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Cem25P: guiding in Dec problem?

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#1 ippiu

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Posted 27 June 2019 - 01:54 AM

Hi.

 

Here some things i've already managed to eliminate dec belt backlash

https://drive.google...HK5TUUqCZalP5n8

and also to fix play in dec worm.

https://drive.google...U_HmqqaD0Q42HVf

 

My guiding setup is:

Asi 120 mini + guidescope 60/228 mm

 

I usually guide with success around RMS 1,2 or less with no problem:

https://drive.google...tl_GplEPn-qvOJ3

 

Did polar alignment either via Sharpcap and via IPolar (always difference about 2.5-3' from each other)...

 

Since some days, i'm experimenting a bad behaviour in Dec: after some minutes phd2 sends Dec pulse guiding to cem25 (i clearly hear the sound) but the mount doesn't react and doesn't correct. Dec starts drifting and drifting: i have to stop guiding.

This is the "less worse":

https://drive.google...FIFqX_I4vjRBRsC

Attached File  PHD2_GuideLog_2019-06-23_232545.txt   53.96KB   2 downloads

 

But this is the worst case:

https://drive.google...sTxD8vpxgKyoYPQ

https://drive.google...owdMn2kfwSyP_L3

Attached File  PHD2_GuideLog_2019-06-26_223734.txt   131.37KB   4 downloads

 

Another worst scenario:

https://drive.google...HhtG2GSMFtgKiWX

Attached File  PHD2_GuideLog_2019-06-18_225211.txt   148.83KB   3 downloads

 

No loose cables, no wind (extremely static atmosphere), everything tight well (i checked everything more times before guiding), tripod and mount well stable on stone courtyard....

 

Thanks for help


Edited by ippiu, 27 June 2019 - 02:26 AM.


#2 OldManSky

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Posted 27 June 2019 - 08:47 PM

Looking at your logs, one thing I would suggest would be to make your min-moves smaller.  If you open one of the log files in the PHD2 log viewer, click on a guiding session, and then check the "Limits" box at the bottom, you'll see lines on the graph that show the limits of the min-move settings -- your error has to go above/below those limit lines before PHD2 will send a correction.  In the 6/18 log, second guiding session, you have the DEC min-move set to 0.7 -- 70% of your guide camera's pixel size in DEC.  Since your guide camera is at 3.39"/pixel, you're letting the guide star get about 2.5" away from center before it will even start to issue a correction.  Then it has to fight to get it back in place.

Drop your min-move to around 0.1 or 0.2, and see if that helps, and at the same time try lowering the DEC aggressiveness just a bit (you have 80%, I'd suggest 50%).  That should get you smaller moves more often, which is what I find works best.



#3 ippiu

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Posted 28 June 2019 - 06:14 AM

Thanks so much, Oldmansky.

I save your post and looking forward trying your suggestions...

 

And regarding Dec continuous drfiting with no mount reaction to phd2 pulse, what do you think? 


Edited by ippiu, 28 June 2019 - 06:24 AM.


#4 OldManSky

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Posted 28 June 2019 - 08:05 AM

I think my suggestions above will help prevent that.

The image below is from one of your logs.  I noted the upper/lower limits based on your min-move settings (red dashed lines marked top and bottom).

In the area I highlighted, your mount gets more than 2.5" off in DEC without issuing a single pulse correction -- because the min-moves are too high.

When the mount finally gets to about 3" low, PHD2 starts issuing corrections, but it looks like they're mostly taking up backlash.  They finally stop the low drift, and bring the mount back in line, at which point things start going close to 2.5" low again without any corrections being issued.

 

Tighten up the min-move -- a lot -- and I think you'll see better results.  With my CEM25P, lots of smaller moves keeps things nicely going.

Good luck!

 

Attached Thumbnails

  • phdsetting.jpg


#5 ippiu

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Posted 28 June 2019 - 08:22 AM

I have understood perfectly what you said in your first reply.

And now with your second reply i confirm myself that i have understood all.

But probably i'm not so clear what i mean about "drifting forever with no mount recovery and reaction even if phd2 issues pulse"...

 

I mean this

What's happened?

 

dec_2.jpg

 

dec_2_1.jpg


Edited by ippiu, 28 June 2019 - 08:24 AM.


#6 OldManSky

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Posted 28 June 2019 - 11:33 AM

That's usually what you see when your calibration was on one side of the mount, and your guiding run is on the other side -- and PHD doesn't know that's the case.

 

What happens then is that PHD starts madly issuing corrections to get the DEC back to where it should be, but instead the corrections are going the opposite way, and it gets into a runaway situation.

 

If you're using ASCOM pulse guiding (I think you are, right?), UNCHECK the box in PHD settings to "Reverse Dec Output after meridian flip" in the Advanced Setup tab.  You only need that checked when you're using ST-4 cable guiding.  If you leave it checked when using pulse guiding (and you're connected to the mount so PHD2 knows where it's pointing and what side of the meridian it's on), what will happen is exactly what you're seeing.

 

In that log (I downloaded it), the calibration was done with "Pier side = East", and the runaway guiding session says "Pier side = West."  

There's the problem :)


Edited by OldManSky, 28 June 2019 - 11:34 AM.

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#7 ippiu

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Posted 28 June 2019 - 03:09 PM

you are simply FANTASTIC...

 

In your 3 replies you open my PHD2 world...:)

 

Thanks


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#8 OldManSky

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Posted 28 June 2019 - 03:14 PM

I was in the same place 7 months ago.  Other people helped me. :)


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#9 ippiu

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Posted 28 June 2019 - 03:18 PM

If you're using ASCOM pulse guiding (I think you are, right?), 

of course...

 

I take advantage of your kindness and i ask you this: "assume Dec ortoghonal to RA" has to be checked? Or unchecked?

 

Immagine.png



#10 OldManSky

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Posted 28 June 2019 - 03:50 PM

Leave it un-checked.

The calibration will figure out the orthogonality (or lack thereof). And adjust for any problems.



#11 ippiu

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Posted 30 June 2019 - 06:06 PM

Drop your min-move to around 0.1 or 0.2, and see if that helps, and at the same time try lowering the DEC aggressiveness just a bit (you have 80%, I'd suggest 50%).  

I tried the above settings.

 

Mediocre results.

 

West side: 

1.jpg

 

 

Attached File  1.txt   75.42KB   6 downloads

 

East side: 

2.jpg

 

 

Attached File  2.txt   63.59KB   4 downloads

 

 

What else can i try to achieve a better RMS around 1"?


Edited by ippiu, 30 June 2019 - 06:07 PM.


#12 Tristarcapt

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Posted 01 July 2019 - 08:54 AM

Hi ippiu,

 

I've been using PHD2 for about a year now with my CEM25P and have gotten my RMS down to .7" total RMS consistently (that's about as good as I'm going to get since I'm seeing limited in my area, however I think the mount is capable of better in areas with better sky's than I have).  So I believe you're capable of, at least, achieving that.  It looks like your DEC has a moderate amount of backlash (just like mine).  Your guide logs show, during DEC reversals, it takes several guide pulses before your mount responds.

 

Three things I'd note that may help:

 

1).  Try to do your calibration near the intersection of the meridian and celestial equator, if possible.   I believe that's the recommended area of the sky to do your calibration in.  Looks like yours was done around 32 degrees DEC.

 

2).  You must have run the Guiding Assistant at some point because I see the recommended backlash compensation guide pulse of 1470ms.  But you have Backlash compensation disabled.  I would Enable backlash compensation.  Better yet, I would run the GA again   (make sure you have "measure backlash" checked) and take all the recommendations, including those for min movs.  Just run GA for a couple of minutes otherwise it may recommend too large of min movs.

 

3).  As far as Guiding Algorithms - I changed my RA to PPEC.  Since the mount has a moderate amount of PE, PPEC rapidly identifies this and my RA guiding has significantly improved over Hysteresis.  If you decide to change, just use the default settings when you switch (except for min mov, which you'd get from GA).  For DEC I see you switched to LowPass2 from the default of ResistSwitch.  I'm curious as to why you did this?  ResistSwitch has always worked well for me.  

 

Hope these help, let us know how it turns out.  Your major issues seems to be your DEC backlash, enabling Backlash comp should help considerably.  My mount has about the same amount of backlash (around 1300ms) and I run with Backlash comp enabled and ResistSwitch.  I usually get around .5" RMS in DEC.  Also, using PPEC in RA usually gives me an RMS of <.6".

 

Scott

 

I tried the above settings.

 

Mediocre results.

 

West side: 

attachicon.gif 1.jpg

 

 

attachicon.gif 1.txt

 

East side: 

attachicon.gif 2.jpg

 

 

attachicon.gif 2.txt

 

 

What else can i try to achieve a better RMS around 1"?


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#13 ippiu

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Posted 01 July 2019 - 10:16 AM

First of all, big thanks, scott, for your reply.

 

 

1).  Try to do your calibration near the intersection of the meridian and celestial equator, if possible.   I believe that's the recommended area of the sky to do your calibration in.  Looks like yours was done around 32 degrees DEC.

 

Usually i calibrate near 0 Dec, as i did in the above example: 8.4 dec in 1.txt. In 2.txt it was too late and i wanted to do a last test, i forgot to go to dec 0 to calibrate.

But usually i calibrate between +10 and -10 dec, as phd2 advises.

Do i have also to be at Az 180°? Or is it more important and crucial just 0 Dec (and any Az)?

 

 

2).  You must have run the Guiding Assistant at some point because I see the recommended backlash compensation guide pulse of 1470ms.  But you have Backlash compensation disabled.  I would Enable backlash compensation.  Better yet, I would run the GA again   (make sure you have "measure backlash" checked) and take all the recommendations, including those for min movs.  Just run GA for a couple of minutes otherwise it may recommend too large of min movs.

I usually run GA for 5 minutes as you can see here: Attached File  PHD2_GuideLog_2019-06-26_223734.txt   131.37KB   2 downloads

But GA isn't able to calculate backlash: it always stops and says "unable to establish a consistent south moves.....etc", and finally it doesn't advise anything about backlash. I tried in the past one night to check "backlash compensation" as GA suggested, but things got worse above 2" RMS. I tried everything GA suggested, also including min movs and guiding alghorithms, but nothing brought a substantial improvement: just always around 1.7 - 1.9" RMS. 

 

3).  As far as Guiding Algorithms - I changed my RA to PPEC.  Since the mount has a moderate amount of PE, PPEC rapidly identifies this and my RA guiding has significantly improved over Hysteresis.  If you decide to change, just use the default settings when you switch (except for min mov, which you'd get from GA).  For DEC I see you switched to LowPass2 from the default of ResistSwitch.  I'm curious as to why you did this?  ResistSwitch has always worked well for me.

 

I obviously tried PPEC with no success: error above 2" RMS. For Dec i used Lowpass2 because GA suggested it. May be next time i switch back to resistswitch.

 

So, for the next time i tried your above settings with my new asi290 mini: calibration at 0 Dec (as i usually do) but at 180 Az too??

 

Guiding Assistant for 5 minutes?? Or better 2 minutes??

 

Settings all GA recommends? And for impossibile to measure backlash?? I did at least 3 times this: https://drive.google...HK5TUUqCZalP5n8

 

Ra PPEC and Dec ResistSwitch with default settings (min movs from GA)??

 

Any other recommendations?


Edited by ippiu, 01 July 2019 - 10:20 AM.


#14 Tristarcapt

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Posted 01 July 2019 - 11:25 AM

 

Do i have also to be at Az 180°? Or is it more important and crucial just 0 Dec (and any Az)?

Recommended is within 1 hour of the Meridian.  But I don't think this is really your problem.

 

 

it always stops and says "unable to establish a consistent south moves.....etc", and finally it doesn't advise anything about backlash.

This seems odd, yet you're getting good calibrations without any errors.  How difficult is it move the DEC gear switch on your mount?  Is it substantially different from RA?  The reason I'm asking is I had to send my mount back for servicing.  It was very difficult to rotate the DEC gear switch (with the tensioning knob fully released).  My mount was behaving similarly to yours.  This is what they found.

 

 Your mount was shipped back to you yesterday, UPS tracking # 1Z88F2R20398487906.
Repair log:
1. DEC frame prevented the clutch fully release, trimmed the frame to fit.
2. Re-meshing DEC gears.
3. DEC bearings replaced (click sound).
4. Enhanced motor torque by modifying the board.
5. Tested per protocol.

 

Ever since then DEC and the gear switch is behaving normally.

 

 

I obviously tried PPEC with no success: error above 2" RMS.

Not sure why this is the case.  I helped my cem25p a lot.  You do have to let it run a little bit before the best results show as it learns your mounts PE.

 

 

So, for the next time i tried your above settings with my new asi290 mini

Make sure you use the "New Equipment Profile" wizard to setup your profile.

 

One last thing, what guide rate do you have set in the mount?  Mine's set to .5 sidereal for both RA and DEC, I believe that's the default in the hand controller.  I tried .9 once for both, and my guiding went crazy.  Looked like it was overcompensating.

 

I'm definitely no expert, but to me, your problem looks like classic backlash (and maybe some stiction, but I don't think so).  Once you figure out how to get a good backlash run with GA and enable backlash comp, I think things would get a lot better.

 

Scott


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#15 Tristarcapt

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Posted 01 July 2019 - 11:46 AM

  FYI, here's a recent guiding session to show what I'm getting.  As I said, I live in an area with fair seeing at BEST and normally much worse.  So, I think you should be able to achieve at least what I'm getting.

 

Attached File  PHD2_GuideLog_2019-06-29_220558.txt   447.14KB   4 downloads

 

Best viewed in PHD2 Log viewer which I'm guessing you have.

 

Here's my PHD2 settings algorithms page.

 

Screenshot (7).png

 

This is what's working for me and my cem25p.

 

Scott


Edited by Tristarcapt, 01 July 2019 - 11:48 AM.

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#16 ippiu

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Posted 01 July 2019 - 01:54 PM

Thanks scott for your suggestion and experiences.

 

How difficult is it move the DEC gear switch on your mount?  Is it substantially different from RA?  

It always has been different: RA very tight Dec very soft. I also followed this procedure and checked everything twice: https://drive.google...U_HmqqaD0Q42HVf

No play, gear mesh perfectly, no grinding sound while slewing, everything seems good

 

 

 

You do have to let it run a little bit before the best results show as it learns your mounts PE.

How long do i have to let it run? I tried 30 min before gave up: above 2" RMS

 

 

One last thing, what guide rate do you have set in the mount?  

0,75 both for RA and Dec. Should i lower it?

 

So among all these possibilities, for you, which is the optimal sequence to try next time?

What would you try, if you were me?

1) Change settings: Ra PPEC and Dec ResistSwitch with default settings ( but min movs from GA)??

2) calibration at 0 Dec and 170° Az?

3) Guiding Assistant for 5 minutes?? Or 2 minutes??

4) Change settings according to what GA recommends??

 

Any other recommendations?

 

Ps. I opened your log in phd logviewer but i don't find the calibration session: just guiding session...


Edited by ippiu, 01 July 2019 - 03:31 PM.


#17 Tristarcapt

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Posted 01 July 2019 - 05:25 PM

 

 

How long do i have to let it run? I tried 30 min before gave up: above 2" RMS

 

 

0,75 both for RA and Dec. Should i lower it?

 

So among all these possibilities, for you, which is the optimal sequence to try next time?

What would you try, if you were me?

1) Change settings: Ra PPEC and Dec ResistSwitch with default settings ( but min movs from GA)??

2) calibration at 0 Dec and 170° Az?

3) Guiding Assistant for 5 minutes?? Or 2 minutes??

4) Change settings according to what GA recommends??

 

Any other recommendations?

 

Ps. I opened your log in phd logviewer but i don't find the calibration session: just guiding session...

I can only tell you what's been working for me.  But I do have a very similar setup to yours, even use the zwo290Mini mm too.  

 

For PPEC, it should adjust pretty rapidly.  It seems about a worm cycle (for us 10mins).  So 30 mins should have been more than enough to see improvement in RA.

1).  Yes

2).  Yes, doesn't have to be exact +/- 10 degrees on DEC and +/- 1 hour on RA are what's recommended.  (but really don't think this is an issue, your calibrations seem ok)

3).  Around 2 mins, maybe a little more.  I was recently told by one of the developers that there's not really much reason to go much past 2 mins unless you're evaluating PE.

4).  Yes, Especially if you can get a good backlash run.

 

In my experience changing the mount guide rate to anything greater than .5 for RA and DEC resulted in worse performance.  So I'd change that back to .5 too.  

 

If you want, since I know my settings work and I have a very similar setup to yours, you can try to just copy my settings and give it a go.  May or may not work.  But still strongly feel it's more of a mechanical (backlash) issue rather than a specific software setting.  If you can somehow dial in and enable backlash comp in PHD2 (preferably through the GA) I think that would help greatly.

 

At some point you've done so many random changes that you feel like you're running in circles.  It becomes better to just start from scratch and reset everything to their defaults.  Run the new equipment wizard, do a fresh calibration, run the GA, and just start from the beginning.  Changing one thing at a time until you start seeing some improvement.

 

In regard to my guide log.  I had done a previous calibration earlier and nothing had changed in my setup so I just reused it.

 

Here's are two excellent references from PHD2's website.  You may have already seen them, but just in case you haven't.

 

https://openphdguidi...stPractices.pdf  (Available in French and Italian too)

 

And this one on evaluating guiding results is very good.

 

https://openphdguidi..._Guide_Logs.pdf  (Available in French too)

 

The guys on the PHD2 help forum are WAY better at this than I am and VERY helpful.  So if none of this works, I'd post something over there and they'll probably zero in on your issue instantly.  Just be sure you upload your guide and debug logs via the help tab in PHD2.

 

Good luck,

 

Scott


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#18 ippiu

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Posted 04 July 2019 - 06:31 PM

So, another test.

 

Changed guiding rate from 0,75 in 0,5 RA and Dec

 

PPEC and Resist Switch, exactly the same as you, @Tristarcapt. For period lenght i don't know why it automatically changed the value in 581,95: i wrote exactly 598,89 (as it is in your settings) but it changed it.

PHD2_algorithm.jpg

 

Run GA for 150s and apply min move number.

See my huge backlash, even if i did the above ioptron procedure to minimize it

GA_backlash.jpg

 

Here the log: better than before, but not fantastic.

For me the only "problem" seems Dec backlash.

Attached File  PHD2_GuideLog_2019-07-04_235133.txt   129.25KB   3 downloads

 

What do you think about backlash compensation? do i have to enable it?


Edited by ippiu, 04 July 2019 - 06:38 PM.

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#19 OldManSky

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Posted 04 July 2019 - 06:56 PM

That’s a fair amount of backlash. All I can suggest is to make sure you’re really well balanced in DEC, try doing the iOptron adjustment again (testing as you do it), and try again. If that doesn’t help, enable backlash compensation in PHD, and try guiding in one DEC direction only.


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#20 ippiu

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Posted 05 July 2019 - 11:05 AM

@oldmansky In another thread you said "I’m running a 6” f/4 Newtonian on mine...with an external guidescope I was getting 0.6” RMS guiding the last two nights."

 

How is possible to achieve 0,6 RMS???

 

Could you please share with me all your settings, if you don't mind? I'd be very grateful to you, because i was able to achieve, at best and just once, 1" RMS. Usually 1,5" RMS is my best  


Edited by ippiu, 05 July 2019 - 01:26 PM.


#21 Tristarcapt

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Posted 05 July 2019 - 11:45 AM

Hi ippiu,

 

Good job, you've got a good handle on this.  You know what your issue is and it's backlash.  Not any particular setting in PHD2.  Your settings are good, you're getting sub arc second in RA and almost that in DEC, despite the moderate amount of backlash.  Don't despair I've seen way bigger backlash that other's have fixed, heck mine was over 10000ms when I first got it and it's now at about 1000ms.  Which I think is pretty common for this mount, others have got it down to the mid hundreds but I think that's about as good is you could get it.  PHD can easily deal with 1000 - 2000ms of backlash with backlash compensation enable.  As far as enabling backlash comp:  you could try with my settings (remember those are optimized for my amount of backlash) and see if it helps.  But, your backlash is beyond the amount that PHD BL comp can effectively deal with.  GA doesn't even recommended using it, only guiding in one direction.

 

Your best option is to try to continue to get that DEC belt tensioned properly.  The bad news is (at least for this mount) that's it's kind of an "art" to get that tension right.  There's a fine line between too loose and too tight.  Your options, as I see them are:

 

1).  Continue to follow the iOptron belt tensioning procedure experimenting with different amounts until you get something acceptable (might be time consuming until you find the right adjustment).  Do you have anybody nearby that has some more experience with the mount that could help you?

 

2).  Send it back to iOptron for adjustment.  They could upgrade the firmware so the motors put out higher torque too, just like they did for me.  At least you know it should come back right, but that may be time consuming too depending on where you live.  I had mine back in less than a week but I live near Chicago so not too far to the Boston area.  Mine was under warranty but even if yours isn't their prices are very reasonable.

 

3).  Not sure how you acquired the mount, but if you got it new from a retailer, talk to them and see if they'll exchange it.  Beware the mounts, while overall are good, they do come from the factory with varying degrees of QC.  So you may end up with one that has better, worse, the same amount of BL, or even other issues.  

 

If it were me, I'd continue to try to adjust it or send it back for adjustment.  You have a know quantity with what you have and it appears that you don't have any other real issues other than backlash (very common and normally very fixable).  And just as Paul said make sure you're well balanced in DEC.  How much weight do you have on your mount?

 

Your calibration looked excellent and your PHD settings look like they'll give you very good results once you get your backlash fixed (or at least more manageable).

 

Don't worry about the Period Length changing.  That will automatically adjust as PPEC learns the worm period.  It should settle around 598ms once you get a few longer guiding runs under your belt.

 

Good luck, and keep us posted.

 

Scott

PS - I'm by no means an expert on any of this.  More of a novice really.  Others my have better recommendations.


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#22 OldManSky

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Posted 05 July 2019 - 01:40 PM

@oldmansky In another thread you said "I’m running a 6” f/4 Newtonian on mine...with an external guidescope I was getting 0.6” RMS guiding the last two nights."

 

How is possible to achieve 0,6 RMS???

 

Could you please share with me all your settings, if you don't mind? I'd be very grateful to you, because i was able to achieve, at best and just once, 1" RMS. Usually 1,5" RMS is my best  

That kind of guiding performance is routine for my setup.  I haven't been over 1" RMS in many months.

It certainly helps that my mount is on a permanent pier -- I don't take it down and set it up each time.  And I spent a few hours refining my polar alignment so it's very accurate.  And the whole thing is very solid and stable on the pier, so there's no tripod shake or anything of the sort.

 

I'm a bit hesitant to share *my* PHD settings -- simply because what works for me may not work for you.  I'd rather see you spend a few hours one night (I know, clear nights are hard to come by, but it's worth doing) fine-tuning YOUR settings, finding out what works for YOUR setup, rather than copying mine.  The way you do that is to get everything setup well, stable, and balanced, and then run a guiding session, keeping track of your settings.  Let it go for at least 10 minutes -- your log will keep the results for you.  Then try adjusting *one* thing (aggressiveness, min-move, backlash, exposure time, etc.) and see if that makes things better or worse.  If better, keep note of what you changed, then change another setting, and so on, until you dial in the parameters that work for you.  If worse, go back to what you had, note the change that made things worse, and try another setting.  If you do that in 10-minute (or so) increments, in a couple of hours you will have zeroed in on what your setup needs to guide well.

 

We others here on the forum can guess at what might help all day, but in the end you need to methodically test, experiment, and find out what works for you.  That's how I got down to consistent sub-1" guiding -- when I started things would vary between sessions, and swing from 2.5" RMS to 0.9" RMS (but never better than that), often in two different sessions only a few minutes apart!  So I gave up one night (it was a nearly full moon anyway), and just tuned guiding.  It paid off. :)


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#23 mewmartigan

mewmartigan

    Viking 1

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  • Joined: 02 Jul 2008
  • Loc: outside of Pittsburgh, PA

Posted 14 July 2019 - 08:21 AM

ippiu,  one thing to suggest because I had this problem last night. 

 

I have a CEM25P also and ran the guide assistant. It told me my backlash was about 9500 and terrible and I should only guide in one direction and the world was ending.

 

Turns out I had the declination locking screw in too tight even though it seemed to slew around correctly. I reset everything, adjust the declination locking screw out about half a turn and the next guiding assistant gave me backlash of about 1500 and was able to recommend settings to correct it out.  I think the screw being too tight didn't allow the small dec corrections at all and it basically just gives up.

 

My biggest issue is that the guiding assistant tells me I am 58 arc minutes off the pole even though my iPolar says I am right on.


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