Author Topic: Encoder and belt drive questions  (Read 418 times)

Manuel Ghumare

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Encoder and belt drive questions
« on: December 24, 2017, 05:37:20 PM »
Hey folks,

I am seeking advice and information to get a mount that fits my requirements.  I began this year with a AVX (after expert advice!)  And I am very disappointed.  Telling more is waste of time.
What I found from the forum and the net:

- GM 1000 HPS GoTo by 10 micron.  It's told with the enconders about the axes you can take 5 minutes subs without guiding.  That needs a very good polar alignment, I guess.  Onyone who has tried?
To be honest, I am not sure if I wish to invest that much money.  And I do not need satelite monitoring.

- AVALON LineAR that's run by belt drives and belt drives just.  It need guiding and thats nice.  They claim it's zero backlash.  Really?  Is anyone doing photography for this one?

- losmandy .  When I got it right you purchase the mechanisms from one men and then have somebody else add the drives.  The fotos I seen seemed a little semi professional although folks appear to be happy with it.
Has it/one of the driveway options got encoders?  Belt drive?

- Atlas EQ/AZ.  Cheap compared to the others.  It's encoders and belt drives.  There must be a reason why the GM 1000 as well as the LineAR cost so much money.
Has it got belt drives just like the Avalon or a worm gear and belts between stepper and pig?  Some wrote about adjusting it and that seems like pig gear for me.
Does encoder mean nice enough to operate like the 1000 HPS without guiding or just fine enough to not loose positon when manually transferring it?  Or is it the difference between central european and chinese work?

Maybe there are better options for me.  What I've: please refer to this signature.  The payload (extent, DSLR, reducer, guidescope and guidcam) is 7kg just and I am using the reducer most of the time  it is over 930mm focal length (and that is why I believed the AVX should be fine with hardly any load and a not overly long focal length).  I am happy with 5 minutes subs and neither am I really going to narrowband using 20 minute subs nor any longer focal length to get the not too distant future.  Maybe a faster scope that might be thicker.  Can't say for sure.  I like galaxies and nebulae.  I am not attempting objects smaller than 10 arcmin today but I'd be glad if I can do so in future.

Any advice is quite much appreciated.

the Elf


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Re: Encoder and belt drive questions
« Reply #1 on: December 27, 2017, 01:43:49 PM »
It actually might be helpful to describe what you don't like about the AVX, because it reveals what is important to you, and what problems you're trying to solve. Many people seem quite happy with the AVX, so that least a question about why you are not.

7kg: That's really not much mass. All of the mounts in your list are overkill, but will be nice if you do decide to get heavier stuff. (Most people do seem to decide to get heavier stuff, until they reach some kind of personal limit!)

Unguided: I disagree with the philosophy of unguided imaging. Feedback -- measuring the results of your efforts and making corrections based on the error -- is a simple and effective technique that generally works very well, and is applied to a wide variety of technologies. Combining feedback (i.e., a guide camera) with predictions based on the consistent behavior of the mount (i.e., PEC, and/or more detailed modeling) seem more than sufficient to me, and in general, a more technically-sound approach than blind open-loop tracking based on modeling. The high resolution encoders used in these designs are very expensive. Personally, I would not spend a lot of money in the persuit of eliminating the guide camera just to get similar performance. Other people disagree!

Avalon: Every time I look at a picture of one of these mounts, I see a long, springy lever arm. I cannot imagine how it would be stiff enough for my personal standards; I expect it to be more bouncy than most other designs in the same general class. I also disagree with allowing the mount's fundamental motional errors to be so high; I think other designs that provide at least reasonable accuracy without guiding are a better way to start. Those two factors would keep me from considering one of these mounts. Other people disagree!

Losmandy: Huh? I'm not sure what you have read. Losmandy sells complete mounts with complete control systems, and they're very nicely made out of the box. Mr. Losmandy is an excellent machinist, and designed the mount at a time when it was hard to get a good mount, let alone for a reasonable amount of money. They're beautifully made. The mechanics, in general, are very precise, solid, and smooth. They're not perfect, though, and people have invested time and effort in making modifications to them to improve them, including changing the motor gearboxes and the worm gears. These apparently sometimes make an improvement, but sometimes not so much. (The major improvement for me was that the new gearboxes are much quieter than the old ones during slewing.) Explore Scientific has recently produced an alternative control system for the mount; we shall see how well it works as more people buy it and try it.

Atlas: I have no experience or strong opinions, here. My weak opinion is that the price is lower because of mass production in a foreign country where it's less expensive to make things than, say, California, and that the design and manufacturing are intentionally lower-cost from the beginning. My impression is that most people are at least satisfied with, and sometimes very happy with, this class of mount.

Jeremy Swaine

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Re: Encoder and belt drive questions
« Reply #2 on: December 29, 2017, 11:55:22 PM »
guiding smooths out atmospheric and mechanical deviation. I gave up on AP but am still versed on the mechanics. All non stepper goto's have encoders but most on the motors. Absolute encoders of quality directly on the axis are expensive, like more than most mounts in total just for the encoders. I have a Losmandy GM8,G11 and a AP Mach1 GTO non with absolute encoders and have no burning desire for them.


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Re: Encoder and belt drive questions
« Reply #3 on: January 11, 2018, 01:47:19 AM »

thank you very much for your answer. I agree, it helps when I explain what I don't like.
- There was play on the dec axis, no matter how strong I engaged the "clutch" (a simple plastic bolt).
- I measured PE with PHD2, guide output turned off and just took the curves after a precise polar alignment. The result was 60'' peak to peak in RA while Dec was flatline.
- With exposure times of 3 to 5 sec on the guider I could not guide that PE out. There was a deviation of about 6 to 8 '' peak to peak left. This ruins an image at 1370mm and at 930 as well.
- When pointing straight up the huge play in the dec axis lead to a pendulum movement. Even with an agressive setting it took PHD 20 seconds or more to catch up. Increasing backlash correction in the mounts menu did not help much.
- Sometimes I made 3 to 5 subs of 5 minutes without any dec corrections and RA as stated above. Then all of a sudden it started oscillating for no reason. Yes, there was a very little wind sometims, but not much and the scope is small.
- I sent it to an expert who put better grease in and adjusted some things. When he was finshed he said the saddle for the dec bearding is just to large, the bearing has got play in the mount. Now, that someone took it apart waranty is void.
- After adjustment dec play is less but still not guidable. I drilled a hole in the counter weight and added a small 500g off axis weight to have some force to one direction when imaging near the zenit. That works!
- I got PEC data and repeated the above test with PHD2 without guide signal: 10'' peak to peak. A strong improvment. I expected to have that out ouf the box. Now PHD need to correct less.
- The tripod is poor. The upper area that carries the mount is not even. The legs have kind of egg shaped ends that easily slip on paved ground. The controller cable is stiff and by far to short.
- Polar alginment is horror. You can hardly look in the polarfinder and have the adjusting screw in your face. There is no kind of "where is polaris" in the mount software. I need to check the position on the laptop first. I had to create my own mark for the RA position that makes the reticle (clock scale, no stars) stand upright. (Yes it has got calibration star based polar alignment but it takes an hour until it is all set. After the polar alignmet you just start from scratch, that is what the german manual by Baader Planetarium says.)
- There is no bubble level and no place to put one. I put a big one on the tripod without mound and adjust the legs, but when the mount and scope is on top the legs have moved a little for sure. Need to sharpen the feet somehow. I thing I go for a berlebach ash wood tripod.

So, what I would like to have:
- tripod stands solid on pavement or stone
- just balance RA and dec and guiding is within 2'' peak to peak.
- no play in the axes
- polar finder without screw in my face
- an option for about 1500mm with 5 micron pixels in the future.
Im willing to invest something between 3 and 4 kEur for that.

Thank you for the detailed review of the different mounts. To explain, where I come from: I'm working in a lab and there is kind of a 100kg mount, called goniometer. That thing has got worm gears and encoders on the worm and the gears under strong tension. It's by Newport spectraphysics and costs 30.000 Eur. With this large machine made for 100kg the have an absolut position of 1.4 milli degree which is 5 arcsecs. The repeatabilty is far better and outside of what I can measure there. That said I was thinking encoders on the worm and a propper worm gear is the way to go as opposed to guiding. Feedback and control is a good concept and I know quite a lot about it from my work in the industry. What I know is, that play or backlash or dead time or howerver you call it, kills any loop or makes it slow. That's why I like belt drives. With 1) no or little play and 2) no sudden movements or rattle guiding will work for sure.

I'll try to find the losmandy that is advertised by a german dealer and post the image soon and you'll see, what I mean.
Thank you again.

the Elf

@Bruce: thank you for your information. I'll redo my losmandy research in the internet.

David Reynolds

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Re: Encoder and belt drive questions
« Reply #4 on: January 11, 2018, 02:50:05 AM »
The Losmandy should be complete with all you need from the manufacturer.

You mention 3-4K Euro, but you list 10 Micron and Avalon Linear. My advice: If you can afford a premium mount (afford doesn't mean you *WANT* to pay that much, but that you *CAN* pay that much) then *BUY* a premium mount. I would recommend the Mach1 GTO if you were US-based, but they cost more in Europe. I think the Avalon Linear may be a great choice for you. Lots of people have done AP with it with tight stars. You should contact them and ask for references, plus you can find posts on Cloudy Nights and probably other (Stargazers Lounge?) astro-sites.

Losmandy is a very good mount, but it's in the "sub-premium" category.

The AVX is a beginner mount, and it appears that your copy was even worse than mine (I thought I had the record). My problems were all in DEC--RA was quite manageable.

Jomega Ceo

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Re: Encoder and belt drive questions
« Reply #5 on: January 12, 2018, 09:31:56 AM »
Hey Don(?), hey Bruce,

this is what I found about the G11:
- the worm gear open, no housing, the wires open:

A gray box with an antenna and long cable dangling:

Again mechanics quite exposed to the elements:

A system called Gemini 2 sold separately:
I understand you buy a pure mecanics mount and this system and attach it to the mount?

It seems to be available as a set as well:

I'm a bit confused. Have you got a link to the system you use/recommend? Thank you!

the Elf

Jayarajan Mcloven

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Re: Encoder and belt drive questions
« Reply #6 on: January 13, 2018, 11:28:57 PM »

thank you. As you see here:
we pay 7500 Eur for that thing. Perhaps the free trade agreement with canada can help me... Or my next visit in the US.
Well, what can I afford? The question is, what are my priorities? I'd like to be happy.... Save money and worry about poor results is not wise. But there is no guarantee that more money leads to good function. In a shop in Skagway, Alaska I read the nice words: "The bitter taste of poor quality lasts long after the thrill about the price is gone." True. Thank you for your advice, I'l try to contact people who have one of the mounts and check the photos they made. Nice to read you made the step from AVX to a premium mount. I'll let you know, when I follow.CS
the Elf