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ETX 90 Tracking speed, weight, or slop?

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#1 jrussell

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Posted 19 March 2024 - 10:03 AM

Took my 90 out yesterday afternoon for a quick solar test run with the sun funnel I made over the weekend. I didn't have a lot of time since I set up in the back yard and trees were to soon be a problem. One thing I noticed that I hadn't noticed last time I had it out at night was the tracking seemed to have trouble keeping up with the sun. So, that made me wonder, was the problem that I need to adjust my tracking speed for the sun, or could it be the additional weight of the sun funnel (which can't be that much I wouldn't think), or is it just normal slop in the ETX drives? I do know the drives have the typical ETX backlash, but I wondered if anyone else has encountered this while doing solar observing?



#2 deSitter

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Posted 19 March 2024 - 11:00 AM

I don't know if I would add any extras to an ETX in any case, but yes there is slop in the motions which can be somewhat improved by disassembling the entire thing and tweaking the worm gears.

 

Don't overtighten the altitude axis! Too much of this can break the ABS plastic axle. Grease can work its way into the clutch and make it slip.

 

As for tracking - the Sun moves roughly its own diameter in 12 hours. So sidereal tracking with occasional tweaks are all that is needed. If you have tracking so off that you can't keep up, either your polar alignment is way off or there is a problem in the gear train. The plastic gears can strip and break.

 

-drl


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#3 DAVIDG

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Posted 19 March 2024 - 11:17 AM

 An easy way to test with the additional  weight is an issue or there is a problem in the drive is put the Autostar in the mode were it reads out the Alt and Az of the scope. Now slew it through 15 degrees or more on both axis  and compare the reading on the Autostar to that on the analog circles. They should agree almost perfectly if  there is no issue.

 

             - Dave 


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#4 michael8554

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Posted 21 March 2024 - 04:28 AM

Are the clutches tight ?

 

Once the slop in the gearbox is taken up the tracking should be steady, discounting any Periodic Error .

 

David's 15 degree test checks the optical encoders, which are used for accurate GoTo's, but not during tracking.

 

Check the tracking rate is on sidereal, not moon or custom.

 

In sidereal it should track the sun for some time before requiring recentering.

 

Or set the tracking rate to custom:

 

Here's what expert AndrewJ said previously:

 

"Basically, inside the firmware there is sidereal, lunar, custom.

To change the rate for solar, you need to use the custom option and enter -3.

( ie 99.7% of sidereal )".


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#5 DAVIDG

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Posted 21 March 2024 - 08:47 AM

 One bit of information  that wasn't provided, was what setup mode was the mount in ? Was it polar aligned equatorial mode or Alt/Az mode ? In either case since the scope was tracking the Sun ie in the daytime the simple reason why the tracking was off was that the alignment was off since there are no stars visible to do an accurate alignment.  The difference between Solar rate and the default Sidereal rate is very small so if the scope was correctly aligned  you would only see the drift after about an hour and it would still be small.

 

                             - Dave 


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#6 deSitter

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Posted 21 March 2024 - 09:27 AM

 One bit of information  that wasn't provided, was what setup mode was the mount in ? Was it polar aligned equatorial mode or Alt/Az mode ? In either case since the scope was tracking the Sun ie in the daytime the simple reason why the tracking was off was that the alignment was off since there are no stars visible to do an accurate alignment.  The difference between Solar rate and the default Sidereal rate is very small so if the scope was correctly aligned  you would only see the drift after about an hour and it would still be small.

 

                             - Dave 

I do a daytime alignment in alt-az mode and just assume the alignment stars are present in the FOV. If you point it accurately north and get it level to begin with, this is plenty good for long tracking, if not go-to operation. Of course all subsequent motion of the scope must be done by slewing with the handbox.

 

-drl


Edited by deSitter, 21 March 2024 - 09:28 AM.

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#7 Thrifty1

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Posted 21 March 2024 - 09:39 AM

Not sure if you have done this already or not, but you can add the sun as an asteroid to the AutoStar and it will then track it.  Here is the info to use for adding it:

 

Name:  Sun
epoch:  1.5 jan 2000
eccentricity:  0.016710
semi major axis:  1
inclination: -0.0001
long asc node:  168.7394
arg of peri:  114.2078
mean anomaly: 357.5172
abs mag: -12
mag slope: 0



#8 DAVIDG

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Posted 21 March 2024 - 11:00 AM

I do a daytime alignment in alt-az mode and just assume the alignment stars are present in the FOV. If you point it accurately north and get it level to begin with, this is plenty good for long tracking, if not go-to operation. Of course all subsequent motion of the scope must be done by slewing with the handbox.

 

-drl

 I agree this works but any drift in tracking the Sun can easily be explained on how  accurate the  calibration alignment was done. So I would look for the most obvious reason and then go from there. If the readout mode test of slewing  the scope and comparing the results from the hand box to the analog circles show no problem  with the Sun Funnel attached then that shows there is no mechanical reason for the tracking drift.  The difference between Solar rate and  the  default Sidereal rate result in a 1 degree difference in 24 hours. With the Sun being a 1/2 degree in diameter it would take 12 hours to drift out the field of view if the alignment was perfect. So  the drive rate doesn't seem to be the problem. 

 

                       - Dave 



#9 jrussell

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Posted 21 March 2024 - 11:13 AM

Thanks for the additional info and suggestions. Unfortunately I haven't had the chance to try it out again. RL getting in the way and we've had pretty much nothing but clouds lately. There have been times during the day that the sun's been out, but by the time I've gotten home clouds have been starting to, or have moved in again.

 

Randy, I have done that but I wasn't using it the day I was testing though. I had just done the easy alignment then slewed to the sun.

 

Regarding my setup I was in Alt/Az mode.

 

ETA: I probably shouldn't have even been trying the sun funnel on it since it's not recommend to use them with any kind of reflector but I was wanting to try it out and try tracking the sun. Basically trying to cram 50 lbs of crap into a 5 lb bag time-wise. Hopefully in the days leading up to the eclipse we'll get some sun and I can try again.


Edited by jrussell, 21 March 2024 - 11:18 AM.


#10 deSitter

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Posted 21 March 2024 - 11:21 AM

Thanks for the additional info and suggestions. Unfortunately I haven't had the chance to try it out again. RL getting in the way and we've had pretty much nothing but clouds lately. There have been times during the day that the sun's been out, but by the time I've gotten home clouds have been starting to, or have moved in again. I'm hoping this isn't foreshadowing of the weather on April 8.

 

Randy, I have done that but I wasn't using it the day I was testing though. I had just done the easy alignment then slewed to the sun.

 

Regarding my setup I was in Alt/Az mode.

There is a setup step that sometimes is overlooked - you must manually rotate the scope in azimuth counterclockwise until it stops (hard stop inside), THEN rotate clockwise back to N. Otherwise the scope may run into a hard stop during operation. This would probably not damage the scope but the tracking would end. The orientation of the base should be such that the control panel faces due west. So setup steps are

 

1) Level tripod and install base such that the control panel faces due west

2) Unlock azimuth and rotate CCW until the stop is encountered, then rotate CW back due N and lock

3) Level OTA barrel and lock altitude

4) Do a simple alignment and just hit ENTER when prompted to center alignment stars.

5) Use the handbox to slew to the Sun

 

Also - remember to allow for magnetic north if using a compass! Scope has to be pointed true north in home position.

 

-drl


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#11 jrussell

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Posted 01 April 2024 - 03:21 PM

Great. Now the gears are skipping again. This scope is really trying my patience.



#12 jrussell

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Posted 03 April 2024 - 10:55 AM

So if I crack open this thing again and clean all the grease out of it (assuming that's the problem), what should I use to replace the grease?  Also, what would be the best thing to clean the grease? I've used Mean Green to clean grease before, but not on plastic parts.



#13 DAVIDG

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Posted 03 April 2024 - 11:18 AM

 If the gears are slipping something else is wrong. Either one of the gears has had the teeth chewed up or  a gear is slipping on it's shaft  or the plastic housing assembly is cracked.  I have seen all these problems in the ones I have repaired.

 

                      - Dave 



#14 jrussell

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Posted 03 April 2024 - 04:20 PM

I'm going to work on it tonight. I checked the gears last time I did but I'll take a closer look this time. Would the housing not fitting down tight enough possibly be a cause? IIRC the screw in the center of the housing was stripped.

 

 

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#15 Thrifty1

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Posted 03 April 2024 - 04:36 PM

The other 2 screw holes are more important (the ones you have empty in that photo) as they hold the gear box stable in the base.  When those are tightened snugly and you move the motor with the clutch locked do you see the gear box flexing? If not, then that screw may not matter much that it is stripped.  

 

I see a LOT of grease on the worm gear and some is on the large metal gear surface where it meets the round metal disk.  I would remove some of that grease and clean all those clutch parts as grease on those flat surfaces will cause slippage.  

 

When you have the clutch tightened and you test the motors back and forth, also look at the large white gear on the end of the worm gear.  I just had one last week I fixed as that white gear was slipping at times.  It is press fit onto the tapered brass shaft of the worm gear and then a screw is on the end of it to help hold it in place.  The one I fixed had grease that had seeped down into it and was causing slipping intermittently.  I cleaned it off, press fit the white gear back onto it, tightened it down with the screw... I applied Loctite to the screw to keep it in place.  At the same time, be sure there is little play in the worm gear in its bracket.  If too loose, you can tighten the nut at the other end and use Loctite on that too.



#16 jrussell

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Posted 03 April 2024 - 07:27 PM

Thanks for the info Randy. I have it open now and it was the large gear that the one on the motor turns that was slipping. I thought I had recalled someone saying the worm drive gear bring too tight could be a problem so I backed off the three screws for that a bit. It stil skipped but then stopped and started turning. It’s only turning to the left that’s been a problem. That seems to be fixed again at least for the moment. I checked the cover for the gear box and it’s actually the screw at the bottom of the picture next to the post that the tripod screws go into that’s stripped and the cover does flex a good bit, especially turning to the left. I’m going to remove the excess grease from the worm drive and see if I can fix the stripped screw.

If y’all have any other ideas what else I should take care of let me know.

Edited by jrussell, 03 April 2024 - 07:29 PM.


#17 jrussell

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Posted 03 April 2024 - 10:45 PM

So here's where I ended up tonight. Cleaned up the excess grease from the gears and tried to come up with a way to take care of the stripped screw problems. Turns out both the first screw I suspected as well as the other one weren't going as tight as they needed. The one in the middle had split badly and was missing a piece and I had epoxied it back together the last time I worked on it. The other one I had tried to epoxy but wasn't able to get the two pieces to come back together. Decided to give it a try without doing anything about that problem and when I tried it, it turned ok at first, then started the skipping again. I had checked all the gears and they looked fine. I tried it a couple more times and I noticed the torque on the gear box was getting worse. I opened it back up to find the piece for the center screw had broken again even worse than the last time. The epoxy from last time obviously didn't hold up and I'm not sure as bad as it is that it's worth trying again. My only other idea is really a last resort and that's to glue or epoxy the two halves of the gearbox together. I'm pretty much out of ideas other than that. I guess if it doesn't work it's time to throw in the towel on this mount.

 

Also as an added bonus when I was deforking the OTA a small plastic piece broke off and I can't find where it came from, but I can hear another small plastic part rattle inside the fork arm. Of course it's the side with the DEC drive so that may be shot too I just haven't looked into it yet.

 

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#18 Thrifty1

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Posted 04 April 2024 - 05:41 AM

My suggestion for the broken screw posts is to epoxy them back together with a metal sleeve around the post so they can’t split. Try to find a metal tube of the correct size that could fit around the screw post.

For example, something like this cut to the length you need if it was the correct diameter.

https://a.co/d/2kPbhia

#19 jrussell

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Posted 04 April 2024 - 07:03 AM

I had wondered about that but I feel like I barely got them back together the last time. If I put the screw into the post, epoxied the post around the screw so that it fit around the screw correctly , and used a sleeve would I be able to get the screw back out? Is there anything I could coat the screw with that would keep the epoxy from bonding to it?

#20 WilsonFlyer

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Posted 04 April 2024 - 07:07 AM

That part appears to be fairly straight forward from what we can see of it. I'd draw that thing up in Fusion and 3D print a CF reinforced PLA part and just replace it. Do you or a friend have a 3D printer?



#21 jrussell

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Posted 04 April 2024 - 08:51 AM

I don't have one but I have a friend and someone at work that do have a 3D printer. I'm actually kind of surprised that as many ETX's as there are out there with miscellaneous problems with the gear box that a full set of gears and a gear box haven't been drawn up for 3D printing. At least if there is a file or files out there that exist I haven't found them.



#22 OzAndrewJ

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Posted 04 April 2024 - 04:10 PM

Gday jrussell

There are several designs of gearbox and i havent heard of people printing them yet

but several people have tried printing the gears for Meade gearboxes ie

https://www.printabl...eplacement-gear

Not sure how robust they were tho.

Andrew Johansen Melbourne Australia



#23 jrussell

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Posted 04 April 2024 - 09:25 PM

Ok. Got some aluminum tubing and made sleeves and epoxied them. The pieces for the center post came off the rest of the way so they’re kind of dropped in with epoxy so I may need to drill it out. If this doesn’t do it I guess my Hail Mary will be gluing the two gearbox halves together.

#24 jrussell

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Posted 13 April 2024 - 06:32 PM

Finally had a chance to sit down with this thing. Gearbox goes together fine now but gears are still skipping. Tried adjusting this, tweaking that but it’s a no go. Quite literally a no go. Guess I’m throwing in the towel and going to look for a different mount or a different scope. Still really like the optics on this thing though so IDK.
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