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Equipment Discussions >> Mounts

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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Raginar]
      #5757757 - 03/26/13 09:27 AM

Chris, yes am on Google Plus. orly dot andico.

I won't be able to make any more updates to this project till Friday, as I'm on business travel.

Seems that there are no alternatives to Heidenhain. The local Baumer dealer has been deafeningly silent. I have asked Heidenhain if they have a cheaper encoder (maybe 2500 ppr).

But speaking of the MI250... I don't see this solution being that useful for mounts of higher-tier than a G11. Neilson in the other thread reports 3" p-p PE for his CGE. If you're already getting that level of performance, an encoder will not provide much incremental benefit.

In fact, the very reason I lost interest in this whole encoder thing was because when I got the Mach1 I discovered its peak-to-peak PE after PEC was 0.5".

I imagine an MI250 would have very low PE, and whatever it had, could be removed with PEC.

I really see this encoder having the most benefit for EQ6 / Atlas and CGEM. Because lots of people have these mounts, and they generally have terrible PE.


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Starhawk
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Reged: 09/16/08

Loc: Tucson, Arizona
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5757820 - 03/26/13 09:57 AM

Thinking cheap, one could maybe use a computer mouse reading off a disk, but they seem to have relative movement problems.

All and all, this is very cool- a ready to go solution for going out in the field is a very good thing. And since I have a CGEM, I'd definitely be interested in giving this a try.

-Rich


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orlyandico
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Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5757844 - 03/26/13 10:06 AM

An optical mouse.. is about 3600 dpi for a really high-end one. If you have a 6" diameter ring, that would give you 6" x 3600 x 2pi = 135k dots. Not enough.

Another approach is using one of those Renishaw linear encoder read heads. I have one of those too! :P

The problem with these is the tape is darn expensive.. over $250 for an appropriate length (about 20"). And it's more of a hassle because you have to wrap the tape around a large cylindrical portion of the mount.


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brave_ulysses
super member


Reged: 04/19/09

Loc: far outside the wire
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5758029 - 03/26/13 11:44 AM

maybe this with a us digital codewheel could be a start

http://elcodis.com/parts/2239977/HEDS-9710-150.html


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: brave_ulysses]
      #5758069 - 03/26/13 12:10 PM

As it has an analog output, I assumed we could do as high a level of interpolation as we wanted. But Agilent's own blurb says it is only good for 1440 lpi with external interpolation.

http://www.agilent.com/newsroom/German/press_releases/optic_encoders.html
Quote:

Agilent's HEDS-971X-150 is an analog-output encoder providing 360 LPI resolution, higher resolution than any analog encoder designed for office automation applications. The HEDS-971X-150 delivers on effective resolution of up to 1.440 LPI when used with external interpolation techniques.




The data sheet does show that it has sinusoidal (current) outputs. Not sure how good those sinusoids are going to be.. definitely would be worth looking into, although I don't think it is precise enough, to be honest.


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Startraffic
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Reged: 02/12/06

Loc: Lat. 39.143345, Long. -77.1748...
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5758276 - 03/26/13 01:36 PM

Orlando,
PM Sent.

Clear Dark Skies
Startraffic
39.138274 -77.168898


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Stew57
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Reged: 05/03/09

Loc: Silsbee Texas
Re: Encoder-based PE Correction on the cheap new [Re: Startraffic]
      #5758559 - 03/26/13 04:11 PM

A Telescope Drive Master at less than half the price...Count me in for one!! I am tired of waiting for Celestron, this would be perfect.

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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: Encoder-based PE Correction on the cheap new [Re: Stew57]
      #5758914 - 03/26/13 07:14 PM

This looks like a really nice idea. Shame the list price of the encoders is so high but precision is expensive.

Would the Arduino cope with counting pulses when the scope is slewing? I'm thinking that the encoder could provide a really accurate absolute position.

Chris


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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: cn register 5]
      #5758965 - 03/26/13 07:40 PM

Chris, I am not sure that the Arduino can count pulses while the scope is slewing. I have some ideas for that (increment a "slot count" whenever the arc-tangent crosses zero).

But right now my plan is to just stop correcting if the encoder turns too fast, and start correcting again when it settles.

It would be possible to add comparators to convert the A and B outputs to digital "in parallel" - and use these digital outputs to sent interrupts to the Arduino. That would allow the Arduino to keep up even while the scope is slewing. However it would still not be an absolute position, since the encoders aren't absolute and don't have a home index. It would be like the DSC encoders.

In any case, adding position feedback takes the design out of the realm of generic (you can't meaningfully transfer the position to the GoTo controller via the ST-4 port!). Most GoTo controllers (except say the Gemini 1/2) don't have provision for supporting encoders anyway.

I don't want to compare this to the TDM, and the only way this thing ends up costing "less than half a TDM" is if you put it together yourself.


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Starhawk
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Reged: 09/16/08

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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5759347 - 03/26/13 10:48 PM

Actually, I've been thinking:

The king rate is most significant near the horizon. The sweet spot for imaging is the zenith. So, sidereal is a good solution. If someone is hard over on looking low to the horizon, a little tracking error will be the least of their problems.

So, no position necessary. If you like, ad a 45degree and 0degree king rate switch for those who won't be deterred.

-Rich


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wolfman_4_ever
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Reged: 07/15/11

Loc: El Segundo, Ca, So. Cal
Re: Encoder-based PE Correction on the cheap new [Re: Starhawk]
      #5759442 - 03/27/13 12:20 AM

Everything I have read, it's about 45degree's... I'm sure it's where you live, conditions, etc..

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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: wolfman_4_ever]
      #5759516 - 03/27/13 02:14 AM

Well looks like the blind-shaft ERN480 is the way to go at $325..

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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5759552 - 03/27/13 03:59 AM

Quote:

I'm wondering if I should blow the $220 for a sample....



Put it towards the CGEM motor/driver replacement fund

I'm not really serious, it wouldn't be nearly as much fun as this is.

Chris


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orlyandico
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Loc: Singapore
Re: Encoder-based PE Correction on the cheap new [Re: cn register 5]
      #5759559 - 03/27/13 04:12 AM

Chris, yeah I don't need the CGEM to work fine. I have a Mach1 for when I need a mount that "just works."

The 2048ppr Heidenhain is $265 US. The 5000ppr is $325 (this is for the version with a 12mm blind shaft hole, it's cheaper than the one with a 12mm hollow bore).

I've decided to refine the design by adding a dual comparator (TLC3702). This increases cost by $1. It will also allow the interpolator box to not lose ticks if the mount is slewing rapidly.


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jbalsam
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Reged: 07/06/12

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Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5760066 - 03/27/13 11:47 AM

One reason for putting encoders on a high end mount is so you can disengage the clutches and push the mount somewhere else fast, re-engage them, and the mount still knows exactly where it is.

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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: jbalsam]
      #5760079 - 03/27/13 11:53 AM

Mounts with built-in encoders on both axes (like the ASA mounts) would work, although I don't think these mounts can be manually pushed.

My old AP600 mount has encoders on both axes, but they are not high-resolution encoders.

I was simply referring to the phenomenon of putting the (single axis) commercial encoder solution on the PME. This would not allow the use of pointing. Besides the PME can't be manually pushed.


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galaxy_jason
Vendor


Reged: 05/22/07

Loc: Texas
Re: Encoder-based PE Correction on the cheap new [Re: orlyandico]
      #5760296 - 03/27/13 01:31 PM

Dumb question but other than a cool science project what is the point of this? If the mount has PEC then simply train it with PEMPro. If it doesn't then it may be time for a new mount. The older mounts without PEC don't track that well anyway and may not be worth the effort.

What am I missing?


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Starhawk
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Reged: 09/16/08

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Re: Encoder-based PE Correction on the cheap new [Re: galaxy_jason]
      #5760323 - 03/27/13 01:37 PM

8/3 resonance.

-Rich


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Stew57
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Reged: 05/03/09

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Re: Encoder-based PE Correction on the cheap new [Re: galaxy_jason]
      #5760336 - 03/27/13 01:39 PM

The PEC on the CGEM can not take out the infamous 8/3 error as it is non integer. Either a 3 worm rotation PEC would have to be instituted (not a 3 rotation average) or new motors. For some mounts the non integer componet is larger than the first harmonic.

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orlyandico
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Re: Encoder-based PE Correction on the cheap new [Re: Stew57]
      #5760363 - 03/27/13 01:45 PM

I agree, this is just a cool science project.

Seriously, mounts with a decent PEC routine (and that includes the Atlas!!) would probably not benefit that much.

That said, even the Atlas has significant fast gear noise, which PEMPro can't remove. I have yet to find out (but I will this weekend) if this scheme can remove fast gear noise.

Of course AP (and others) have solved the problem of fast gear noise. AP at least makes their own gears, and I can testify that it really knocks gear noise way down.

Whether or not encoder-based PE correction is worth doing has been argued about a bazillion times so no need to repeat the arguments here.


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