For MX/ME Users. Expectations re Guiding and T-Point

[PRejto (Peter)]

Originally Posted by Terry B

I don't have a PMX but have seen similar problems with Dec guiding with my tak mount.
If I turn dec guiding off and only guide on RA there will be a very slow dridt one way depending on how good the polar alignment is. I can go for ~20 mins with little drift evident. Superimposed on the drift will be almost random movements both directions due to seeing changes. If the Dec guiding is set too sensitive the guider will then bump the mount 1 direction. Sometimes it will overshoot and then try to bump it back. This will take time though due to the backlash in the dec system requiring more then 1 bump of the guider to move it back. This can set up the pattern you see.
I have also had occasions when a cosmic ray hit or interference has occurred on the guider and the mount has reacted wildly to this pushing the mount a long way and then it overshoots correcting it.
I mostly turn dec guiding off to overcome this problem.
Cheers

Terry

Thanks Terry. That's very helpful input! I'd better tighten up my polar alignment. I'm finding that azi is quite difficult to nail when one is close. On the MX often I will move the required tics (from T-Point) and the mount actually hasn't moved. I might need to run 50 points in T-Point just to figure that out and then start over. I am able to chart moves photographically in altitude but when I tried the same trick in azi it failed big time. I'm not quite sure I understand why. Can anyone here provide a method for verifying say a 1 arcmin move in azi photographically?

Thanks,
Peter

[Joshua Bunn (Joshua)]

Originally Posted by PRejto

Can anyone here provide a method for verifying say a 1 arcmin move in azi photographically?

I would have thought do the same as what you did for Alt, but for Azi instead, this is probably what you tried?
A move in Azi would result in the scope moving in RA Depending on where in the sky you were pointing, if im not mistaken. I think this would apply if your scope was pointing at the meridian. So take your image then image link it. Jog your mount 1 arcmin in RA, then move your Azi knob the required amount to get back to the plate solve.
Does this sound right???

Josh

[PRejto (Peter)]

Originally Posted by Joshua Bunn

I would have thought do the same as what you did for Alt, but for Azi instead, this is probably what you tried?
A move in Azi would result in the scope moving in RA Depending on where in the sky you were pointing, if im not mistaken. I think this would apply if your scope was pointing at the meridian. So take your image then image link it. Jog your mount 1 arcmin in RA, then move your Azi knob the required amount to get back to the plate solve.
Does this sound right???

Josh

Hi Josh,

I posted this very question at SB and received quite a response. You might want to read: http://www.bisque.com/sc/forums/t/19832.aspx

Peter

[Joshua Bunn (Joshua)]

Thanks Peter, Quite a response.

[PRejto (Peter)]

Just a bit of an update.

Got under my deck yesterday and discovered to my horror that the bolts holding my pier down were only finger tight! Heat/cold cycles I guess?

So, spent most of last night just redoing T-Pointing and finally dervied a new model that looks pretty good. RMS=8.5 (200 points), MA= 23.5, and ME = 15 (from refracted pole). I managed to do this in two runs. The first was about 50 points that I took care to make sure included the extremes of what is visible from my location. I had never previously taken points that low down but I'm assured by Patrick Wallace that the MA term requires these for more accuracy. I then moved the mount according to the T-Point super model recommendations but I confirmed both moves photographically. Again, from Patrick Wallace I learned how to do this for MA: cos x altitude angle (on meridian pointing North) x arcsec move indicated from T-Point PA = actual move to make. Thus, if T-Point says move 1 arcmin in MA and the scope is pointing 40 degrees dec then: cos(40 degrees) x 1 arcmin = .766 arcmin (the amount you move the scope photographically). It will work anywhere in the sky but on the meridian it's easier because the declination control will move the scope in a way where it is easiest to measure the actual move (in a vertical line if camera has dec=Y).
I used my ST-i in focus mode and the jog controlls to do this.

After making the moves I ran a 200 point model and was pretty happy to see how close it ended up. For the 200 point model I started out by taking about 10 points manually again selecting the extreme points in the sky visible, but for the remainder of the 190 points I confined the run to about 35 degrees and higher.

Should I worry about the MA term, or is it close enough?

Peter

[Joshua Bunn (Joshua)]

Well thats good news Peter.

Originally Posted by PRejto

Should I worry about the MA term, or is it close enough?

Peter

Personally i think your close enough.

Getting to the perfect zero in Alt and Azi may not be the best thing. You may have seen this over at SB. If you type in "Polax" to your command line in tpoint and execute, you will get ideal Azi ans Alt settings to minimize certain undesirable points.

hers mine:

Polar axis settings: ME MA
+82 +12 current setting
+84 +0 refracted pole
+48 +0 minimize field rotation
+76 +0 minimize unguided declination drift
+77 +36 minimize unguided total drift


The polar axis is currently 82 arcsec above and 12 arcsec
to the right of the true pole.


To reach the refracted pole, raise the polar axis by
2 arcsec and rotate the mount anticlockwise by 15 arcsec.


To minimize field rotation, lower the polar axis by
34 arcsec and rotate the mount anticlockwise by 15 arcsec.


To minimize unguided declination drift, lower the polar axis by
7 arcsec and rotate the mount anticlockwise by 14 arcsec.


To minimize unguided total drift, lower the polar axis by
6 arcsec and rotate the mount clockwise by 29 arcsec.


Quite a neat feature


Josh

[PRejto (Peter)]

Originally Posted by Joshua Bunn

Well thats good news Peter.



Personally i think your close enough.

Getting to the perfect zero in Alt and Azi may not be the best thing. You may have seen this over at SB. If you type in "Polax" to your command line in tpoint and execute, you will get ideal Azi ans Alt settings to minimize certain undesirable points.

hers mine:

Polar axis settings: ME MA
+82 +12 current setting
+84 +0 refracted pole
+48 +0 minimize field rotation
+76 +0 minimize unguided declination drift
+77 +36 minimize unguided total drift


The polar axis is currently 82 arcsec above and 12 arcsec
to the right of the true pole.


To reach the refracted pole, raise the polar axis by
2 arcsec and rotate the mount anticlockwise by 15 arcsec.


To minimize field rotation, lower the polar axis by
34 arcsec and rotate the mount anticlockwise by 15 arcsec.


To minimize unguided declination drift, lower the polar axis by
7 arcsec and rotate the mount anticlockwise by 14 arcsec.


To minimize unguided total drift, lower the polar axis by
6 arcsec and rotate the mount clockwise by 29 arcsec.


Quite a neat feature


Josh

Josh,

Many thanks for that tip. I wasn't aware of it!!

Peter

[PRejto (Peter)]

It looks like I perhaps should go one more step. It seems MA ought to be at least "0" or on the positive side rather than -23.

If I understand things correctly I can do this by making the recommended changes and then synching back into a "portable mount." I think if I did 50 points I might be able to save my 200 point model. Have I got that right?

Polar axis settings:

ME MA

+67 -23 current setting
+88 +0 refracted pole
+48 +0 minimize field rotation
+82 -0 minimize unguided declination drift
+82 +20 minimize unguided total drift


The polar axis is currently 67 arcsec above and 23 arcsec
to the left of the true pole.


To reach the refracted pole, raise the polar axis by
21 arcsec and rotate the mount clockwise by 28 arcsec.

To minimize field rotation, lower the polar axis by
19 arcsec and rotate the mount clockwise by 28 arcsec.

To minimize unguided declination drift, raise the polar axis by
15 arcsec and rotate the mount clockwise by 28 arcsec.

To minimize unguided total drift, raise the polar axis by
15 arcsec and rotate the mount clockwise by 53 arcsec.

Peter

[Joshua Bunn (Joshua)]

Yes, you need to use the portable recalibration option to save your large Tpoint model. And, i would agree that it would be better to be on the +ve side rather than -ve of the refracted pole in MA.

Josh

[PRejto (Peter)]

Originally Posted by Joshua Bunn

Yes, you need to use the portable recalibration option to save your large Tpoint model. And, i would agree that it would be better to be on the +ve side rather than -ve of the refracted pole in MA.

Josh

Josh,
I've got another question for you about the recalibrating into a portable mount option. I've remembered doing this in the past and not being too happy with the result. Let's say you do a lot of points to recalibrate back. At the end, though, the RMS is certainly not even close to what it was with the original super model, and, you cannot run the super model again (unless I'm doing something wrong to start with). In fact, the PA reports are quite different between a non-super T-Point run and Super model T-Point run, so for example if the main purpose was to re-establish ME and MA you seem to end up with numbers that cannot be compared to the numbers you were trying to modify. I've noticed that MA doesn't change so much between a super/non-super PA report, but that ME varies quite a lot. Is there a way out of ths situation or is the only answer to start over from scratch with a new model?

I'm imagining that somehow you could reverse the super model and continue to add points to the original model, and then run the super model again. I suppose that just isn't possible.
Peter

[Joshua Bunn (Joshua)]

Originally Posted by PRejto

I'm imagining that somehow you could reverse the super model and continue to add points to the original model, and then run the super model again. I suppose that just isn't possible.
Peter

You cant do this. Once the supermodel/model is finished you cant add points to it.

I hear what your saying about the RMS values not being as good. Is that the only issue, is it just that the RMS values are not as good or is the mount not pointing how you want it to? Ive wondered about this too and posted about it on the Sb forum some time ago. I was asured my mount was using the larger 2,3,4,500+ original model i initially made and not the short 20 point model. I guess the not so good RMS results you see for the recalibration are just for reestablishing the terms for the large model which is doing the work. In short, all is good and the larger model is working as it should.

I wouldn't use the 20 point recalibration data to gauge your PA, this needs to be done using a 50 point model (see the last post on this page). But if you do end up doing another larger tpoint model, you will get a better approximation of the PA and can make the recommended adjustments with more confidence as there are more samples to work from.

After you have done the 20 point recalibration, does the scope point how you want it to?

Does this answer your question?

Josh

[Bassnut (Fred)]

You guys sound like you know what yr talking about, so here's a question.

The super model corrects on the fly, so does PEC, looks like potential conflict to me. What gets priority?, can't have both correcting at the same time, or do they talk to each other and behave?.

[Joshua Bunn (Joshua)]

Well, they are suppose to work together, I use them together and it works for me. What do others think?

here is a post that describes it.


Josh

[Bassnut (Fred)]

Access to this Resource is Denied

mmm, must be secret, thought as much.

[Joshua Bunn (Joshua)]

Originally Posted by Bassnut

Access to this Resource is Denied

mmm, must be secret, thought as much.

Yeah, you need to be registered.

here is a snippet, hope im not violating anything:

This is the best link for an explanation as to what PrecisionPEC is and does versus Protrack,
http://www.bisque.com/sc/forums/p/9710/35396.aspx#35396
From the link the following is per Patrick Wallace and I quote.
"ProTrack and PrecisionPEC are complementary, each specializing in different parts of the same job.
PrecisionPEC eliminates the all-important high-frequency irregularities in tracking rate that are caused by the tiny residual imperfections in the RA worm drive. Protrack applies the TPoint all-sky pointing model to mop up the much slower drifts caused by such things as flexure.
Although Protrack could be made to do at least part of what PrecisionPEC does, it would require an inconveniently dense TPoint mapping run, and the diagnostic displays and tools offered by PrecisionPEC would be lacking."
There are also 2 additional links to provide examples showing how Protrack improves un-guided tracking by eliminating errors like polar miss alignment, refraction, and flexure. One is here from a customer,
http://www.bisque.com/sc/forums/p/8034/28616.aspx#28616

It should be noted that by not compensating for refraction and or tube flexure you can't expect pixel level tracking for extended periods without guiding. Extended being in many cases longer then about 1 minute with any appreciable focal length. Often times the sagging OTA will cause several arcseconds of error in only a few minutes.

Another more drastic example of what Protrack can do was posted here by me. By showing images before and after Protrack was enabled and after introducing a 30 arcminute polar alignment error to demonstrate, actually 28.5 arcminutes off the pole before and after ProTrack using 67 points and Super Model.

http://www.bisque.com/sc/forums/p/12...280.aspx#46280
By employing both of these even while guiding you will compliment the overall tracking performance.
And I almost forgot What is Protrack??
http://www.bisque.com/help/paramount...s_protrack.htm

[Bassnut (Fred)]

Excellent, thanks for that Joshua.

[PRejto (Peter)]

Originally Posted by Joshua Bunn

I hear what your saying about the RMS values not being as good. Is that the only issue, is it just that the RMS values are not as good or is the mount not pointing how you want it to?

I wouldn't use the 20 point recalibration data to gauge your PA, this needs to be done using a 50 point model (see the last post on this page). But if you do end up doing another larger tpoint model, you will get a better approximation of the PA and can make the recommended adjustments with more confidence as there are more samples to work from.

Josh

Hi Josh,

you have been busy fielding all sorts of questions!!

Thanks for the clarificationn. I'm not really so concerned with pointing, but if the aim is to improve PA perhaps doing a recalibration (with 50 points) will work just fine. But, the real issue for me in doing that is that you end up with recommendations that cannot be super-modeled. Given that the original data was super-modeled, and one is trying to refine those numbers, after a recalibration you end up with numbers that really don't tell you if your adjutment moves actually helped. Am I making any sense here? So, it seems that if you want to compare super model numbers before with super model numbers after the only way is to start a new model from scratch.

The only way I can imagine out of this situation might be to save the original super model of many hundreds of points. Start over with a 50 point run and run the super model. That would tell you if the adjustment was correct and PA actually better. If so, discard that new model model and re-load the original super model and do a 50 point recalibration into that model. The PA report wouldn't necessarily show an improvement but you would know that it was actually better. Can you think of another way?

Peter

[gregbradley]

I don't quite understand this thread. I am not sure why you would need to know about converting arc minute errors into how much you adjust your mounts PA? Super Model or PA report does that automatically and tellls you how many ticks to adjust. Is this for those whose mount is not an SB so they have a way of knowing how much they need to adjust their mount?

I also have a question as I have only just started using ProTrack to help with guiding.

I did a full new callibration run the other night. I adjusted the PA just the small amount it said to. I ran another large model this time 330 points. It took about 3 hours to do (perhaps my 10 second exposure length at 3x3 binning is too long).

It came up with the same adjustments needed to the dec and RA axes but also said they were good enough.

I did a 5 minute unguided exposure with Protrack turned on and it had stars duplicated 3 times like a correction was applied at 3 points.

But then when I did my normal PEC autoguided image there was a noticeable improvement in the roundness of the already quite good stars. Everything was tighter and the final image will show better resolution and sharpness because of it.

Do you find you can do a 5 minute sub with sharp round stars with just ProTrack?

Martin Pugh at AAIC said you had to experiment and see what combination works, Protrack on PEC on, PEC off Protrack on, Protrack on PEC on. He said for his mount Protrack with PEC gave a worse result. My limited test here seems to show an improvement (although I have not done a 15minute sub with PEC on and Protrack off yet). Per that thread they are complementary and work on different aspects off error. Its nice to have a tool that sophisticated to take the tracking to the next level.

As far as adding to an existing model when you try to do an automated callibration tpoint run it gives you several options. Can't you just use the sync function or short mapping run if something has been adjusted manually to bring the model back to reflect the current state of the mount after physical adjustments?

Greg.

ps. RMS f 8.5 is very low. Mine is 21.1 after 330 points at 3 metre focal length.

[PRejto (Peter)]

Hi Greg,

I cannot answer all of your questions, but I can attempt it with a few.

1. The initial point of this thread is probably lost somewhat in the current discussion, though it is all ultimately about getting better guiding in the end. When I discovered that the bolts on my pier to the concrete base were loose I needed to start my model over and that has led to the current discussion.

2. I think the problem you experienced is exactly the main point under discussion. That is, you spend many hours doing a long T-Point run, apply the super model and then discover that the mount requires 1 or more smallish physical adjustments. Yes, our paramounts have tics to make this easier, but as you can see in your own application, moving the mount a small amount seems not to have resulted in a change in your new T-Point model. Small moves may actually not really move the mount no matter what the tics say! So, many of us verify the change photographically so that we know without question that very small changes have actually taken place. There was a recent statement at SB about this. Namely that if you make a physical move, and use the same points to compare the before and after results, that the change WILL be refected in the PA report (as long as there are enough points collected). Therefore I would hazzard a guess that when you thought you had moved the mount a little bit it actually didn't change.

I'm sorry if I'm being pedantic and have not understood your message!!!!

3. My issue is that you and I may run a very lengthy T-Point that we then super model, correct? If you look at the PA reports before and after running the super model they are frequently quite different values, especially the ME term (elevation). The point I was making in my last post is that if you have a large T-Point model that you wish to save and continue to use, but you need to refine the PA as well, is can this be done?. The answer is yes...you can make the adjustments recommended by T-Point (probably best to photographically confirm those adjustments) and then pick the option to "recalibrate into a portable mount." That option will supposedly figure out the new ME and MA terms but preserve all the other terms in the original super model. This sounds great until you actually try it. I'm not saying it doesn't work. I'm sure that it does. However, what you see (after say doing a 50 point recalibration) is not the original super model refined. What you see are just the non-super model results of the 50 point run. The super model runs behind the scene (I guess) and you still get great results. However, if the reason that you recalibrated was to check on the final results of the recalibration by reading the new PA report the numbers reported for polar alignment cannot be directly compared to the numbers in the initial super model report. So, it appears that in the end (if you recalibrate) you have to take it on faith that the numbers for PA actually improved. The only way out of this situation would be to do a full T-Point run starting from scratch every time you make physical adjustment to the mount because this would allow one to make direct comparisons between each run as to PA accuracy.

I'm sure if I've got this wrong somebody will jump in. I'm actually hoping there is a different answer! And I hope I have not confused this even more!! It reflects my current understanding of this.

PS. I would think that your RMS value might be a bit better. Are you running the super model? The reason I ask is that prior to running the super model (and accepting the result) my RMS was around 60. After super model it went to 8.5 If you have been skipping the super model (and I kind of doubt you are) be sure to look at the before and after values in the polar alignment report. I did my last run at ca 1750 mm, but I've got no mirrors to move at all.

PSS. My memory of what Martin said was just a tad different. I'm not remembering him making any comments about PEC being turned of to use ProTrack. I think ProTrack requires PEC to be working. I think he said to experiment with guiding having ProTrack on or off. I know of several people that have more trouble with guiding with ProTrack on. I cannot comment for myself about this because I have made a decision a while back to "uncomplicate" things re guiding by doing my experiments with Protrack off (but PEC on for sure).

Peter

[Joshua Bunn (Joshua)]

Originally Posted by PRejto

I'm not really so concerned with pointing, but if the aim is to improve PA perhaps doing a recalibration (with 50 points) will work just fine. But, the real issue for me in doing that is that you end up with recommendations that cannot be super-modeled. Given that the original data was super-modeled, and one is trying to refine those numbers, after a recalibration you end up with numbers that really don't tell you if your adjutment moves actually helped. Am I making any sense here? So, it seems that if you want to compare super model numbers before with super model numbers after the only way is to start a new model from scratch.

If your aim is to improve PA, and you want to preserve your 200 point model, yo will need to save / export your model, i think as a dat file, then start a new pointing run which will clear everything (it wont delete your save data). Run your 50 modeling points, make the recomended adjustments until your happy with it. Its not an iterative approach but you may want to run 2 maybe 3 runs. Make sure you get pointing samples low down.

Once your happy with your PA, clear the model then import your saved 200 point model and super model it. Now you need to do a recalibration run using the portable mount option. Choose about 20 points from the SCP area up to Dec 0, over to the west and to the east, and scater some down near the horizon. once the points are captured, press finish, no super modeling required. Your Done, and although the scatter plot and RMS values may not look like or as good as your 200 point data, the mount will be using your 200 point data.

Originally Posted by PRejto

The only way I can imagine out of this situation might be to save the original super model of many hundreds of points. Start over with a 50 point run and run the super model. That would tell you if the adjustment was correct and PA actually better. If so, discard that new model model and re-load the original super model and do a 50 point recalibration into that model. The PA report wouldn't necessarily show an improvement but you would know that it was actually better.
Peter

Thats it, although 50 points for recalibration is not necessary, 20 is fine. 6 is what the manual says i think, but 20 has been mentioned on the forum.

Josh.