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Hello,
Attached is my guiding log from last night. I'm looking for some input on a sinusoidal or sawtooth pattern I'm seeing in DEC. Guiding in RA seems reasonable -0.54" RMS. The sawtooth pattern is not something I've seen before. Apart from double checking DEC balance, could there be other culprits? Typically, DEC is better for me than RA. If I had the same performance in DEC as in RA, I'd be where I want to be, so feels like I'm close, just need to solve this "special" cause. For example, if DEC had also been 0.54", I'd be at 0.75" RMS over a 4 hour period, which I'd consider good.
Info on my setup
Stellarvue SVX080T-3SV (80mm f/6)
OAG with ZWO ASI 290MM Mini (2.9 micron pixels)
Mount is a GM811G, main cam is a ZWO ASI1600MM Pro. Other accessories mounted are dew heater, motorized focuser. All cables are bundled into a cable organizer and secured at two points to the DEC axis to prevent drag.
Thanks,
Arun
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Hi Arun,
Possibly backlash. It takes a while for DEC to reverse direction. When it does, you see the large jump resulting in a saw-tooth-like pattern.
Regards,
? ? ?-Paul
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Thanks, Paul. Should I use a different backlash setting in PHD2?
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On Tue, Jun 16, 2020 at 10:21 PM, Arun Hegde wrote:
Thanks, Paul. Should I use a different backlash setting in PHD2?
Didn't ask, but do you have a spring-loaded worm? If yes, backlash is less likely to be an issue.? I don't use PHD, but there's a tool there to measure the amount of backlash and to then automatically adjust for it during guiding. At least that should tell you how large the backlash is on your mount. After you measure it, you could also try to using the TVC setting in Gemini, although PHD is probably better to handle backlash. If backlash is really large, you may want to adjust the worm assembly.?
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Paul,?
I have a spring loaded worm so from what I am reading these are less susceptible to backlash?. Last night I ran? "baseline guiding". Tonight looks to be clear as well. I will repeat that and post both guide logs tomorrow. I imaged last night, but it was the Lagoon which is so low in the sky here (about 22 degrees altitude max, less than 20 much of the night) that guiding results for that would not be representative.?
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As long as it is set to use backlash compensation in PHD you should be good because it adjusts itself as time goes on. You can increase it so that takes less time. I'm usually somewhere close to 1000ms. I've found to err on the side of loose DEC for my GM811G spring loaded worm, and have very good PA, and my DEC is usually better than RA. There is also a backlash setting in the Gemini/HC that I believe you should make sure is off as it confuses PHD2. I've personally seen issues when my worm is either too loose or too tight, and it seems to have similar behavior. -- Another thing that I've recently done is add some blue locktite to the non-spring worm adjustment screw, this keeps the adjustment from drifting. One last thing, is that I usually do my adjustment at night, or early in the morning so that the temperature is the same as when I'm imaging. There is some expansion of metal somewhere in the system that affects the worm adjustment.?
I usually get anywhere from 0.7 to 1.1 RMS but have seen it as low as 0.5 and up to 1.3 with clear sky's, but maybe some seeing conditions.?
Jamie
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Thanks, Jamie. That range is similar to what I get. I will post my baseline guiding log tonight, done with the scope perpendicular to the polar axis. A I recall, this gave me numbers close to 0.9" RMS. What was strange was the DEC oscillation was not something I've seen or noticed before.
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actually that's why i want to see your baseline guiding.?
on one section of your guiding, it looks like pretty standard backlash that can be easily adjusted out
on another run, it looks different: there's maybe some stiction-type behavior. not sure what is the difference between the runs
Thanks, Jamie. That range is similar to what I get. I will post my baseline guiding log tonight, done with the scope perpendicular to the polar axis. A I recall, this gave me numbers close to 0.9" RMS. What was strange was the DEC oscillation was not something I've seen or noticed before.
-- Brian?
Brian Valente portfolio
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Brian,
Here is the guidelog from yesterday. The first calibration and 12.5 minute guide run are with the scope perpendicular to the polar axis. The second calibration was done because I rotated the focuser to frame the object correctly. The second calibration was also done similarly to the first, with the scope perpendicular to the polar axis. The extended run is of the Lagoon Nebula which was pretty close to the horizon. I do see the 3 minute period oscillation in DEC in the extended run as well, similar to yesterday. It appears less prominent here I think because the RA guiding is worse with the object so low.
Arun?
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Brian:
Another one, this time I used the GA then ran 20 mins or so with the scope perpendicular to the polar axis.
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Arun, did you try to turn on PHD2 backlash compensation? I see in your logs that it was disabled for all the runs:
Backlash comp = disabled, pulse = 20 ms
Also, make sure to disable backlash compensation in Gemini by setting TVC to 0, so it doesn't cause an overshoot.
You may also want to watch Bruce Waddington's PHD2 presentation recorded yesterday. It starts around 2:00 mark in the video. He has some examples that look similar to your guiding log, and says it may be stiction rather than backlash (two sides of the same coin):
Regards,
? ? -Paul
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Hi Arun
Can you point me to the log where you followed the baseline guiding?
here's what i expect to see:
calibration successful run, followed by GA run, followed by 15-30 minutes interrupted guiding
anything after that is optional
just to clarify, the goal of scope position is not just perpendicular to axis, it's pointing to the intersection of celestial equator and meridian.?
Brian:
Another one, this time I used the GA then ran 20 mins or so with the scope perpendicular to the polar axis.
-- Brian?
Brian Valente portfolio
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Hi Arun
okay i think this is pretty close to a baseline guiding
Here's some observations/recommendations
- upgrade to the latest 2.6.8 PHD release
- turn on auto backlash compensation, start with an initial value of 800ms. PHD will adjust it to a more accurate amount, and continue to adjust it through the sessions to tune it as needed
- Try the PPEC algorithm for RA. set your initial value to 239.34 - that could help tune up RA a bit, although your PE looks quite low
- i see you experimenting with exposure time. looking at the difference between 1.5s and 2.5s there isn't a tremendous difference once you remove the settling (total RMS 0.81 vs 0.84)
- your calibration has more steps than i expected - the default is about 12. just confirming you used the new profile wizard, and didn't change any of those values?
?
Brian:
Another one, this time I used the GA then ran 20 mins or so with the scope perpendicular to the polar axis.
-- Brian?
Brian Valente portfolio
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Thanks, Brian and thanks Paul.
Yes, I always do the calibration close to the intersection of the equator and meridian because the apparent motion of stars is greatest there which would mean a good calibration. You'll see it was done at RA =14 hours and DEC=-7.8. Not exactly at the intersection, but pretty close to it!
The first thing I did when I started guiding was turn TVC to 0. However, I entirely neglected to turn on backlash compensation in PHD2 - the results were good enough without it. I assumed I had it set, but never checked! I did watch part of Bruce Waddington's excellent video yesterday and plan to finish it tonight. The first thing I looked for the shape of the RA curve when I ran GA and was happy to see that the PE seemed low.?
Brian - the reason I changed the exposure time lower was because it seemed to help with the amplitude of DEC guiding, but again, I only observed it over a short period of time.??
What I will say is this - when I checked PHD2 this morning, I was pleased to see that the last part of the guiding seemed very stable with an RMS of 0.69" over a fairly extended period of time. I'll follow your recommendations on PPEC and DEC backlash and report. I expect based on this and also Jamie's observations below that DEC backlash compensation will make a significant difference.
Thanks to all for the prompt feedback.
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Great -?
yes i saw the good guiding there, both axis contributing roughly equal
so that's something you'll want to watch for. If it ends up that you improve DEC considerably, you want to make sure RA is keeping up, which is why i suggested PPEC
Brian
Thanks, Brian and thanks Paul.
Yes, I always do the calibration close to the intersection of the equator and meridian because the apparent motion of stars is greatest there which would mean a good calibration. You'll see it was done at RA =14 hours and DEC=-7.8. Not exactly at the intersection, but pretty close to it!
The first thing I did when I started guiding was turn TVC to 0. However, I entirely neglected to turn on backlash compensation in PHD2 - the results were good enough without it. I assumed I had it set, but never checked! I did watch part of Bruce Waddington's excellent video yesterday and plan to finish it tonight. The first thing I looked for the shape of the RA curve when I ran GA and was happy to see that the PE seemed low.?
Brian - the reason I changed the exposure time lower was because it seemed to help with the amplitude of DEC guiding, but again, I only observed it over a short period of time.??
What I will say is this - when I checked PHD2 this morning, I was pleased to see that the last part of the guiding seemed very stable with an RMS of 0.69" over a fairly extended period of time. I'll follow your recommendations on PPEC and DEC backlash and report. I expect based on this and also Jamie's observations below that DEC backlash compensation will make a significant difference.
Thanks to all for the prompt feedback.
-- Brian?
Brian Valente portfolio
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Hi Arun,
Well, a 3 minute period is not the 4 minute worm period, so its not the RA worm, Oldham coupler, or gearbox.? And it's a DEC period not an RA period anyway.? ?It's also not PEC as this only corrects the RA drive.??It's a good puzzle.?
Did you drift align your polar axis before imaging?? If not, redo the drift alignment.? If your polar axis is off (and it has to be off) you'd need these DEC corrections.? It seems that PHD2's correcting, maybe overcorrecting, then has to correct again.
_____ and/or_______:
Second question is: can this be a mechanical issue, not PHD2 software per se?? To check that, grab your scope front and back and try to wiggle it on the DEC axis.? Can you move it?? If so, you should try to eliminate this mechanical hysteresis.??
There are two causes of the mechanical hysteresis: 1) the worm to ring gear spacing.? You adjust that on non SLW units by? a) use a feeler gauge to measure the space between the right side worm bearing block and the vertical plate next to it.? Write that down. b)? Try to loosen the 2 bottom hold-down bolts, and gently rotate in the worm using the left hold bolt as a pivot point.? Use the feeler gauges to determine if you got closer. But don't force the worm in too tightly.? c) retighten the mounting bolts and? d) check the worm for "Stall" or "Lags" messages on Gemini.
2) second cause of hysteresis is caused by the worm sliding along it's axis.? See if you can observe this when you rotate the scope.? If you have this worm shifting, and an SLW unit, contact the factory for advice.? ?If you don't have an SLW on this axis, see my earlier PDF on how to rectify that issue.
All the best, (and happy imaging and good health!!!)
Michael
Brian,
Here is the guidelog from yesterday. The first calibration and 12.5 minute guide run are with the scope perpendicular to the polar axis. The second calibration was done because I rotated the focuser to frame the object correctly. The second calibration was also done similarly to the first, with the scope perpendicular to the polar axis. The extended run is of the Lagoon Nebula which was pretty close to the horizon. I do see the 3 minute period oscillation in DEC in the extended run as well, similar to yesterday. It appears less prominent here I think because the RA guiding is worse with the object so low.
Arun?
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Hi Michael,
You're correct that the PA can be better. PHD2 reports the error to be about 3 arc minutes, much worse than the Polemaster is rated to be. So I think that is certainly contributing. I've never drift aligned, but next time, I'll try Sharpcap. Lots of people seem to be getting better results with it than the Polemaster.
I have tried to check for looseness in the DEC axis. I can certainly move the scope in DEC with enough force, but it is a smooth deliberate motion, not slop. I think that's how the clutch is supposed to work.
Based on this I'm thinking that I will see improvement by better PA and mostly by using backlash compensation in PHD2. I was hoping to try it tonight and report, but clouds moved in earlier than expected. I can't complain too much though. We've have a week of clear skies right before the New Moon. That's only happened once before in SE Wisconsin since I started imaging, and that was two years ago. So I'll have to wait a few days.
In the meantime, good health and clear skies to all of you.
Arun
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Yeah sharpcap with polemaster is a good combo, my 'quick and dirty' go-to right now :)
Hi Michael,
You're correct that the PA can be better. PHD2 reports the error to be about 3 arc minutes, much worse than the Polemaster is rated to be. So I think that is certainly contributing. I've never drift aligned, but next time, I'll try Sharpcap. Lots of people seem to be getting better results with it than the Polemaster.
I have tried to check for looseness in the DEC axis. I can certainly move the scope in DEC with enough force, but it is a smooth deliberate motion, not slop. I think that's how the clutch is supposed to work.
Based on this I'm thinking that I will see improvement by better PA and mostly by using backlash compensation in PHD2. I was hoping to try it tonight and report, but clouds moved in earlier than expected. I can't complain too much though. We've have a week of clear skies right before the New Moon. That's only happened once before in SE Wisconsin since I started imaging, and that was two years ago. So I'll have to wait a few days.
In the meantime, good health and clear skies to all of you.
Arun
-- Brian?
Brian Valente portfolio
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Dear Arun,
The polar axis tools (video camera type, or visual polar scope) are an excellent start.? It is after that is completed that you would do a Drift Align method.?
If you have not done Drift Align before, expect to take awhile to get used to it.? I use the nice PHD2 Drift Align tool to watch for the angle of the DEC axis tracking.? You make adjustments until that DEC line is level flat.??
You start Drift Align?on a star near the meridian and equator, and at that pointing position, you only will observe and correct AZ error, not Elevation.? After that is done, you move to a star near the equator and either horizon.? At that position, you only adjust for elevation error, not AZ.???When done your mount will be very very accurately aligned and your autoguider will have very few DEC corrections...ideally only RA corrections to the small number of PE irregularities.?
To help me make these Elevation and Azimuth adjustments, I put little cut "arrows" ...just thin wedges of label maker white adhesive tape...on the black adjustment knobs.? I put a pointer at 12 o'clock on my Elevation front knob, and my East side Azimuth adjustment knob.? This helps me see how much knob rotation I use when doing the polar adjustments.? I am running a very long FL like 3950mm 14 inch f11 SCT.? If you are running a wide angle scope you may not have as critical a polar alignment task.
(By the?way, once you do an accurate drift align, it's a good time to run a PE test on a star near the meridian and equator, using PHD2 with the autoguiding turned off after it calibrates.? Drift in DEC can mess up a PE evaluation run, even though the analysis software will try to subtract out a linear drift in the star movement, it's better to just have none there. )
Back to the mechanical "hysteresis" question.??
Each clutch disk, RA or DEC,? will "slip" after you apply some amount of torque to the scope.? It should require a strong effort to cause the clutch to slip.? Contact me if your clutch is "too slippery."? ?I have a different clutch disk for you to use for imaging.??
But I mean to have you wiggle the (DEC) axis at a lower force than causing the clutch to slip.? Again, look for worm movement along its axis, if you can see it (if your axis has a removable cover, take that off to observe the worm).? If it is able to move along it's axis, you should try to eliminate that movement...that certainly is a cause of time delay ("hysteresis") in the drive response.? It represents time that the (PHD2) correction program is wasting trying to get a correction.
Hope you resolve your imaging tracking soon...then send us some pretty pictures!
All the best, Michael
Hi Michael,
You're correct that the PA can be better. PHD2 reports the error to be about 3 arc minutes, much worse than the Polemaster is rated to be. So I think that is certainly contributing. I've never drift aligned, but next time, I'll try Sharpcap. Lots of people seem to be getting better results with it than the Polemaster.
I have tried to check for looseness in the DEC axis. I can certainly move the scope in DEC with enough force, but it is a smooth deliberate motion, not slop. I think that's how the clutch is supposed to work.
Based on this I'm thinking that I will see improvement by better PA and mostly by using backlash compensation in PHD2. I was hoping to try it tonight and report, but clouds moved in earlier than expected. I can't complain too much though. We've have a week of clear skies right before the New Moon. That's only happened once before in SE Wisconsin since I started imaging, and that was two years ago. So I'll have to wait a few days.
In the meantime, good health and clear skies to all of you.
Arun
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This is a great thread!?
I tried phd2 pec guiding last night with the period you gave and RA RMS seemed to have a nice improvement. Hard for me to tell if it was just seeing, but I went from 1.1 the previous night to 0.8. I've never gotten pec to work with the GM811 before.?
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Paul Kanevsky
Jamie Amendolagine
Michael Herman