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Need some help fixing DEC tracking that failed - hardware issue?

Astrophotography Meade
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#1 rpwooste

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Posted 08 July 2021 - 08:34 PM

I'd really appreciate some help tracking down how to fix a new DEC problem when autoguiding with PHD2.

 

I'm using a Meade LXD75 mount, Meade DSI camera for guiding on a second OTA, and PHD2 on a Windows laptop for guiding using ASCOM to both the DSI guide camera, and with ASCOM via the handbox controller with a USB to Serial dongle and the additional 4 pin cable into the AutoStar #497 hand controller to control the LXD75 mount, and the traditional 8 pin cable from the AutoStar back to the mount. 

 

It seemed to be working fine June 30th, I ran the PHD2 Calibration routine, and successfully guided for over an hour. However my next two attempts on 7/1 and 7/6 failed during calibration with:

 

"DEC Calibration Failed: star did not move enough"

 

I also tried PHD2's Star-Cross test with a long exposure on the camera. This is supposed to move the mount during the exposure so that each star draws a cross. I only got a straight line with a blob in the middle. So it went back and forth in one axis, RA presumably, and then failed to do anything in DEC. Could I have accidentally disabled DEC guiding? I don't see how, and fear this to be hardware issue, but anything's possible. 

 

The mount slews in RA and DEC using the four arrow controls on the hand box, and the goto function during the 3-star alignment routine does the same job, as it used to, of getting fairly close to the targets. I also tried going back to the Meade Autostar Envisage guiding software, that seemed to calibrate, but then when I took several 120 second images for over an hour my target progressively moved in one direction out of the field of view. So the guiding did a horrible job, perhaps only stabilizing RA with DEC presumably still broken.

 

Given the mount, motors, and handbox's four-way arrows still work, how can I troubleshoot this? My 8 wire cable from the handbox to the controller looks to have aged badly with the rubber on the plugs looking torn and could be a source of problems. Are different wires used for guiding corrections versus slewing? Could it simply be that my 8 wire cable has one broken wire? I tested all 8 wires inside and they showed connectivity, but it's possible that when the cable is being stretched and the plugs bent in the mount outside that the cable fails in a manner that testing it inside doesn't show the failure.  Or could the problem be something else? 

 

I've attached the PHD2 logs from all three nights so you can see how the calibration routine worked on 6/30, but failed on 7/1 and 7/6. 

 

Thanks so much!

Roland.

Attached Files



#2 OzAndrewJ

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Posted 08 July 2021 - 09:27 PM

Gday Roland

Just to put yr mind at rest initially.

If the mount can slew in all directions "manually", then the motors and their cards are OK

as is the Hbx to Mount comms.

If gotos get pretty close, then the encoders and their feedback loops are probably OK.

 

From what i read, you are using the PC to Handbox cable to connect for pulse guiding ( ie not ST4 )

so it uses simple serial commands to do the guiding.

If the RA guiding was working, then the comms is working.

That said, i see 4 failed cals on the 6th and they all failed in RA ????

I note they all had 3000ms exposures and the one that worked had 1000ms.

In the next 2 days data, you used 1000ms exposures and the RA worked but DEC didnt

( altho DEC did show some movement in the RA direction, but that may be just PE )

Soooo, need to dig some more.

 

What firmware are you running ( and is it patched )

 

Andrew Johansen Melbourne Australia



#3 nitegeezer

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Posted 08 July 2021 - 11:01 PM

I want to add a comment about DEC and PHD2. Under guide controls, there is a pull down that selects Off, Auto, North, and South. I normally set guiding in one direction, usually north, so I don't have any backlash problems. I don't guide with DEC Off so I never try to calibrate that way, but wondering if that is part of your problem.

#4 OzAndrewJ

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Posted 08 July 2021 - 11:58 PM

Gday Chuck

Under guide controls, there is a pull down that selects Off, Auto, North, and South.

Yep, but that is ignored for calibrating.

I just plotted his first run ( supposedly when it worked ) and get the following.

The purple line is the "theoretical" path the RA would take based on pulses sent.

The black is the same, but for DEC.

Assuming this is correct, it shows nearly 400 arcsecs of DEC lash

which would explain why the cal and starcross tests didnt work

( assuming he hadn't preloaded the gears before starting )

First thing first is to do a proper lash test, soooooo Roland

have you done a drive train yet ?? ( ie the std Meade train for lash )

If so, what value is it reporting for RA and DEC 

( as these are equivalent to geartrain lash, in arcsecs )

 

Andrew Johansen Melbourne Australia

Guided.jpg

 

 

 

 

 

 



#5 rpwooste

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Posted 09 July 2021 - 02:53 AM

Wow thanks for the advice. So starting with the firmware, under Setup->Statistics I see it say version "43Eg" I haven't done an update in years and don't actually know how to. Is there a link to the download and the method to update it?

 

It has been a very long time since I've used the Telescope, other priorities got in the way, so I think it has been sitting for several years, and obviously been a long time since I've done a "Train Drive" I will have to run through that the next time I set it up, hopefully in a few days. Thanks for the reminder on that. 

 

Should I be doing the Train Drive every time I use the telescope, or is this something only needed once in a while?

 

Thanks so much!

Roland.



#6 OzAndrewJ

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Posted 09 July 2021 - 04:21 AM

Gday Roland

43Eg is the latest (proper) firmware Meade released for the 497 handboxes,

so my suspicions re bugs in the newer models doesnt come into play.

You can update if you want ( using Starpatch as my preference )

but its not required for this specific problem.

 

Should I be doing the Train Drive every time I use the telescope,

No. The drive train procedure only measures the "mechanical" slop in the geartrain, so in reality,

only needs to be done if the mechanical arrangement of the gearing changes.

If your plots are correct, you appear to have a lot of slop in DEC.

The LX75s have a sealed gearbox, so if there is slop now ( ie after the drive train )

it is most likely the motor to worm transfer gears have a lot of slop

or the worm to worm wheel gap is quite large.

Both should be easily detected by taking off the covers and having a look

 

Andrew Johansen Melbourne Australia


Edited by OzAndrewJ, 09 July 2021 - 04:22 AM.


#7 Sammy

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Posted 09 July 2021 - 11:32 PM

The dec motor case on the lxd75 has a slightly slotted hole so you can move the motor in or out to adjust the slop in the gears in the case... if that doesn't get rid of all of it, you can take the motor box off and put a thin washer held in place with tape to get the cogs closer together.

#8 rpwooste

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Posted 14 July 2021 - 02:47 AM

So I pulled the motor boxes off both DEC and RA, tightened up the worm collet on the DEC, and adjusted the worm adjustments for both DEC and RA to eliminate all kinds of worm jitter. It looks good, except I found that both the DEC and RA gears were loose on their worm shafts. I found that I have no Hex Wrenches smaller than 1.5mm, and had to order a kit containing some 1/20th and 3/64th size wrenches with the expectation that the 1/20th should work on tightening the worm drive gear lock pin.

 

https://drive.google...iew?usp=sharing

 

Is this SAE 0.05 hex? i.e. 1/20th?

 

You can see the wobble in the drive gear here, here's how much the gear moves without turning the worm shaft:

 

https://drive.google...ew?usp=sharing 

 

On RA it looks like it wobbles at least 1 tooth worth of distance, and the DEC is only a little less.

 

Is this something I only need to tighten up with a hex wrench, or do I need to also glue/loctite so it doesn't move at all, this looks like it could be a large percentage of my RA and DEC lash. 

 

Roland.



#9 OzAndrewJ

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Posted 14 July 2021 - 03:04 AM

Gday Roland

Cant really help with the socket size, as Meade do use odd sizes at times.

That said, the slop there is massive and will certainly account for a lot of your inaccuracies.

 

Tightening the grubscrew should fix it, but a dab of loctite probably wouldnt go astray.

I know some people have actually bored out the screws and tapped for a larger size

but if you can adjust the grubscrew, put some loctite on the shaft, then slide on the gear

and lock it, and the loctite should take most of the torque.

 

Andrew Johansen Melbourne Australia



#10 rpwooste

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Posted 19 July 2021 - 11:33 AM

Hey Andrew,

 

OK, so I bought a new set of hex wrenches, removed the ridiculously small 1/20th inch set screw, tapped out a larger #6-32 set screws and replaced those. With the worm drive gear now tight I tried adjusting the worm gear collet and the worm adjustment. Initially I got all the play out of it, but the motor failed part way around - it was tight in parts, but exceptionally tight in others where the motor failed, so I backed it off, and can't seem to find any setting that both has no back lack and the gear turns easily, so I compromised as best I could. I also created a new handbox cable as mine was falling apart. 

 

Back outside, the go to functionality works surprisingly well, bright stars are pretty close to the center, so I was pleased with that. I tried the PHD2 Star Cross test, 20 pulses of 2000ms in each direction, the resulting image appears as only 3 sides of a cross, something seems wrong. 

 

I then tried tracking M29, 120 second images, 180 seconds apart, in a series. Most of the images suffer the uni-directional motion blur you see in one of the attached images, two of them (mostly by luck) are better than the others, and it's clear to see that PHD2 is not doing very well tracking the star cluster as M29 continues to move in one direction from one image to the next. I calculate that it's moving 6" (angle) per minute (time). So over an hour the target moves away from the center of my lens quite a bit, and suffers on average 12" of blur in each 120 second exposure. 

 

I also took a screenshot of PHD2 while it was tracking (sometimes it appears MUCH MUCH worse, with errors up to 8", but this particular screen shot was when it was looking very good. It's reporting RMS errors in RA of 0.8" and Dec of 0.33"

 

Can you see from the Star-Cross or log file, or debug file, what might be wrong? The log file probably contains several Star-Cross tests, the picture is from the 20-2000ms attempt. 

 

 

https://drive.google...iew?usp=sharing

 

Thanks,

Roland.



#11 OzAndrewJ

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Posted 19 July 2021 - 04:41 PM

Gday Roland

Something really odd is going on in DEC still, that needs more study.

In general, drift between frames is normally associated with flexure somewhere.

That said, the RA problem is sticking out like the proverbial.

It looks like one of the transfer gears on the RA worm or motor/Gbx is very eccentric

thus creating a good then sloppy fit.

Ref attached plot which shows a section of the residual RA error plot

and the theoretical pulses sent to fix the error.

You can see that it runs well, then a high freq ripple builds then drops again.

The LXD75 has a 144 tooth wormwheel with a 37 tooth gear on the worm

As such, the worm period is 598.4s and the tooth period is 16.2s

The plot is very close to this.

To confirm it properly, do an unguided plot for several revs of the worm

and see what drops out.

Based on the guided error being 25arcsec pk-pk at its worst,

i suspect the unguided will be shocking

It all comes down to how tight the fit is.

Remember, a bit of lash in RA isnt totally a problem as when guided

it never reverses. so maybe loosen the fit a bit so the Xfer gears dont strain

DEC needs more coffee in me than i currently have

as it went very odd towards the end

 

Andrew Johansen Melbourne Australia

RA Tracking Post fix.jpg


Edited by OzAndrewJ, 19 July 2021 - 04:48 PM.


#12 rpwooste

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Posted 20 July 2021 - 02:18 AM

So I tried another ~40 minute session, this time unguided, drift is in the same direction, but about 2.5x worse than when I was guiding. Now about 15-arc seconds drift per minute of time, versus about 6-arc seconds with PHD2 guiding. 

 

Because it was unguided I don't have any PHD2 data from tonight, however going back to the data I shared yesterday and the plots you've made from it, I started looking at the log files. If I look at the RA Guiding Duration and DEC Guiding Duration, it seems that over roughly an hour of guiding the guiding cumulatively added about 35 seconds of South, and the RA about 50 seconds of West (this is taking into account all the guiding to the North and East removed from these totals). It seems that for the RA and DEC to be continuously correcting in one direction must imply something is gravely wrong with the system and that somehow it's running fast or slow on a continuous basis. 

 

What is the blue line in your chart? When I charted the amount of time being guided in DEC or RA both continuously deviated from the zero line, so either I'm mistaken, or I don't understand your blue line. 

 

What kind of errors, either hardware, software configuration, or setup problems could have caused this? I'm pretty confident that I'm looking at the right star for polar alignment, and I've used the Kochab rotation method. I'm using the default alignment (two stars with Arcturus and Deneb), and then using Vega as the target that I'm measuring from California 38.7N, -121W

 

If the gears were too tight and it ran slowly, surely the encoder would tell the motor it's running slow and just apply more power to get to where it needs to go, I'm not yet understanding how it can continuously drift. Unless my alignment is totally out. I start the German North by lining up the two stamped metal triangles on the RA and Dec axis, am I supposed to be lining up Polaris through the main OTA?



#13 OzAndrewJ

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Posted 20 July 2021 - 03:03 AM

Gday Roland

Because it was unguided I don't have any PHD2 data from tonight,

Dont understand?????

PHD can log and plot the unguided performance of the mount ( thats what its for :-) )

 

What is the blue line in your chart?

The blue line is the "as guided" performance registered by PHD  ( ie what you would have seen on the PHD screen )

The purple line is what path a perfect mount would have taken based on the pulses sent at the given guide rate.

ie RA drift is about 400 arcsec in 2800 seconds

 

What kind of errors, either hardware, software configuration, or setup problems could have caused this?

The HiFreq ripple looks like a gear spacing error on the transfer gears.

either one gear is eccentric, or as the worm rotates, it binds a bit and springs the gears apart / together.

Hard to tell from the guided data as the guides affect the logging.

 

If the gears were too tight and it ran slowly, surely the encoder would tell the motor it's running slow and just apply more power to get to where it needs to go,

Yep, but remember tracking rates are affected by polar alignment errors

as well as refraction effects.

The "stamped triangles" dont mean anything, you need to have the scopes RA axle pointing at the pole

To check, do a quick unguided plot at HA = 0 and then +/-6 and look at the DEC drift.

That will confirm how well you are polar aligned.

 

ie in your 18th plot, the DEC was drifting at about 7.6arcsec/min until it cracked the sads

and went haywire, so it wasnt all that close

 

Andrew Johansen Melbourne Australia



#14 rpwooste

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Posted 20 July 2021 - 10:15 AM

Hi Andrew,

 

So that's interesting, I didn't know I could get data into the log file of PHD2 without actually guiding. I looked at the UI and still don't know which button to press to start logging/tracking a star without guiding. I've really only used the buttons at the bottom left to connect, begin looping pictures, and then to guide. But looking at the log, the looping pictures doesn't seem to record any data, only when I'm guiding, so I guess you're talking about another option in the menu, but I'm not sure which. 

 

I use the polar scope built into the mount, which has a visual overlay of Polaris, and Kochab, I rotate the counter weight shaft correctly to align with Kochab. However, once the mount is aligned I've then just been aligning the two pairs of stamped triangles to set DEC and RA. The scope is thus pointing to Polaris, although it wouldn't be centered because anyway Polaris is slightly offset from center, hence the Kochab method in the first place, but when the OTA is on and at German North Polaris would be away from center. 

 

When you say an unguided plot at HA=0, do you mean basically the German North setting, i.e. approximately towards Polaris, the starting position? Or do you mean having the GoTo target Polaris which I would assume would make it move quite a bit to adjust for the offset from center.



#15 OzAndrewJ

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Posted 20 July 2021 - 06:05 PM

Gday Roland

So that's interesting, I didn't know I could get data into the log file of PHD2 without actually guiding.

Yep

Start your session like normal, start looping and set the exposure etc to get a high frame rate with a tight star.

( Doing testing, you can pick any star between DEC = 0 and zenith so can nearly always get a bright one )

Now go into the "Brain", on on the guiding tab, uncheck the "Enable Mount Guide Output" at bottom right

Now start guiding as per normal.

The mount will still calculate and log the errors, but wont send any guides :-)

 

When you say an unguided plot at HA=0,

In its simplest terms

If drift aligning, you normally use 2 positions

First one is with CW bar vertical and the OTA pointing E or W  ( ie OTA pointing to HA = + or -6hrs )

Any DEC from say 0 to 60deg will do

DEC drifting in this position indicates the RA axle is pointing above or below the pole

The next one is with CW bar near horizontal and OTA pointing near meridien ( HA = 0 )

DEC drift in this position indicates the RA axle is pointing left ot right of the pole

Lots of worked examples ( with pretty piccies ) of this on the net

but a simple drift align for a minute or so will show if DEC is drifting.

 

Andrew Johansen Melbourne Australia



#16 rpwooste

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Posted 24 July 2021 - 02:50 PM

Hey Andrew,

 

Thank you so much for your help and advice, and the obvious amount of time you've spent to help out a stranger, it's very much appreciated! With your help and encouragement I've so far removed the worm drive gear set screw, drilled, tapped, and installed larger set screws. Then completely disassembled the DEC and RA to clean and lube the worm gears, and having uninstalled it all I now understand how tight the collet needs to be, and what the other five bolts do for worm gear adjustment.

 

When I started my tracking was so bad that unguided the stars were drifting 5.7pixels/minute on my camera, even with guiding that was only reduced down to 2.5 pixels/min. After all this work, and a better understanding of how to use PHD2 my results last night showed a drift of only 0.13pixels/min when guiding, and that was with an 800mm OTA versus the early 500mm, so I think an improvement of about 30x. 

 

I think there is still improvement in my setup process that could help, let me check with you on the steps I should be doing, and if you see any optimization opportunity, here's what I'm doing each time I setup:

 

1. Polar align the mount, using the Kochab counter weight shaft rotation method using the optical alignment scope built into the mount
2. Unlock the clutches, move to "German North" by aligning the stamped metal triangles on the DEC and RZ axis's. 
3. "Easy" (2 star alignment)
4. Launch PHD2, go into the polar drift alignment tool.
  a. Slew to the location for Azimuth adjustment
  b. Run the calibration routine (I think this takes ~5-10 minutes)
  c. Drift for a few minutes, Adjust (repeat this loop several times)
  d. Slew to the location for Altitude adjustment
  e. Run the Calibration Routine (I think this takes ~5-10 minutes)
  f. Drift for a few minutes, Adjust (repeat this loop several times)
 
5. Unlock the clutches, move it back to German North
6. Do the 2-star alignment again. 
7. Switch on High Precision Mode
8. Use the Meade GoTo to slew to the photograph target
  a. Center on the bright star in the High Precision mode
  b. Now check the camera focus on the bright star
  c. Mini-Slew over to the actual target (close to the bright star)
  d. Run the calibration routine again
  e. Start guiding & shooting.
 

My drift for ~2-5 minutes, then adjusting is not very efficient and not working particularly well. I think today I just realized that I've been making a stupid error here. Given the worm has a 10 minute period, and it's quite eccentric, I need to drift for exactly 10 minutes, and then adjust. Because I've been drifting for less than 10 minutes I'm either getting the up or down on the eccentricity of the worm gear and wasting my time making corrections in the wrong direction half the time.

If the calibration takes 5 minutes each time, and the drift is going to need 10 minutes each time, is there any step above that I can cease doing to make this more efficient?

I'm wondering if I even need step #3 above, or whether the polar drift alignment doesn't even need me to do an alignment beforehand. If that's the case I think I should be reducing this list to:

 

1)  Optical alignment using kochab etc.
2)  PHD2 drift alignment, calibrate (5 mins), drift 10 minutes, adjust to the purple circle, repeat this for both Azi and Alti.
3)  Unlock the clutches, go back to German North, 2 or 3 star align (5 minutes)
4)  Then turn on High Precision, and calibrate again (5 mins) with each new target.

It's still at least 45 minutes in the best case before shooting images, is this right? Honestly I'm probably taking 90-120 minutes currently, so this sounds like a massive improvement.

 

Thanks,

Roland.



#17 OzAndrewJ

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Posted 24 July 2021 - 05:50 PM

Gday Roland

Re the process, do whatever you are happy with

The align model used only helps with pointing.

Doing a 2 star align is a waste of time "tracking" wise, as the DEC never tracks.

If you spend a bit of time to get the mechanical alignment of the RA axle close,

then you only need to do a polar one star align.

Then use synchs in new local areas to refine pointing.

 

Given the worm has a 10 minute period, and it's quite eccentric, I need to drift for exactly 10 minutes,

Nope, that is only required if you want to measure RA "tracking" drift

For drift "aligning" the mount, you only use the DEC trace.

The DEC never moves so any major drift on the sensor is due to mechanical misalignment.

This requires you cave calibrated the sensor to know which way is which.

If i am doing this, i normally spend a bit of time ensuring my sensor axes are aligned with the mount axes

and then i can just watch the live feed on a screen to see drift.

 

It's still at least 45 minutes in the best case before shooting images,

Practice makes perfect :-)

But lots of the new camera based apps can get you very close very quick.

I will allow others to comment on them as each user will have different experiences

 

Andrew Johansen Melbourne Australia




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