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Equipment Discussions >> Mounts

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Chris Purves
super member


Reged: 11/01/08

Loc: Wimbledon, London
Why encoders ? new
      #5494457 - 10/29/12 11:17 AM

Hi Guys,

Over dinner at this past weekends IAC we were discussing the new paramount and astrophysics mounts that come with optional encoders. More specifically, with protrack and tpoint like models running in software, is there really any benefit to having encoders especially when they can run you many 000's of $$$?

I would welcome any thoughts / opinions on this topic.

Cheers
Chris


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pfile
Post Laureate


Reged: 06/14/09

Re: Why encoders ? new [Re: Chris Purves]
      #5494497 - 10/29/12 11:40 AM

tick management perhaps? though i don't know if those encoders have enough resolution.

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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Why encoders ? new [Re: Chris Purves]
      #5494498 - 10/29/12 11:41 AM

Quote:

Hi Guys,

Over dinner at this past weekends IAC we were discussing the new paramount and astrophysics mounts that come with optional encoders. More specifically, with protrack and tpoint like models running in software, is there really any benefit to having encoders especially when they can run you many 000's of $$$?

I would welcome any thoughts / opinions on this topic.

Cheers
Chris




Chris:

I believe the advantage of encoders that on the axes themselves rather than counting motor revolutions is that you are measuring what you actually want to measure rather than counting motor revolutions and then dealing with the various possible errors in the gear train etc.

I one time tested a Bushnell North Star GOTO mount, it used encoders but the resolution was very course and it only had one slewing speed which was way too fast. I suspect they used encoders on the two axes because they could use cheaper motors. It was not very accurate...

Jon


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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Why encoders ? new [Re: Chris Purves]
      #5494499 - 10/29/12 11:42 AM

In the case of AP, I believe that the main benefit to the encoders is for people who do remote, unattended imaging. With the encoders, the mount "knows" its physical orientation at all times and it cannot get lost.

There was a mention on the AP user's group a week or so ago that the encoders could also be used for backlash compensation. For example, it would be possible to eliminate declination backlash issues while guiding.

I am not familiar with the Bisque system.

-Wade


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Chris Purves
super member


Reged: 11/01/08

Loc: Wimbledon, London
Re: Why encoders ? new [Re: WadeH237]
      #5494516 - 10/29/12 11:55 AM

Thanks guys - I imagine the two systems are pretty much the same idea.

I agree it means the mount knows (to some accuracy) exactly where it is at all times and can check after a slew that it actually got to where it thought it was going. Clearly periodic errors should be cut significantly - by how much is yet to be seen as PEC is already very good. Would be nice if they inlcude a relay to trigger a park on the ME without the PC - e.g rain detected -> immeadiate park.

Chris


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RAKing
Postmaster
*****

Reged: 12/28/07

Loc: West of the D.C. Nebula
Re: Why encoders ? new [Re: Chris Purves]
      #5494562 - 10/29/12 12:23 PM

I have encoders on my Losmandy GM-8. I also have the Gemini system on it and the advantages are as you mentioned. I feel the encoders are more accuarte and my Gemini is always checking with the encoders to see if they match. It will report any errors or slippage so I can make corrections.

I am still a visual astronomer and I like having the ability to unlock the clutches and move the scope manually without losing alignment. When I am done scanning the Milky Way with my little refractor, I can just lock the clutches and issue a Goto command to my next area with no problems.

Cheers,

Ron


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Startraffic
professor emeritus
*****

Reged: 02/12/06

Loc: Lat. 39.143345, Long. -77.1748...
Re: Why encoders ? new [Re: Chris Purves]
      #5494576 - 10/29/12 12:33 PM

Chris,
I put encoders on my Losamandy G11 originally for use with a Sky Commander. The original Analog Setting circles weren't incredibly accurate, & I never got the hang of using them. The Digital Setting Circles were easy, Polar align, set the time, synch to 2 stars & go. To further increase their accuracy, I upgraded the encoders themselves from 4k to 10k encoders. The accuracy was phenomenal. Then I upgraded to Gemini-2 for Goto capability, but the encoders weren't supported. I've upgraded mounts once again to a HGM-200 with Gemini-1 & I'm back to built-in encoders. I've swapped the Gemni-2 over to the HGM & put the Gemini-1 on the G11. The original Gemini-1 supports the optional encoders, but not the 10k resolution, so I've reinstalled the original encoders. The G11 will be sold in the near future as a complete kit.
I'm waiting for the external encoder support in the Gemini-2 to be implemented.
The primary reason I want this is because of outreach. I do quite a bit of outreach with the BSA. Cubscouts (boys 6-10) are are allowed into my POD with another adult & get to look through the scope as a part of their Astronomy belt loop award. The boy invariably put their hands on the tube & move the scope from the object we're observing. With the Sky Commander or the Gemini-1 it's simple to get back on target by readjusting or doing the Goto again. Without the external encoders, you have to manually reacquire and resynch on the object. If the scope gets bumped again then you have to do the same thing again. Boyscouts (Boys 11-18) also are allowed to use the obs with another adult and tend to be better than Cubs, but they also will bump the scope off target. They however, are required to reacquire the target themselves as apart of their astronomy badge.
The exception to this is when they kick the tripod, move the legs & all bets are off. This is far less likely to happen with the HGM since the mount & tripod weigh ~100 lbs. Add in the OTA, GS, finder & cameras & its' close to 150 lbs. It takes a really determined kick to move it. I'm still putting in a pier this upcoming spring, but that for more floor space & less tripping. The POD is 8ft in diameter, put the HGM/G11 tripod in there & your footing is something to really pay attention to.

Clear Dark Skies
Startraffic
39.138274 -77.168898


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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Why encoders ? new [Re: Chris Purves]
      #5494811 - 10/29/12 03:09 PM

Quote:

Thanks guys - I imagine the two systems are pretty much the same idea.

I agree it means the mount knows (to some accuracy) exactly where it is at all times and can check after a slew that it actually got to where it thought it was going. Clearly periodic errors should be cut significantly - by how much is yet to be seen as PEC is already very good. Would be nice if they inlcude a relay to trigger a park on the ME without the PC - e.g rain detected -> immeadiate park.

Chris




When I suggested that it knows where it is, I was not referring to it's position relative to the sky. I was talking about it's absolute position mechanically.

For example, if you have some issue where the mount ends up losing its sync, you can end up having to go on site to correct the problem (or risk crashing expensive equipment into other expensive equipment). With the encoders, this is not an issue, as the mount can never get lost relative to itself.

Yes, the encoders can and do improve PE. Astro-Physics publishes numbers on their site for the AP1600 with and without encoders and with and without PEC. The encoders essentially reduce PE to 0.2 arc seconds. Off the top of my head, I believe that PEC with no encoders is 1.4 arc seconds.

As you've surmised, it's possible that your unguided tracking accuracy may be better than your seeing. And if you are guiding, I don't know that the encoders offer much practical benefit to the actual mechanics of imaging.

It's probably worth mentioning that if you are going to try long unguided exposures, the encoders are not the most important thing. Due to refraction, flexure, etc., the correct tracking rate is rarely exactly sidereal rate. The new APCC software that is due to be released "rsn" (IBM speak for 'real soon now') will be able to build a pointing model and vary the tracking rate to account for all of this - with or without encoders.

For what its worth, I have an AP1600 on order and I did not opt for the encoders. I believe that the mount and associated software will give me pointing, tracking and guiding that exceeds what my sky will give me in nearly all cases. If I was going to be setting up a remote observatory, I would have made a different decision.

Finally, I believe that after-market encoders can potentially work wonders on lesser mounts that don't have the mechanical qualities of the AP and Bisque mounts. But in that case, the benefits are totally different than what I've described above, in that there are big improvements to be made on tracking. I would think that one caveat is that the encoders would have to be compatible with guiders to account for the non-sidereal rate issues that I listed above.

I hope that this makes sense,
-Wade

PS: One other final comment: You put "to some accuracy" in parenthesis above. I would think that it would be better to say "to extreme accuracy". The encoders that AP uses have 67 *million* pulses per rotation of the axis. In other words, the mount knows the position of the axis to within .019 arc seconds.

This is enough accuacy to do all kinds of interesting things, like taking out backlash as I suggested in my first reply. My personal feeling, though, is that you do get diminishing returns on all this technology, once your mechanical performance exceeds other factors and is no longer limiting. Hence my decision to skip the encoders for the AP1600 at this time.


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jjongmans
super member


Reged: 02/11/12

Loc: The Netherlands
Re: Why encoders ? new [Re: WadeH237]
      #5494827 - 10/29/12 03:20 PM

I just ordered a ASA Direct Drive mount, no gears, no backlash, no periodic error. It uses Renishaw Encoders which resolve to 0,01-0,02 seconds of arc. And corrects 100 times a second, compensating wind gusts. It depends on accurate pointing and tracking models, with refraction and flexure taking into account. No guiding is needed. That's what their sales dept says.. I'm going to find out.

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BPO
sage


Reged: 02/23/10

Loc: South Island, NZ
Re: Why encoders ? new [Re: jjongmans]
      #5495738 - 10/30/12 03:53 AM

There are many images around captured with the aid of these mounts. Often in the 30 minutes+ unguided category. The learning curve is steeper than for the old mount designs, and place more demand on the skill and knowledge of the user, but once those issues are dealt with they certainly appear to be the future.

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frolinmod
Carpal Tunnel


Reged: 08/06/10

Loc: Southern California
Re: Why encoders ? new [Re: WadeH237]
      #5495788 - 10/30/12 06:20 AM

Quote:

When I suggested that it knows where it is, I was not referring to it's position relative to the sky. I was talking about it's absolute position mechanically.

For example, if you have some issue where the mount ends up losing its sync, you can end up having to go on site to correct the problem (or risk crashing expensive equipment into other expensive equipment).



Absolute encoders will certainly give you that, but so will a homing sensor at far less cost. Paramount mounts since at least the GT-1100S circa 1999 have had homing sensors. With the ME-II they're also offering optional (and quite expensive) absolute encoders.

By the way, with absolute encoders there are no positioning ambiguities and therefore no park position restrictions. But I wouldn't buy them just for the benefit alone.


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orlyandico
Postmaster
*****

Reged: 08/10/09

Loc: Singapore
Re: Why encoders ? new [Re: frolinmod]
      #5495840 - 10/30/12 07:43 AM

the trouble with "regular" gray-code absolute encoders is that their resolution is not that high (up to about 14-bit or 16K tics only).

a more generic solution is a very high resolution analog encoder which has indexing marks. These are usually every 5-10 degrees or so.

So when the mount powers up it has to do a (slight) slew to find its absolute position, which is much less than the slew-to-home required by the Paramounts and CGE/CGE Pro.


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Ray Gralak
Vendor (PEMPro)


Reged: 04/19/08

Re: Why encoders ? new [Re: frolinmod]
      #5496467 - 10/30/12 04:38 PM

Quote:

Quote:

When I suggested that it knows where it is, I was not referring to it's position relative to the sky. I was talking about it's absolute position mechanically.

For example, if you have some issue where the mount ends up losing its sync, you can end up having to go on site to correct the problem (or risk crashing expensive equipment into other expensive equipment).



Absolute encoders will certainly give you that, but so will a homing sensor at far less cost. Paramount mounts since at least the GT-1100S circa 1999 have had homing sensors. With the ME-II they're also offering optional (and quite expensive) absolute encoders.

By the way, with absolute encoders there are no positioning ambiguities and therefore no park position restrictions. But I wouldn't buy them just for the benefit alone.




RA Encoder advantages:

1) Essentially perfect RA tracking when tracking rate modeling is used.
2) No need for homing.

Dec Encoder advantages:

1) Essentially perfect Dec tracking when tracking rate modeling is used (comets, satellites, asteroids, and tracking rate compensation for refraction, flexure, etc.).
2) The encoder can be used to completely eliminate declination gear backlash.
3) No need for homing.

RA/Dec encoder disadvantage:
1) Extra cost.

-Ray


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Ray Gralak
Vendor (PEMPro)


Reged: 04/19/08

Re: Why encoders ? new [Re: jjongmans]
      #5496477 - 10/30/12 04:43 PM

Quote:

compensating wind gusts.




The encoders will only correct mount errors they can measure. The encoders can't, for instance, correct pointing errors caused by the OTA vibrating or bending from the wind, because those errors are not "visible" to the encoders.

-Ray


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frolinmod
Carpal Tunnel


Reged: 08/06/10

Loc: Southern California
Re: Why encoders ? new [Re: Ray Gralak]
      #5496552 - 10/30/12 05:27 PM

Thank you Ray. It'll be interesting to see if we can come up with more benefits (even little ones) and if we can quantify them. Otherwise it's looking like a case of paying a lot more for very little benefit. Par for the course in our hobby.

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jjongmans
super member


Reged: 02/11/12

Loc: The Netherlands
Re: Why encoders ? new [Re: Ray Gralak]
      #5496591 - 10/30/12 05:53 PM

Quote:

Quote:

compensating wind gusts.




The encoders will only correct mount errors they can measure. The encoders can't, for instance, correct pointing errors caused by the OTA vibrating or bending from the wind, because those errors are not "visible" to the encoders.

-Ray




A wind gust will disturb the rate of rotation, so it can be measured and can be compensated, especially when that's measured 100 times/sec with a resolution of 0,01-0,02 arcsec. But I agree that the complete setup has to be very stiff.

Some experiences here. "I have tested this myself by achieving round stars in unguided 10 minute exposures at RTMC in 2011 with a 30 mph crosswind."

Watch this at 2:55: youtube

Edited by jjongmans (10/30/12 06:01 PM)


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Why encoders ? [Re: Chris Purves]
      #5497173 - 10/31/12 12:50 AM

Quote:

Hi Guys,

Over dinner at this past weekends IAC we were discussing the new paramount and astrophysics mounts that come with optional encoders. More specifically, with protrack and tpoint like models running in software, is there really any benefit to having encoders especially when they can run you many 000's of $$$?

I would welcome any thoughts / opinions on this topic.

Cheers
Chris




I think the idea behind high resolution encoders is to allow closed loop, unguided long exposures with minimal tracking errors. The point of absolute encoders in particular is to allow the mount to always know how it is oriented (a definite benefit for remote observations).

In order to get the benefit of he high resolution encoders when taking unguided exposures, you will need a sophisticated pointing model that corrects for polar misalignment, tube flexure, atmospheric refraction, etc..

Are the results better than auto guiding? I'm skeptical, but it's possible. I suspect, though, that the main benefit is that once you have taken the time to develop a good pointing model, you can avoid the hassle of finding a guide star, worrying about losing your guide lock from a passing cloud, etc.. I wouldn't bother unless I had a permanent site.

Personally, I prefer the combination of a good mount and an adaptive optics system with an off-axis guider (to put the guide camera in front of the filters). That has the potential to help with seeing (at least lower frequency seeing) as well as being less expensive than high resolution encoders. Both systems are closed loop and can provide seeing limited results when all components of the optical train are optimized.


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