iEXOS-100 stepper motor nonlinearity

[drjolo]

I would like to share with you my measurements of iEXOS-100 mount stepper motor tracking rate recorded in short time scale. The reason we did such measurement was permanent problem with elongated stars, even at short 1-3s exposures during focusing. Together with friend of mine we took off  RA stepper motor cover and noticed that stepper motor movement during tracking is not linear. I put Allen key to the RA worm shaft to better record this effect:

 

It did not look good, so I decided to actual measure the period of that wobble. I measured that during 24s of movie axis wobbles 18 times, so the period is 1.3s

iEXOS-100 has 1:144 worm gear ratio and 1:4.5 stepper gear ratio. It gives stepper revolution time equals 133s. That means that this wobble corresponds to 1/100 of stepper revolution. Assuming it is 200 steps per revolution motor, then it corresponds to 2 stepper motor steps. That non linearity may have two sources. Either chosen stepper motor model is non linear itself when microstepped. Or microstepping controller is not able to control the motor linearly.  We contacted with manufacturer and we got the response it is known problem and it is expected behavior, and it does not affect the images captured with this mount.

So I decided to measure amplitude of that effect. I exported 18 frames of the movie to Photoshop and measured at 400% zoom level the distance that Allen key lever travels over one 1.3s period.

Then after some calculations it gave me the following results. Orange line is perfect tracking rate 15"/s. Blue line is actual measured axis movement. Gray line is an error. Horizontal axis is time in seconds. Vertical axis is RA main axis movement in arc seconds.

So the maximum error is about 5.5 arc seconds. It may cause elongated stars even at few seconds exposures, when you have imaging setup with 300-400mm FL telescope and modern small pixel CMOS camera. At higher declination this effect will be of course smaller. And this is only one of the error sources in the mount. It will sum up with worm periodic error and any worm gear inaccuracy. 

According to manufacturer that is expected behavior and do not affect imaging results. However I would expect better behavior in the first step of the transmission in the astroimaging mount. 5.5" error does not look large, but it has 1.3s period and cannot be compensated with guiding or periodic error compensation. Then it sums up with all other errors and I would not risk statement, that it does not affect the images captured with this mount. 

 

[Mattee]

Thanks Drjolo,

we await an explanation from the Company (Explorer Scientific) !!!!
  the rotation error is very noticeable!
 I also noticed it.
 After a few seconds of shooting elongated stars!
 Even having a perfect polar alignment it is really useless to use this mount for astrophotography.  Even with ridiculous equipment (weight of about 2.5Kg) the frame behaves very badly!
 I believe the Explorer Scientific Company, better do a recall campaign and modify the project!
 It does not work this way!!!!!
 Matteo Belloli

[Adam J]

I was seriously considering one of these from FLO but if this is normal no way am I going to make that purchase. That oscilation is larger than my proposed image scale. 

[Devonshire]

Drjolo,

What software are you using to control the mount?  Explorestars?  Something else?

 

[drjolo]

It is my friends' mount, and as far as I remember he uses Cartes du Ciel.

Eventually some time ago we found out the root cause of this problem. It is the stepper controller chip (DRV8824 or DDR8834 - I do not remember now) that provides non-linear current when microstepping. When we connected both motors to another board (AstroEQ) then they work perfectly fine. 

[Kiwi_Brad]

I am having similar issues and PHD2 always throws up errors after calibration about not being able to make sufficient adjustment or similar. 
 

I have managed to get some decent images from this as an entry level mount for the price but the support and help is non existent.  They reply but have not actually offered parts of solutions to any of my issues. Terrible binding wobble and backlash.  I am still a beginner so don’t know what is ‘normal’ and not but I suspect a lot with this mount is not. 
 

it just adds to the learning curve of all the things you need to get across mount troubles are something you could do without.  
 

if you are across mounts already for the price as a beginner it’s good enough but don’t expect help fixing it. 

[Deiviejr]

I too am having this issue. Any suggestions?

[Cyril]

On 08/02/2021 at 18:44, Kiwi_Brad said:

I am having similar issues and PHD2 always throws up errors after calibration about not being able to make sufficient adjustment or similar. 
 

I have managed to get some decent images from this as an entry level mount for the price but the support and help is non existent.  They reply but have not actually offered parts of solutions to any of my issues. Terrible binding wobble and backlash.  I am still a beginner so don’t know what is ‘normal’ and not but I suspect a lot with this mount is not. 
 

it just adds to the learning curve of all the things you need to get across mount troubles are something you could do without.  
 

if you are across mounts already for the price as a beginner it’s good enough but don’t expect help fixing it. 

A good way of putting a beginner off? The price is okay but there are other mounts that will get you up and running/ viewing the heavens with less fussing about trying to sort things that should have been sorted at the design  and manufacturing stages.

Perhaps the real problem lies in where a lot of this gear is now being produced, China. My new super fast American company laptop has a small label on the back, Made in China.

Theres a lot of help on the PMC-8 group pages https://espmc-eight.groups.io/g/MAIN

[NWAastronomer]

I happened to find this thread.

I didn't see anyone post photos they've taken with the iEXOS-100. So, I figured I would do that to show what people are doing with the mount.

https://espmc-eight.groups.io/g/IEXOS100BEGINNER/topic/astrophotography_iexos_100/83998012?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,83998012

https://espmc-eight.groups.io/g/IEXOS100BEGINNER/topic/first_photo/83354596?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,83354596

https://espmc-eight.groups.io/g/IEXOS100BEGINNER/topic/nord_america_from_italia/76972764?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,20,76972764

https://espmc-eight.groups.io/g/IEXOS100BEGINNER/topic/competition_between_camera/70167412?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,80,70167412

https://espmc-eight.groups.io/g/IEXOS100BEGINNER/topic/first_image/49585496?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,100,49585496

I guess the little mount does take decent images -- event with the aforementioned pulsing. 

I suggest that if you think you are having a problem with your mount you contact Explore Scientific customer service at https://explorescientific.supportsync.com/ so the customer service staff can work with you.

And as noted above, the PMC-Eight Users Group has a wealth of information and is a great resource. https://espmc-eight.groups.io/g/MAIN

Kent Marts

Explore Scientific
 

[S t e f a n]

Just came across this thread. When tracking Jupiter, I see a very regular periodic wobble in RA with an amplitude of about 1/10th of Jupiter's diameter, so (Fall 2022) about 5", and a period of roughly 1.2s (I counted 12 seconds for 10 wobbles). I assume this is the same issue? Is this correctable by firmware, or is this behavior (as seems indicated by an earlier post) baked into the driver chip on the PMC mount controller board?

I just started using this mount (IEXOS-100-2), so I assumed this was due to something I was doing, and maybe caused by bad balancing or some other newbie error, but then I came across this thread.  If this (5 arcsecond drive wobble) is inherently not correctable, that would be kind of bad. I mean, it's about a tenth of the diameter of a pretty large planet...