Encoders: yay or nay?

[Nikodemuzz]

Hello all!

I am looking for a new mount, which I hope will be one that will last me a long time. I'm trying to follow the "buy once, cry once" advice, to the extent that I can and what is reasonable for my use. I would love to buy a Mesu and be set for life, however that goes beyond my financial limits. I am looking at iOptron and their CEM60 and CEM120 as possibilities. Both come with or without encoders.

If I'm understanding correctly, encoders reduce (or eliminate?) periodical error. But how significant that is for a mount's performance if you are guiding anyway? I have read that the models (at least for the CEM60) with encoders have issues with guiding.

I'm a bit confused, as I would be willing to consider an investment into added quality, but I'm not really sure if I'll get that with the added expenditure in encoders. What do you think?

[Davey-T]

There was some problem with encoder version software don't know if it was sorted or not, I think @souls33k3r had encoder version and swopped it for non encoder one.

Dave

[Astrosharkey]

I have the CEM60 without encoders as I guide all the time and love it, it appeared that the encoder software would fight the guiding causing issues, I knew I wouldn’t use the encoders so save myself the extra cash.

[souls33k3r]

I had a right nightmare with the CEM60-EC, I couldn't guide no matter what I did. The firmware back in the day (well it was last August) was littered with issues. In the end I had to send it back and had it swapped with the vanilla CEM60. Now the 120EC was better, firmware had a few issues initially but that was all sorted out. Maybe I had a lemon, but then again I wasn't the only one having such issues. 

There's a thread I started a while back on CN and it has a plethora of information from people testing it and whatnot. From the latest info, the newer firmware now has a handle on things but not to the extent of saying its fixed but more of a workaround. 

https://www.cloudynights.com/topic/658841-cem60-or-cem60-ec/

Its a massive thread so make yourself comfortable but I'm sure you'll find all the info that you need right there for the 60EC version. 

The Cem60 vanilla has been a joy. 

Hope this helps. 

[vlaiv]

Non guided use - encoders

Guided - no encoders

[Nikodemuzz]

Thanks all! It seems to make sense to save the money and potential headache and go without encoders.

Now to decide on the model. Not sure if it is going to derail the topic completely to discuss it here, though. The main scope for the foreseeable future will be a 120-130mm APO, used primarily for imaging. But I don't want to be having to upgrade the mount even if I would get a bit heavier/larger scopes.

I'm sure the CEM60 would be able to handle that, so I'm not sure how much there is to gain by going to heavier mounts, such as the CEM120 or the more premium ones. One thing to consider is the fact that I will have to set up for each session in the backyard, but at least the hauling by car will be mostly in the past.

[vlaiv]

1 hour ago, Nikodemuzz said:

Thanks all! It seems to make sense to save the money and potential headache and go without encoders.

Now to decide on the model. Not sure if it is going to derail the topic completely to discuss it here, though. The main scope for the foreseeable future will be a 120-130mm APO, used primarily for imaging. But I don't want to be having to upgrade the mount even if I would get a bit heavier/larger scopes.

I'm sure the CEM60 would be able to handle that, so I'm not sure how much there is to gain by going to heavier mounts, such as the CEM120 or the more premium ones. One thing to consider is the fact that I will have to set up for each session in the backyard, but at least the hauling by car will be mostly in the past.

If you will be doing setup each session - limit your choices to CEM60.

I would advise CEM120 otherwise regardless of the fact that you will be using relatively light scope (less than 15Kg total), but mount head is 26Kg alone .... I'm sometimes fed up with setting up Heq5 each session .

If you can - maybe build a pier if you have an option to leave mount on the pier when not in use (as opposed to being stored away). In this case I would say that CEM120 is still an option.

 

[Jkulin]

I have had the 60EC and now have the 120EC, I had perfect guiding with the 60EC and when I upgraded to the 120EC I knew there were issues but was prepared to see it through, I am now guiding down as low as 0.22RMS error

Most of the issues with the EC versions were that people weren't prepared to upgrade the firmware, to me that is part and parcel of imaging, it takes 5 mins to upgrade the firmware and you can always revert back.

[Nikodemuzz]

13 hours ago, vlaiv said:

If you will be doing setup each session - limit your choices to CEM60.

I would advise CEM120 otherwise regardless of the fact that you will be using relatively light scope (less than 15Kg total), but mount head is 26Kg alone .... I'm sometimes fed up with setting up Heq5 each session .

If you can - maybe build a pier if you have an option to leave mount on the pier when not in use (as opposed to being stored away). In this case I would say that CEM120 is still an option.

 

I see what you mean, however I'm now used to setting up a CGEM for each session, which sits between CEM60 and CEM120 in terms of weight. As long as there is no need to haul things up and down flights of stairs, I think I would manage.

For that to work, a proper case or other solution would be needed to keep the mount safe and tidy. CEM60 ships with one, the CEM120 doesn't.

I would definitely be interested in a pier solution. However, I would need to solve two questions to make it viable. First, we are renting so a non-destructive installation would be needed. Second, I would need to find a way to adequately protect the mount from the elements, as an obsy will not be possible. I'll have to look into this to see if might actually be feasible.

[Nikodemuzz]

7 hours ago, Jkulin said:

I have had the 60EC and now have the 120EC, I had perfect guiding with the 60EC and when I upgraded to the 120EC I knew there were issues but was prepared to see it through, I am now guiding down as low as 0.22RMS error

Most of the issues with the EC versions were that people weren't prepared to upgrade the firmware, to me that is part and parcel of imaging, it takes 5 mins to upgrade the firmware and you can always revert back.

This is interesting. When you have the encoders, the needed guiding corrections are probably smaller. I wonder how the guiding accuracy is without them. I'm sure I will find the answer in the thread that souls33k3r linked, I'll just have to find the time to read it.

Edited March 15, 2020 by Nikodemuzz

[Nikodemuzz]

To provide a conclusion for this thread I can happily announce that I went with the CEM60, no encoders. Here´s hoping I still have a chance to give it a proper spin before the season is completely over. 😃

Thank you to everyone for their contributions and advice!

[Anthonyexmouth]

Would encoders be a replacement for guiding? or is guiding superior? 

[vlaiv]

13 minutes ago, Anthonyexmouth said:

Would encoders be a replacement for guiding? or is guiding superior? 

Both have positive and negative ....

Encoders can't compensate for dynamic things nor for other types of errors. Poor polar alignment can't be corrected with encoders. Sudden displacement of telescope tube (wind, kick or whatever) - won't be compensated by encoders. Encoders can't correct for atmospheric refraction - you need elaborate sky model for that.

Guiders have quirks - need to be calibrated, are always dynamic and depend on seeing and other factors. If not configured properly can hurt more then help (chasing the seeing, over corrections - causing oscillations, etc ...)

One thing is certain - guiding is more cost effective way of doing it (arguably - will depend on justifying some expenses on other tasks as well - such as imaging, for example use of computer), and more people opt to do it that way.

I think encoders are good thing - if done properly. One of the problems associated with encoders is that it is really hard to get good precision on encoders. You want your encoders to have rather good resolution. For example, I would say that resolution of encoders needs to be around 0.1-0.2" to get really good results (just imagine DEC axis - you want your DEC error to be rather low - and that is dictated by precision in DEC encoder, because DEC is for the most part stationary).

Now if we want to have that much precision - we need encoders that divide circle in 10 * 60 * 60 * 360 = 12,960,000 steps. That is 23.6 bits of precision, so you need something like 24 bit precision encoder.

You can also see how tricky this is if you for example imagine that your encoder has 10cm diameter. Circumference of it will be 31.415.... cm = 314.159... millimeters = 314159.265um = 314,159,265.358... nm - you need single "tick" on such encoder to be 24nm wide. You can't even use photons to read such a tick because regular photon is about x20 larger in wavelength.

Now you can see why encoders are very expensive for that sort of resolution, not to mention level of craftsmanship needed to make perfect circle and avoid what we combat all the time - imperfection in manufacturing of round things (periodic error).

In the end - I think maybe best design for a mount would be to accept periodic error of worm gear - minimize backlash by using spring or magnetic loaded automatic tension system on worm and have encoders on worm shaft rather than main shaft. This way all other imperfections would be minimized (motor imperfections, gear system imperfections, belt system imperfections) and motion of worm would be properly timed and smooth enough so guiding would take care of worm period error with ease as it is smooth changing. Because mounts have at least 120+ teeth on worm gear - that is about 7-8 bits less precision needed - 16bit encoders are readily available at reasonable cost.

 

 

[oymd]

On 01/04/2020 at 17:25, vlaiv said:

Both have positive and negative ....

Encoders can't compensate for dynamic things nor for other types of errors. Poor polar alignment can't be corrected with encoders. Sudden displacement of telescope tube (wind, kick or whatever) - won't be compensated by encoders. Encoders can't correct for atmospheric refraction - you need elaborate sky model for that.

Guiders have quirks - need to be calibrated, are always dynamic and depend on seeing and other factors. If not configured properly can hurt more then help (chasing the seeing, over corrections - causing oscillations, etc ...)

One thing is certain - guiding is more cost effective way of doing it (arguably - will depend on justifying some expenses on other tasks as well - such as imaging, for example use of computer), and more people opt to do it that way.

I think encoders are good thing - if done properly. One of the problems associated with encoders is that it is really hard to get good precision on encoders. You want your encoders to have rather good resolution. For example, I would say that resolution of encoders needs to be around 0.1-0.2" to get really good results (just imagine DEC axis - you want your DEC error to be rather low - and that is dictated by precision in DEC encoder, because DEC is for the most part stationary).

Now if we want to have that much precision - we need encoders that divide circle in 10 * 60 * 60 * 360 = 12,960,000 steps. That is 23.6 bits of precision, so you need something like 24 bit precision encoder.

You can also see how tricky this is if you for example imagine that your encoder has 10cm diameter. Circumference of it will be 31.415.... cm = 314.159... millimeters = 314159.265um = 314,159,265.358... nm - you need single "tick" on such encoder to be 24nm wide. You can't even use photons to read such a tick because regular photon is about x20 larger in wavelength.

Now you can see why encoders are very expensive for that sort of resolution, not to mention level of craftsmanship needed to make perfect circle and avoid what we combat all the time - imperfection in manufacturing of round things (periodic error).

In the end - I think maybe best design for a mount would be to accept periodic error of worm gear - minimize backlash by using spring or magnetic loaded automatic tension system on worm and have encoders on worm shaft rather than main shaft. This way all other imperfections would be minimized (motor imperfections, gear system imperfections, belt system imperfections) and motion of worm would be properly timed and smooth enough so guiding would take care of worm period error with ease as it is smooth changing. Because mounts have at least 120+ teeth on worm gear - that is about 7-8 bits less precision needed - 16bit encoders are readily available at reasonable cost.

 

 

If I got that right, are you suggesting that not all encoders are equal in their precision?

that is, encoders on say an IOptron CEM25P EC would be less precise than the ones on a CEM120EC, which would be less precise than those on Astrophysics and Paramount mounts etc?

the more expensive mounts would have a higher number of clicks/counts etc?

[Davey-T]

12 minutes ago, oymd said:

If I got that right, are you suggesting that not all encoders are equal in their precision?

that is, encoders on say an IOptron CEM25P EC would be less precise than the ones on a CEM120EC, which would be less precise than those on Astrophysics and Paramount mounts etc?

the more expensive mounts would have a higher number of clicks/counts etc?

10Micron have there own encoders produced to their spec, my mount can do 30 minutes " unguided " and is really only limited by the imaging system / conditions.

Dave

[vlaiv]

15 minutes ago, oymd said:

If I got that right, are you suggesting that not all encoders are equal in their precision?

that is, encoders on say an IOptron CEM25P EC would be less precise than the ones on a CEM120EC, which would be less precise than those on Astrophysics and Paramount mounts etc?

the more expensive mounts would have a higher number of clicks/counts etc?

First part is correct - not all encoders are of equal precision - in fact, one of main specs of encoder is its resolution (number of data bits it produces). Encoder precision is directly related to encoder price.

I can't say if encoders on CEM25EC are less precise than those on CEM120EC - that is up to manufacturer to decide (which ones they will use) and to publish or not relevant information.

 

[oymd]

23 minutes ago, vlaiv said:

First part is correct - not all encoders are of equal precision - in fact, one of main specs of encoder is its resolution (number of data bits it produces). Encoder precision is directly related to encoder price.

I can't say if encoders on CEM25EC are less precise than those on CEM120EC - that is up to manufacturer to decide (which ones they will use) and to publish or not relevant information.

 

well, it would make sense that with higher end mounts the encoders would also be higher spec...no?

i think I read somewhere that the encoders used by AP are the top end ones...

I think they are called “Reninshaw” 

pretty sure I got that name wrong...

 

[vlaiv]

1 hour ago, oymd said:

well, it would make sense that with higher end mounts the encoders would also be higher spec...no?

i think I read somewhere that the encoders used by AP are the top end ones...

I think they are called “Reninshaw” 

pretty sure I got that name wrong...

 

Could be. I don't know that much about encoders to assert things with any certainty other to do basic math of what might be needed.

In fact, I might be wrong on my calculations above as well - as I assumed simple model of operation, but that might not be the case.

I said above that we need something like 24bit encoder to have precision of 0.1", and calculated single division of circle to be around 24nm. I also noted that such precision can't be done in optical configuration.

In above post I also gave a screen shot of ACURO AC58 absolute rotary encoder with 22bit max resolution. That is single turn resolution. Datasheet for this product, says following:

Now, I would expect 22bit resolution encoder to have 4,194,304 ticks. Like we said there is 60*60*360 arc seconds per full turn = 1,296,000 from this simple math says that there is about 3 and a bit more ticks per arc second or to be precise - single tick is 0.308990... arc seconds.

However - absolute accuracy is actually x100 less than that?

There is probably a lot of engineering involved in selecting and using absolute rotary encoder - knowledge and skill that I simply don't have.

What I do "know" is that you need really precise encoders if you want them to work properly and make mount precise enough and those tend to be expensive.

[Dr Strange]

2 hours ago, oymd said:

well, it would make sense that with higher end mounts the encoders would also be higher spec...no?

i think I read somewhere that the encoders used by AP are the top end ones...

I think they are called “Reninshaw” 

pretty sure I got that name wrong...

 

Correct. They are Renishaw (you spelled it right too) high resolution absolute encoders. Key words there being high resolution and absolute. This means that they are very accurate in their positioning and cannot get "lost" because each tick on the encoder wheel has  a specific code identifier. These encoders are much more expensive. And much more accurate. 

The problems that iOptron faced were in how they handled the SDE (Sub-Divisional Error aka interpolation error) in software and how poorly they handled all the owners who found a problem and wanted help with it. At short focal lengths the encoder version of the CEM60 and CEM120 did really well. When people tried to image over 1,500mm in focal length potato stars started to appear. This was due to a high frequency oscillation that would occur that wouldn't show up at shorter focal lengths but did in the longer ones. There are reports that they have addressed the SDE problem in software that was missing before but problems are still being reported so I am not sure this was addressed or there is some other additional problems. In addition this doesn't include the other issues they have had and still do have in the variability in manufacturing process, materials, and quality control. 

For its price point the CEM60 and CEM120 are good mounts but I would recommend the standard versions not the EC versions.

[Dr Strange]

Oh and I ran into this high frequency oscillation SDE problem during my Masters degree work. Different application but same problem. This was back in the mid 90's so the money for absolute encoders were more than the University was willing to spend for a non-PhD candidate that didn't have additional grant funding for their research.