Poor RA Guiding

[BigDan]

I have a Losmandy Titan 50 I bought new in 2015.  I am only a weekender at my dark site, and I'm in Florida.... there's so much moisture down here..... clouds and rain, this mount has received light usage.  I recently re-greased the mount with the help of Michael Herman.  My RA guiding has gone to pot.... really terrible.  My overall RMS error tends to be over 1.0.  My DEC guiding can look pretty good.  The mount has huge DEC backlash.... I've started making DEC corrections in only one direction, so I don't get all these huge DEC corrections due to backlash.  But no matter what I do, I can't seem to hammer down the RA guide error.  Here is what all I've done:  Excellent polar alignment, using QHY PoleMaster, verified by the Guiding Assistant.  I've run PEMPro, taking great effort to get it right, and send a good PEC curve to the mount (and checked whether curve needed to be inversed.... all that checked), I try guiding with or without PEC turned on, I recently re-greased the mount, my PHD2 calibrations look pretty good (orthoganality low, plots on axes or close, actual rates close to expected), I've tried different algorithms (was using Hysteresis, now using PEC algorithm.... and let it run beyond 2 worm cycles), used different exposure times, and I even used my imaging camera for guiding.... an entirely different camera, and was having the same problem with RA.  I am guiding with an ONAG-XM, so I'm guiding with near infrared.  I do not have an astigmatism corrector.  But when I said I tried using the imaging camera for guiding, I just set up a new profile in PHD2, so I was guiding with the imager through the imaging port, and got the same result.

I am desiring to upgrade my imaging camera to the QHY 600, with larger filter wheel, and larger filters, but I'm not going to spring for that until I get my guiding and/or mount problems figured out.

Attached are logs from my last 2 guiding sessions, and a screen shot of my last calibration.  The DEC plots are a little bit off the axis.... I've done better on that previously, but I don't think that would blow out my RA guiding.

Any help with this will be appreciated.

[wave...@talktalk.net]

Hello BigDan, I'll take a shot for you and hopefully learn something myself when the experts take a look. From my interpretation of the guide logs and your report about recent lubrication of the drives, I suspect this servicing may have actually caused the current problem. As example, in the 17 minute guide session shown below (RA only), the moves in both East and West direction require umpteen pulses to correct.

From my perspective, it looks like it's sticking for a short time, then releasing and overshooting. This then repeats to the end. So, I'm wondering whether the servicing involved tightening up the worm mesh and/or the worm thrust bearings? However, I'm also prepared to be quite wrong1
Cheers,
- Jack

[BigDan]

You know... you brought up about the only thing I didn't try.  Adjusting that damn worm mesh.  I'm too tired tonight to try, but tomorrow night I will try playing with that.  What a pain.  I did see an RA stall reported on the hand controller the other night.... very rare.  I will try backing off on those wing screws on the worm block.  Pretty much the concept is....you make adjustments until you have all 3 just right:  windage, or move scope back and forth by hand, and feel no play, get no stalls or funny sounds when slewing, and get good guiding.  It's not easy to do.  I'll try playing with it and report back.  I've tried using a spark plug feeler gauge..... been there, done that.

[Brian Valente]

Hi Big Dan

I saw the previous response re: possible RA stiction. If you are getting RA stalls in addition to this, it's a good bet the worm mesh may need to be 

RA main areas are 21 seconds and about 151 seconds. they may be worm meshing, but the 151 sounds like it might be a loose oldham coupler. Did you disassemble your mount when doing the greasing?

I see you were fiddling with the calibration step size, you really should leave this setting at what the profile wizard calculates it. 

One thing that would be helpful in diagnosing is a longer guided and unguided run, so we can see what the mount is doing without  guide pulses

For this i suggest the baseline guiding, which lays out how to do this in some simple steps

https://www.dropbox.com/s/zrf0au7jaisvt85/How%20to%20create%20a%20baseline%20for%20guiding%20results%20using%20OpenPHD2.pdf?dl=0

that log would help

Regarding dec, it doesn't appear to be a constraint in your guiding, I see a couple runs that are about 1/2 arcsecond and under. 

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[BigDan]

What is an oldham coupler?  Never heard of it.  I'm going to play with adjusting the worm mesh, and see if that makes a difference.  Here's what I did when I re-greased the mount:  Removed both worm blocks.  Removed old grease from worm wheel and worm gear, added new grease.  Mated up the worm block with the worm wheel, so as not to have windage play felt when manually moving the scope, and slewed around to determine no abnormal noises when slewing.  I have a power source with built in amperage meter, so I tried getting the amperage as low as possible for both directions on both axes.  I will see if I possibly have the wing bolts too tight on the block.... try backing that off.  Beyond that...... how do you know when it's time for new worm gear(s)?  I see Losmandy sells them on their website, and would the concept be that they are brass, and the worm wheel is steel or stainless steel, which means that the worm gear is fairly sacrificial, and replacing that would be more needed than replacing the worm wheel? 

[Brian Valente]

>>> What is an oldham coupler?  Never heard of it.  I'm

Woop - ignore that, i was on the G11 thought train

those kind of period errors may have more to do with the reassembly than any problem with the worm or worm gear. I would revisit the gearbox assembly and make sure it's well fitted. 

You may do better with this discussion over at the losmandy forums. Mount adjustments are important but probably off topic for the guiding forum here

Brian

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[Dawn, Paul, and Elizabeth Romero]

Quote: those kind of period errors may have more to do with the reassembly than any problem with the worm or worm gear. I would revisit the gearbox assembly and make sure it's well fitted. 

Hi Brian et al,

That's exactly what I also needed to hear! My older mount too has the couplers and I shaved the gearbox shaft too short, along with stripping the set screws on the Oldham couplers.  I'm hoping this will get the DEC error down from something like 35 to  less than  5 arc seconds peak. I hope too because the RA seems to do 90% of the work, from what I'm getting from developer Bruce's TAIC talk. My DEC peak to peak  is less than 2 arc seconds, which, correct me if I'm wrong, is ok.

Clear Skies,

Paul

[Brian Valente]

Hi Paul

I suggest submitting your guidelogs to a new thread if you want an evaluation. It's hard to comment on generalizations that may not be universal. 

If you can do a baseline guidelog that would be great as well

https://www.dropbox.com/s/zrf0au7jaisvt85/How%20to%20create%20a%20baseline%20for%20guiding%20results%20using%20OpenPHD2.pdf?dl=0 

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/7c226259-3324-4192-b7bc-9381d927e09dn%40googlegroups.com.

[BigDan]

I've spoken with Michael Herman.  He also is going to evaluate the attached logs with a PE program.  They are unguided for just over 45 minutes.  The more the merrier in evaluating what's going on.  I'm wondering if I need new worm gears.  I purchased the mount new in 2015 and it's had light usage, but it's 7 years old.  Never switched out the worms.  When I first got the mount, I don't recall huge DEC backlash, but it's had that for a few years.  Now the RA has gone crazy.  I know for certain I've got Titan 50 specified for Mount Type in the hand controller, so one would think the gear parameters are shown correctly in the Gemini 2 gear setup page, but I'm going to check that.

[wave...@talktalk.net]

Hello again BigDan,
You said:
 " He also is going to evaluate the attached logs with a PE program. The more the merrier in evaluating what's going on."
So, for my own education, I'll take another shot - and because I'm totally unfamiliar with the Titan 50,and maybe have more questions than answers! The PHD2 Log Viewer gives the trace of your 45 minute GA run shown below.

The PA error isn't noted on this, but it shows some drift, so maybe it's worth another check using PHD2's drift alignment tool?
The trace contains 17 worm cycles in 45 minutes, indicating a primary period of about 160 seconds. Is that correct? - it seems unusually rapid.  Are the mount driver parameters set correctly?

PHD2's periodic error analysis of the tracegives the following:

Here, the worm primary error at 160 sec (approx) has a Peak-to-Peak size of about 12.5 arcsec and RMS 4.4 arcsec. I think it appears rather large for a high end mount.
Apart from this main peak, there's a smaller one at around 323 seconds (twice the worm period) with a P-P of 3.2 arcsec and RMS 1.1 arcsec. I'd guess this is a gear/belt related overtone, your Titan specialist should know.

The single Dec glitch in this run was at about 19 minutes in. I won't even try to guess why. Whatever it was, the guide star was lost momentarily and the RA didn't really notice.

The only other comments I have is that your calibration is very good, but doesn't require the 150 pixel calibration distance you've set. I'd reset that to the default 25 or maybe 50 and/or adjust the guide rate closer to sidereal.

Goog luck.
- Jack

[BigDan]

Thanks for that.  Off the top of my head, 1 full worm cycle is 319.1 seconds, just over 5 minutes.  This is a long shot, but I recently updated the firmware in the mount, using the Gemini Firmware Updater (GFU).  My recollection is that I checked on my hand controller that a Titan mount was selected... it was selected.  However, there are 2 different Titans..... the original Titan-25, and mine, the Titan-50.  I believe it relates to the gear ratio in the gear box.  

The hand controller, as I recall, only showed Titan as a selection, and not Titan-25 or Titan-50.  There is a table on the Gemini-2 website, that gives the specific gear settings.  I have printed that table out, and will compare to the selections in my mount.  It's a long shot, but I need to check it out.

If that's not it, I may be due for a tune-up at Losmandy.  It would be easy to send them both worm blocks and gear boxes for testing/replacement.

But I need to check those gear settings first.