing that) and find all the problems and correct them. The mount was imaging fine in that I have done 5 completed images with it since I installed it, but I wanted better pointing and this led me down this track to finding out that there were problems in the mount that could have been avoided by installing parts better despite being told everything would be ok. So while I agree that doing a PA by drift defeats the purpose, I had to do it to confirm was something amiss. Simply sending the mount back to Ken or SB did not seem appropriate and I would not have learnt as much about this mount as I have. However, you can do a drift alignment on these mounts and it can be done very quickly to get it to 20 minutes each side (ie meridian and east stars).

Thanks for taking the time to put what you said down. Still appreciated but try not to treat me like a child; I don't really appreciate that. I have invested a lot of time in sorting these issues out and probably I would hazard to say more time than anyone has ever done with one of these mounts. Hence reading documentation several times to ensure I have not missed something. So when I say I have read something you can be assured I have studied the text, and understood what was being said. I did not simply skip things!! Reading is not skimming something. That is all I have to say about that.

[rally]

Paul,

Then why do you bother asking for help if you already know everything and have read everything and comprehend what you have read and then get upset when someone tries to take the time and effort to help you and become defensive, aggresive and call them a liar.

I referred you to the manual because that is THE best place for the explanations and the guides that will help you.
I would not normally bother going to try and cut and past the text without all the tables and diagrams in here when its already in the manual but since you deny they even exist - here they are.

The current Tpoint manual is located on the Software Bisque site
The old manuals are almost identical.
http://www.bisque.com/sc/media/p/28002/download.aspx
You need to be logged in to get it - I assume the link will work since its an aspx engine
That page is located here Filed under: TPoint, Documentation, Classic Docs, User Guides, Telescope Pointing Analysis System
Use those steps if the link doesnt work.

The Manual is entitled
TPoint for Windows
A Telescope Pointing Analysis System
User Guide
Revision 1.32

Pages 29 and 30 refers to the section on Polar alignment, the table Gary posted is there.

It states

"Optimum Location of the Polar Axis
The optimum polar-axis setting depends on what declination and hour angle you're observing. For observations on the meridian, somewhere between the true and refracted pole is best. Ideally, one should shoot for the refracted pole. For typical users in the continental US the refracted pole is 1-1.5 arcminutes above the true pole. For observers in the northern hemisphere, this corresponds to ME = -60 to -90. For southern hemisphere observers it's plus not minus, so for Sydney Australia aim for ME = +90.
The following table shows the angular distance (in arcseconds) between the refracted and unrefracted poles for different latitudes and elevations.

----------
Note - Table is inserted here but I havent included it
----------

If you want to set your polar axis to the refracted pole, and you're in the northern hemisphere, aim for ME = minus the tabulated value. In the south aim for ME = plus the value."

I think the last sentence is pretty clear - "If you want to set your polar axis to the refracted pole" (as opposed to the True Pole) . . .


On Page 40 in the section entitled
Using TPoint to Suggest Telescope Modeling Terms

It states
"Remember, it is best to shoot for the refracted pole, which is about 90 arcseconds above the true pole. This has the same effect as the “Kings Rate” in that it will somewhat average out the effects of atmospheric refraction. See the “Optimum Location of the Polar Axis” on page 29 for more information."

Referring the reader back to page 29

On Page 48 in the section about Polar Alignment errors section
It states

"Polar Alignment Error in Elevation
In the Northern Hemisphere, a positive ME means that the pole of the mounting is below the true (unrefracted) pole. A mounting aligned on the refracted pole will have a negative ME. In the Southern Hemisphere, a positive ME means that the pole of the mounting is above the true (unrefracted) pole. A mounting aligned on the refracted pole will have a positive ME."

This again clearly differentiates between the True Pole and the Refracted pole and explains what readings you will get and why you will get them.

On page 74 in the "ME Term" description section

It states :
"Term: ME
Vertical misalignment of the polar axis of an equatorial mount: a rotation about an east-west axis equal to coefficient ME.
Notes: In the Northern Hemisphere, positive ME means that the pole of the mounting is below the true (unrefracted) pole. A mounting aligned the refracted pole (for most telescopes probably the simplest and best thing to aim for in order to avoid unwanted field rotation effects) will have negative ME.
In the southern hemisphere, positive ME means that the pole of the mounting is above the true (unrefracted) pole, and a mounting aligned the refracted pole will have positive ME."

I would repeat my suggestion that either you didnt read this or you didnt comprehend these clear specific and well explained references to the Refracted and Unrefracted (True) poles and how to set up your mount.

Good luck with it.

[bert (Brett)]

I have used tpoint for the sky6. And I felt like I was stabbing in the dark. Since I have upgraded to skyx, I have found the tpoint side of things to be more intuitive. Skyx clearly states the elevation value to the true pole, and to the refracted pole.

Brett

[Bassnut (Fred)]

What a great thread, what a mine of really usefull information

[bert (Brett)]

Not trying to hijack the thread.... I'm doing a polar alignment on a pme atm.

I seem to get pa elevation with a consistant result of under an arcminute out, I think from memory around 50 seconds from the true pole. But azimuth seems to fluctuate, there is no pattern in the tpoint model that tells me that there is a mechanical problem. I ended up doing a drift align and adjusted till I was getting no pixel drift in 10 minutes (.86 arc sec per pixel). Tpoint still tells me that I am 2 minutes out.

I guess my questions are;

How many arc seconds out do consider a good polar alignment?

How consistant are your result from a 30 point models? (eg within 20 arc seconds etc)

Thanks
Brett

[gregbradley]

The pdf that has the Tpoint tutorial and example goes over the number of points you need before you can introduce extra terms.

From memory I think that was about 50. There is a maximum number of terms suggested you can add with the lower number of map points.

You also need to fit the data and remove outlier to make the model more accurate.

Only then it says is the PA model accurate.

So the idea you can do 6-20 may work but it isn't what is in this tutorial.

In my experience with my setup (limited experience) I manually did models of up to 30 points. I found the PA suggestions sometimes made the guiding worse so I therefore assume further away from accurate PA.

I did 200 point model (it only took about an hour) and used those suggestions. They improved things. Mind you it is easy to adjust the elevation the wrong way so if there is any doubt about which way to turn that check the manual before you do it.

I have also done a few 50 - 100 point models and after fitting the data, getting rid of outliers, adding terms the PA adjustments seemed to improve things. I do an autoguiding straight after an adjustment as final proof I did improve things and not worsen them.

If you aren't doing large enough models, and removing outliers then adding terms then the PA adjustments may not be correct as they will change when you do this.

I use 3x3 binning when using my Proline 16803 or 2x2 binning with the ML8300, 10 second exposures, slew time seems to be about 5-6 seconds, download time is 1 second so the whole thing from slewing to the point, taking the exposure, plate solving it takes 19 seconds on my setup. I could probably reduce that to 15 seconds by using 6 second exposures if it were a moonless clear night.

So try the above and see how it goes.

It does seem a bit elusive to get it exactly perfect. My setup is very close (the PA adjustments said zero in one axes) yet I sometimes get slightly eggy stars still. I am wondering if I am getting flexure in my guide camera in the MMOAG.

Greg.

[Paul Haese]

Final update - Problem solved

Firstly, I want to thank everyone who contributed to the thread. I would like it to be instructive of how to sort problems on an expensive mount if it is second hand and how to get things sorted on the software side.

My recommendations-

Don't use any other programs other than MaximDL, Pinpoint, Tpoint, Tpoint mapper and TS. This combination gives me 100% pointing plates solves except when an obstruction is in the way (ie cloud or roof or trees). Its plate solving has resulted in the model below.

Check your mount thoroughly. In my case I had an incorrectly installed worm gear (grub screws were lose) and the dec axis has a worn spring in the spring plunger assembly (soon to be fixed too but still operates at present).

Read the manuals several times and call mates to help in understanding and nutting out a fairly vague set of instructions (Rally I will have agree to disagree about how things are read). Use this thread as an instructive base for where to start. Each persons gear is different but some common aspects are present.

Check all the connections to camera and gear regularly, sometimes things slip or just work lose (this did not happen in my case but I just checked anyway).

Now for the piece de resitance, the model below is 181 points. Pointing without terms was 62 arc seconds across the sky. I got clouded out and could not finish a 250 point run but the model was reducing in arc seconds on each hit after 150.

The PA information on the first part of the model at 50 points showed no need to move the mount and so i continued on to do a larger run. It now shows that ME is nearing the refracted pole for my observatory and and the MA is just a touch out. Some mild refining could be done later but for now it is not necessary.

The fit information shows that the terms used were as per recommendations (that sigma should be around or less than half of the value applied). I used 6 extra terms to result in a 10.3 arc second pointing of the mount.

The scatter plot confirms that the run does not have the North South divide it had in previous runs, and this would indicate that the mechanical errors have been eradicated from the system. The remaining errors are mostly going to be lost in the noise.

Now onto imaging again. I did a test pointing at the lagoon through high cloud and the mount put the center of the image in the exact center of the lagoons cluster. Very happy with this results and relieved this matter is finally sorted.

Thanks once again for the help.

PS Brett I am thinking 15 seconds in MA is fine and something around your refracted pole is fine. In reality anything under a minute on each is pretty good. Just my opinion though.

[frolinmod]

Originally Posted by rally

Tpoint's Polar Alignment Report corrections relate to the True Pole.

Tpoint for Windows as used with TheSky6 aligns you to the true pole.

Tpoint Add On for TheSkyX Professional aligns you to the refracted pole.

[gary]

Originally Posted by frolinmod

Tpoint for Windows as used with TheSky6 aligns you to the true pole.

Tpoint Add On for TheSkyX Professional aligns you to the refracted pole.

Hi Ernie,

Just to qualify.

Whenever you see the ME term in either version of TPOINT, it is always referring to the true pole.

Both versions provide a verbose alignment recommendation report in English text
which is an interpretation of the numerical MA and ME values.

The TPOINT version that works with TheSky6 gives a verbose report in terms of
aligning to the true pole.

The TPOINT version that works with the SkyX gives a verbose report in terms of
aligning with the refracted pole and the report has an explicit note that alerts to this.
The report in this instance gives both ME and what the report refers to as 'Ideal ME'.
'Ideal ME' is simply ME plus or minus the correction interpolated from the lookup table
in the manual.

The 'Ideal ME' is somewhat of a misnomer as it is not necessarily always the ideal
point to which to align the polar axis anymore than the ME point, as discussed
earlier. Depending on where one is imaging in the sky, the optimal point will
be somewhere in the range true pole to refracted pole.

Since circumstances therefore change as to where one might want to align the
polar axis, the ME value for the true pole is what is important to know as then
one can simply make the necessary offset to it.

The bottom line -
Whenever someone reports ME, it will always refer to the true pole, irrespective
of the version.

[gregbradley]

So if you are using Tpoint for windows you would look up the offset to true pole and go for being a bit above what tpoint for windows says is perfect?

For Sydney is that about 90 arc secs?

Also with X Y errors when autoguiding do the X errors relate to alt?

I redid my PEC tonight and got quite a similar curve to what I got a few months ago. I then double checked (using the auto button to check it) to check the east west button. It turned out my PEC curve was upside down. Now corrected the PEC is helping not fighting the autoguider and I am getting rounder stars which is a relief.

This mount is so accurate this little fine points make a huge difference. My mount has about 2.3 arc seconds of PE peak to valley. That's pretty phenomenal.

Also do what is the consensus about usong Protrack?
If I had say a 250-300 point model of the whole sky would I expect to see an improvement in tracking if I activated Protrack whilst using PEC and autoguiding?

I have turned it on when I had a 200 point model and it seemed to worsen slightly the guiding.
Greg.

[frolinmod]

Originally Posted by gregbradley

If I had say a 250-300 point model of the whole sky would I expect to see an improvement in tracking if I activated Protrack whilst using PEC and autoguiding?

I guess I'm lucky. I get an improvement here with ProTrack and only 190 or so points. Fewer points does indeed appear to make it worse rather than better, at least for me. I'm using TheSkyX and having Tpoint compute a supermodel. I suspect that how many points are needed before tracking gets better rather than worse with ProTrack is probably unique to the particular equipment load and it's characteristics (i.e., the model that gets computed vs. reality and the residuals between them).

[Paul Haese]

Just a small epilogue here people. Last night I went down to the obs and managed to get my PEC sorted (0.5" peak) and then before the cloud came in I got a 12 minute sub. Part of the sub was guided and then I lost the guide star, so 6 minutes or so were unguided. Pointing was just about as close as I could expect. The image attached is of Corona Australus. The two stars with the small amount of nebulosity at center (in between the two stars) is where the sky pointed the mount; and the image center is just a thousanth or so out (most likely the 10" or error). In other words, great pointing. Spica was also pointed at for the PE data and that was so close to center. So proof of concept and pointing has worked. And; as you can see nice tight stars. Pretty happy with that too.

[frolinmod]

You should absolutely be able to image unguided at a focal length of four meters for at least five minutes with nice round stars.