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GEM28 Experience?

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#1 meansrt

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Posted 03 September 2021 - 01:31 PM

Anyone have any experience with the new GEM28? I cant find a review anywhere at all. I love my GEM45 and was thinking about getting the 28 for wider field stuff. 



#2 Tom M

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Posted 03 September 2021 - 04:39 PM

I've got a GEM28EC that I'm happy with but have only had limited time with it due to the weather.

 

This link should get you started but you can also do an advanced search and look for all threads with GEM28 in the title.

 

https://www.cloudyni...st-impressions/

 

 



#3 Relic

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Posted 04 September 2021 - 08:21 AM

Last night was the first time I was able to test my new gem28.
The setup is light enough to carry without the CW on it which is nice. This is my first GEM as I am transitioning to AP. Balancing so far is tricky due to only having a dslr and different lenses, which is an issue on my end not the mount. After I upgraded the firmware the mount seemed to work fine. Real quiet. I look forward to what I will be able to do with it.

#4 Tams

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Posted 04 September 2021 - 02:58 PM

I have a GEM28 and it has not been a good experience.  Mount is basically unusable. Ioptron support recommends I take it apart and 'fix' my brand new $2000.00 piece of crap myself, or send it back to them at my cost.



#5 meansrt

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Posted 05 September 2021 - 09:03 AM

I have a GEM28 and it has not been a good experience.  Mount is basically unusable. Ioptron support recommends I take it apart and 'fix' my brand new $2000.00 piece of crap myself, or send it back to them at my cost.

What’s been your issue with it?



#6 Tams

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Posted 06 September 2021 - 09:00 AM

The saddle tightening knob rubs against the top of the mount if the load is off balance even slightly. (has rubbed the paint off).

 

Goto is useless (main reason for buying a goto mount). Tried all obvious solutions short of shimming the OTA as recommended in the manual. (Wish I had read that before I bought it. Why sell a GEM mount that requires users to 'shim' the OTA to make goto work. WHy not just design it so goto works?)

 

Won't track properly. When I manually centre Jupitor, it drifts out of FOV within minutes.

 

Mount has stalled and HC freezes 3 out of 5 times I've used it. Have to power cycle to get it to respond again.

 

I am using a C5 and canon 6d, so weight is well within limits.

 

Also, the feet on the lite rock tripod are horrible. Prefer the peg type feet on my SE tripod.



#7 markusd112

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Posted 19 September 2021 - 10:39 AM

Hi,

I have bought the GEM28 several days ago. The saddle tightening knob seems to be changed: the new one doesn't rubs anylonger against the mount. But if you leave it in a wrong angle, it clashes to the mount and can damage it. It's not very well designed...

 

signal-2021-09-18-121120_001.jpeg

 

Does anybody know how to remove the DEC axis so I can open the cover of the RA gear? Do I have to simply remove this two screws?

signal-2021-09-19-140243.jpeg

 

Thanks,

 

Markus



#8 Mr.OD

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Posted 20 September 2021 - 03:48 AM

Markus to remove dec axis You need to unscrew 4 screws. Two which You have pointed on Your picture and two which are basically inside. One of the hidden screws is accessible after You unmount shaft adapter, second is accessible after You remove bottom half of dec axis motor plastic cover - it covers a hole through which You can access 4th screw. Both hidden screws should be visible after unscrewing polar axis cap.
The lever arm to clamp saddle should be rotatable, you just pull it outside and rotate to an angle which is convinient for You.

Edited by Mr.OD, 20 September 2021 - 03:57 AM.

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#9 markusd112

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Posted 20 September 2021 - 06:29 AM

Thanks, Mr. OD!

 

One other question: the RA gear switch of my GEM28 has three possible positions: unlock, lock and a third position just after lock position. Does anybody know the reason for the third position? Or is it a defect? The DEC axis only has two positions: lock and unlock....

 

Unlock:

geoeffnet.jpeg

 

Lock:

verriegelt.jpeg

 

and a 3rd position:

3tePosition.jpeg

 



#10 Kevin_A

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Posted 20 September 2021 - 06:40 PM

I had a lot of issues guiding with my gem28. Basically to guide the best you mesh it loose so that the axis rocks slightly just before your lock is in the full lock position. The loosened mesh’s slight rocking movement should just stop exactly right when it locks into position.  If you tighten the mesh too much this mount suffers from worm stiction and your guiding will be very rough. If you are way too loose you will have definite backlash. It is worse on Dec than RA. The belts like to be slightly on the tight side with 1-2mm deflection instead of the usual 3mm. Having the mesh too tight on this mount causes so much stiction that multiple guide pulses do nothing then it shoots. Better to be loose and let the guide pulses do their job. I had a third position until i took my mount apart and fixed the machined bore that is supposed to stop the lock mechanism. I also took out the locks and added superlube to a few internal faces and lubed up the mesh adjustment screws that are part of the locks. I initially contacted ioptron as my guiding was 0.3 to 0.7 most of the time then Dec would jump to 4 gradually then go back to guiding great. They said… loosen the mesh! 


Edited by Kevin_A, 20 September 2021 - 06:42 PM.

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#11 Thilo

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Posted 21 September 2021 - 04:40 AM

I had a lot of issues guiding with my gem28. Basically to guide the best you mesh it loose so that the axis rocks slightly just before your lock is in the full lock position. The loosened mesh’s slight rocking movement should just stop exactly right when it locks into position.  If you tighten the mesh too much this mount suffers from worm stiction and your guiding will be very rough. If you are way too loose you will have definite backlash. It is worse on Dec than RA. The belts like to be slightly on the tight side with 1-2mm deflection instead of the usual 3mm. Having the mesh too tight on this mount causes so much stiction that multiple guide pulses do nothing then it shoots. Better to be loose and let the guide pulses do their job. I had a third position until i took my mount apart and fixed the machined bore that is supposed to stop the lock mechanism. I also took out the locks and added superlube to a few internal faces and lubed up the mesh adjustment screws that are part of the locks. I initially contacted ioptron as my guiding was 0.3 to 0.7 most of the time then Dec would jump to 4 gradually then go back to guiding great. They said… loosen the mesh! 

Kevin, this is very interesting! I had the same issue with my GEM28 (except for the gear switch), and I can completely confirm the beneficial effects of your proposals.

 

Unfortunately, I seem to have a further severe problem with my GEM28. While trying different DEC backlash adjustments, I noticed that in some circumstances the DEC gear switch on my GEM28 would not engage in a specific rotational segment of the DEC axle anymore. I opened the DEC housing and measured the distance between the DEC worm gear and the DEC gear while the gear switch was disengaged. The distance varied during a full rotation of the DEC axle by about 0.2 - 0.3 mm. This lead to the conclusion that the DEC shaft in my case seems to be slightly tilted, wherein the tilted DEC shaft kind of staggers while it rotates, thus varying the distance between the DEC gear and the worm gear. I confirmed that the distance variation is not caused by a geometric/dimensional aberration of the DEC gear itself by also directly measuring the distance between the outer circumference of the DEC shaft and a fixed reference point, say a specific location on the gear housing. Result: The same variation of about 0.2 - 0.3 mm. This makes it quite difficult to find the right DEC backlash adjustment, i.e. the sweet spot, between an engagement between the DEC gear and the DEC worm gear that is not too tight when reaching the smallest distance therebetween and an engagement between the DEC gear and the worm gear that does not have too much play when reaching the largest distance therebetween.

 

Not knowing if the tilt of the DEC shaft can be corrected by adjusting the shaft nut of the DEC shaft at all, do you or anybody else know if there is a trick to loosen the shaft nut? I suppose, a special tool is used for "normal" assembly (which I don't have). I tried to use a pair of scissors to engage the two openings arranged on the outer axial end face of the shaft nut with the tips of the scissors. Unfortunately, I was not able to rotate the shaft nut since it seems rather firmly tight.

 

Thank you!


Edited by Thilo, 21 September 2021 - 10:51 AM.

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#12 Kevin_A

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Posted 21 September 2021 - 08:35 AM

Here are some other juicy tidbits of information regardng this mount. I have completely torn this mount apart into all its base parts and reassembled everything completely from scratch and noticed a few things.
Is there a third position… no there is not…. Kinda.

Let me explain as mine had a third position at first. The lock switch pocket machined into the housing for the lock switch has another very slight machined cutout for a small rectangular key shaped dimple that is on the back face of the lock switch. Once you notice this small rectangular feature on the back of the lock switch you will see how and where its used to stop at the lock position cotout on the housing. It is very small you hardly notice it. This dimple is very shallow and if the very small machined perimeter pocket on the housing was not cleaned up in assembly the dimple on the lock switch can go past it if you apply even slight over pressure. I took out the lock switch and used a scribe to remove excess paint and debris in this very shallow small pocket on the housing hole perimeter and my lock switch now stops hard at the lock position. Inside the machined bore on the housing the worm case housing has a slight machined cup cutout depression where the meshing adjustment ball rotates into. My adjustment was not smooth until i added a dap of superlube into this shallow pocket. I also added lube on the mesh adjustment ball threads which is part on the lock switch to make adjustments smoother. All of these changes helped make adjustments smoother. This mount is very nice but a bit finiky for meshing. It has stiction or backlash and it takes testing to find the sweetspot in between.  I would not east heavy this mount and just rely on phd2 to adjust things for you. It will guide 0.3-0.5 RMS all night long when you find its sweet spot.


Edited by Kevin_A, 21 September 2021 - 08:49 AM.

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#13 Thilo

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Posted 21 September 2021 - 09:01 AM

Here are some other juicy tidbits of information regardng this mount. I have completely torn this mount apart into all its base parts and reassembled everything completely from scratch and noticed a few things.
Is there a third position… no there is not…. Kinda.

Let me explain as mine had a third position at first. The lock switch pocket machined into the housing for the lock switch has another very slight machined cutout for a small rectangular key shaped dimple that is on the back face of the lock switch. Once you notice this small rectangular feature on the back of the lock switch you will see how and where its used to stop at the lock position cotout on the housing. It is very small you hardly notice it. This dimple is very shallow and if the very small machined perimeter pocket on the housing was not cleaned up in assembly the dimple on the lock switch can go past it if you apply even slight over pressure. I took out the lock switch and used a scribe to remove excess paint and debris in this very shallow small pocket on the housing hole perimeter and my lock switch now stops hard at the lock position. Inside the machined bore on the housing the worm case housing has a slight machined cup cutout depression where the meshing adjustment ball rotates into. My adjustment was not smooth until i added a dap of superlube into this shallow pocket. I also added lube on the mesh adjustment ball threads which is part on the lock switch to make adjustments smoother. All of these changes helped make adjustments smoother. This mount is very nice but a bit finiky for meshing. It has stiction or backlash and it takes testing to find the sweetspot in between.  I would not east heavy this mount and just rely on phd2 to adjust things for you. It will guide 0.3-0.5 RMS all night long when you find its sweet spot.

Thanks again, Kevin, for your detailed and helpful explanation which really improve the (guiding) behaviour of this mount.

 

You write you have completely torn the mount apart. Did you also dismount the DEC shaft? I mean, did you loosen the shaft nut which holds the DEC shaft within the shaft bearings and exerts tension thereto?

 

If YES, how did you do this?

If NO, is your DEC shaft properly aligned with the DEC rotational axis, i.e. the shaft doesn't stagger like mine (cf. my post above)?


Edited by Thilo, 21 September 2021 - 09:03 AM.


#14 Kevin_A

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Posted 21 September 2021 - 11:15 AM

Thanks again, Kevin, for your detailed and helpful explanation which really improve the (guiding) behaviour of this mount.

 

You write you have completely torn the mount apart. Did you also dismount the DEC shaft? I mean, did you loosen the shaft nut which holds the DEC shaft within the shaft bearings and exerts tension thereto?

 

If YES, how did you do this?

If NO, is your DEC shaft properly aligned with the DEC rotational axis, i.e. the shaft doesn't stagger like mine (cf. my post above)?

Can you post a picture or utube video as i am having a hard time visualizing how that can be. I did not have to remove any bearings as mine are all good and with correct tension on them.


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#15 Kevin_A

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Posted 21 September 2021 - 11:19 AM

Thanks again, Kevin, for your detailed and helpful explanation which really improve the (guiding) behaviour of this mount.

 

You write you have completely torn the mount apart. Did you also dismount the DEC shaft? I mean, did you loosen the shaft nut which holds the DEC shaft within the shaft bearings and exerts tension thereto?

 

If YES, how did you do this?

If NO, is your DEC shaft properly aligned with the DEC rotational axis, i.e. the shaft doesn't stagger like mine (cf. my post above)?

Does your counterweight bar move side to side… if not i doubt it is the issue. What could be the issue is that your dec worm ring is not centered perfectly on the shaft via the mounting screws. Even if the bearings were not tight that would only allow up and down movement and not side to side. The gear however if not mounted perfect could lead to less than ideal concentricity with the shaft.


Edited by Kevin_A, 21 September 2021 - 11:23 AM.

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#16 Kevin_A

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Posted 21 September 2021 - 11:46 AM

Kevin, this is very interesting! I had the same issue with my GEM28 (except for the gear switch), and I can completely confirm the beneficial effects of your proposals.

 

Unfortunately, I seem to have a further severe problem with my GEM28. While trying different DEC backlash adjustments, I noticed that in some circumstances the DEC gear switch on my GEM28 would not engage in a specific rotational segment of the DEC axle anymore. I opened the DEC housing and measured the distance between the DEC worm gear and the DEC gear while the gear switch was disengaged. The distance varied during a full rotation of the DEC axle by about 0.2 - 0.3 mm. This lead to the conclusion that the DEC shaft in my case seems to be slightly tilted, wherein the tilted DEC shaft kind of staggers while it rotates, thus varying the distance between the DEC gear and the worm gear. I confirmed that the distance variation is not caused by a geometric/dimensional aberration of the DEC gear itself by also directly measuring the distance between the outer circumference of the DEC shaft and a fixed reference point, say a specific location on the gear housing. Result: The same variation of about 0.2 - 0.3 mm. This makes it quite difficult to find the right DEC backlash adjustment, i.e. the sweet spot, between an engagement between the DEC gear and the DEC worm gear that is not too tight when reaching the smallest distance therebetween and an engagement between the DEC gear and the worm gear that does not have too much play when reaching the largest distance therebetween.

 

Not knowing if the tilt of the DEC shaft can be corrected by adjusting the shaft nut of the DEC shaft at all, do you or anybody else know if there is a trick to loosen the shaft nut? I suppose, a special tool is used for "normal" assembly (which I don't have). I tried to use a pair of scissors to engage the two openings arranged on the outer axial end face of the shaft nut with the tips of the scissors. Unfortunately, I was not able to rotate the shaft nut since it seems rather firmly tight.

 

Thank you!

It depends on where you measured this variance. When the worm ring is machined it is macnined from the center point of an aluminum blank, but the pickup point of the center may not correspond to the exact center of the aluminum blank outer diameter they used. So the inner grooves of the cut mesh will be accurate wheras the outer diameter of the worm ring does not have to be as that could have been cut when making the blank material. So if you measure the outer diameter that is not conclusive as that is not a surface that is used in meshing or a critical surface. The worm ring mounts with a bunch of screws, and they have a bit of wiggle room too that could account for it being out of concenticity with the shaft. These mounts are designed to account for this, even my older mount had this feature, it had an adjustable gap and a spring to keep it meshed regardless of concentricity. My gem28 would lock in some places and not in other places around the 360. It was due to the mesh being too tight still. My shafts rock slightly between the worm teeth until the locking knob is just almost at the full lock position, the extra 1/8 of a turn snugs up the mesh and that is where it is good at all 360 degrees around the ring.


Edited by Kevin_A, 21 September 2021 - 11:50 AM.

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#17 meansrt

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Posted 21 September 2021 - 12:00 PM

Wow I'm surprised to hear all these issues with your mounts. I'm certainly reconsidering purchasing one now. It's so frustrating that in this industry it can be so hard to find reviews or feedback on certain kinds of equipment. Maybe it's because the customer base is so small and perhaps less interested in writing or filming reviews but you would think that some of the astronomy magazines would be ready and willing to review new mounts that come into the market. 


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#18 Mr.OD

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Posted 21 September 2021 - 01:07 PM

Gem28 is my 5th mount. I always buy new, none which I have bought worked right out of the box when it comes to precise guiding. I think at this price point it is hard to expect that it will be. It is a precise machine which should be adjusted perfectly at the factory. Is it? I dont think so, but I am glad that I can afford it and it can be adjusted at home without ESA/NASA assistance.
I am still working on mine slowly. It is a bit tricky due to spring loaded worm gear but to be honest I do not regret that I have bought it.

Edited by Mr.OD, 21 September 2021 - 01:07 PM.

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#19 Kevin_A

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Posted 21 September 2021 - 01:11 PM

Wow I'm surprised to hear all these issues with your mounts. I'm certainly reconsidering purchasing one now. It's so frustrating that in this industry it can be so hard to find reviews or feedback on certain kinds of equipment. Maybe it's because the customer base is so small and perhaps less interested in writing or filming reviews but you would think that some of the astronomy magazines would be ready and willing to review new mounts that come into the market. 

Just about all mounts have startup issues and a best setting learning curve. This mount is very well made but sometimes not assembled or setup perfectly. Once dualed in this mount is a solid performer and can run a 0.3-0.5 arcsec rms all night long. It is usually just a matter of finding its sweetspot with the gear meshing.


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#20 Thilo

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Posted 21 September 2021 - 01:51 PM

Can you post a picture or utube video as i am having a hard time visualizing how that can be. I did not have to remove any bearings as mine are all good and with correct tension on them.

Hi Kevin, what I mean is the following (I appologize for having chosen the terms "worm gear" and "worm wheel" (former DEC gear) in a confusing manner in my initiating post):

 
The staggering of the DEC shaft I mentioned is shown in fig. 1 below. I measured the varying distances d1, d2 between a fixed reference point on the DEC housing and the inner circumferential surface of the DEC shaft. d2 is measured after half a turn of the DEC shaft (the yellow/orange circles in fig. 1 show color markings on the DEC worm wheel and a screw I made to better see the 180 ° rotation). Although my measuring technique may not be highly accurate, the variance is more than evident to me. The same distance variation at d1 and d2 occurrs at d1' and d2'. This is why the backlash between the worm gear and the worm wheel also varies during a full rotation of the DEC shaft. Thus, a backlash setting for one specific angular position of the DEC shaft may be good in one segment while at an opposite segment it may be too tight or too loose.
 
Fig. 1:
GEM28-DEC staggering.jpg
 
The next figure refers to the shaft nut. I am not sure if it is possible to correct a tilted DEC shaft (if that is really the case with my GEM28) at all, e.g. by loosening the shaft nut and tilting the tilted shaft axis (fig. 1, purple line) to better align with the true rotational axis of the bearings (fig. 1, green line).
 
Fig. 2:
GEM28-DEC shaft nut.jpg
 
Thank you for your assistance!

Edited by Thilo, 21 September 2021 - 02:55 PM.


#21 Kevin_A

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Posted 21 September 2021 - 03:18 PM

 

Hi Kevin, what I mean is the following (I appologize for having chosen the terms "worm gear" and "worm wheel" (former DEC gear) in a confusing manner in my initiating post):

 
The staggering of the DEC shaft I mentioned is shown in fig. 1 below. I measured the varying distances d1, d2 between a fixed reference point on the DEC housing and the inner circumferential surface of the DEC shaft. d2 is measured after half a turn of the DEC shaft (the yellow/orange circles in fig. 1 show color markings on the DEC worm wheel and a screw I made to better see the 180 ° rotation). Although my measuring technique may not be highly accurate, the variance is more than evident to me. The same distance variation at d1 and d2 occurrs at d1' and d2'. This is why the backlash between the worm gear and the worm wheel also varies during a full rotation of the DEC shaft. Thus, a backlash setting for one specific angular position of the DEC shaft may be good in one segment while at an opposite segment it may be too tight or too loose.
 
Fig. 1:
 
The next figure refers to the shaft nut. I am not sure if it is possible to correct a tilted DEC shaft (if that is really the case with my GEM28) at all, e.g. by loosening the shaft nut and tilting the tilted shaft axis (fig. 1, purple line) to better align with the true rotational axis of the bearings (fig. 1, green line).
 
Fig. 2:
 
Thank you for your 

There maybe an easier way to check if it is tilted and the veriance is big. Proceed slowly and with caution. Rotate the lock switch so the worm ring teeth and worm teeth just clear each other when positioned near a low spot from this tilt.  leave the lock switch unlocked at that rotated position and very gently rotate the ring gear by hand to hear and see if the teeth start interfering with each other… but do it gently, slowly, balanced and unweighted. If it is tilted then at 180 degrees it should interfere. Do this very gently and slowly so the worm doesnt get damaged. I do not recommend this technique but it would confirm the geometry is not good. Do this by turning the worm ring by hand.


Edited by Kevin_A, 21 September 2021 - 03:25 PM.

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#22 Kevin_A

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Posted 21 September 2021 - 03:31 PM

Can you pm me as were kinda hijacking this thread! Haha



#23 Thilo

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Posted 21 September 2021 - 03:35 PM

There maybe an easier way to check if it is tilted and the veriance is big. Proceed slowly and with caution. Rotate the lock switch so the worm ring teeth and worm teeth just clear each other when positioned near a low spot from this tilt.  leave the lock switch unlocked at that rotated position and very gently rotate the ring gear by hand to hear and see if the teeth start interfering with each other… but do it gently, slowly, balanced and unweighted. If it is tilted then at 180 degrees it should interfere. Do this very gently and slowly so the worm doesnt get damaged. I do not recommend this technique but it would confirm the geometry is not good. Do this by turning the worm ring by hand.

Kevin, with all due precaution I did the test. As expected, the teeth - while being just clear in one rotational position of the head assembly (i.e. low spot of tilt) - start interfering when rotating to the opposite position, i.e. about 180 °.

Okay, let's switch to pm... Thx!


Edited by Thilo, 21 September 2021 - 03:40 PM.

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#24 Kevin_A

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Posted 04 October 2021 - 12:07 PM

If anyone is still having erratic guiding issues and both the belt tension is good as well as the mesh, this document should help fix any dec or ra random spikes. Stiction in the shaft bearings is a known issue and ioptron has a fix…

 

https://www.ioptron....nAdjustment.pdf


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#25 Stevemr2t

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Posted 04 October 2021 - 12:58 PM

I'm having some pretty serious communication issues with this mount. I use the ASI Air Pro and the mount keeps disconnecting itself / losing coms with the ASI Air (Both Wired and Wireless / tested on multiple ASI Air's)

 

I had some marginal success after changing out the power supply for the mount and also using a USB hub, but it still basically only works (GOTO) maybe 30% of the time.

 

I have CEM 40, GEM45 and CEM 70 and those work flawless. 


Edited by Stevemr2t, 04 October 2021 - 01:04 PM.



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