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Closing a Can of Worms: Eliminating Periodic Error

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#1 Olivier Biot

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Posted 21 April 2012 - 05:36 AM

Closing a Can of Worms: Eliminating Periodic Error

By Stephen Ruhl.

#2 malachykidd

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Posted 21 April 2012 - 11:13 PM

Very slick. A couple of CNers implemented the early version of the some time ago, iirc.

As a G11 user, I'm curious about the particulars of your setup. Looking at the vendor's site, it appears this replaces Gemini for total control of the mount? Did you start with a Gemini equipped unit? Did you keep or replace the motors and gear boxes?

#3 bluedandelion

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Posted 21 April 2012 - 11:55 PM

The link to gpi-encoders should be fixed. It should be http://www.gpi-encoders.com/ but in the body of the article, the link points to http://cloudynights....i-encoders.com/

Nicely implemented and written by the way.

Ajay

#4 sfruhl

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Posted 22 April 2012 - 12:19 PM

I started out with a set of steppers and the digital drive. I took them off and sold them. My installation has Pittmans with the sitech controller mentioned in the article. The documentation from sitech includes several people who have gone through the process of conversion so there are details on sourcing and making the parts.

I have not done this but if you have a gemini, the gemini servo motors can be used instead of the pittman. The user group has details on converting the din pin layout from the gemini into the sitech controller.

Steve

#5 ASTERON

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Posted 24 April 2012 - 05:17 AM

Very nicely implemented solution and outstanding results !
Closed -loop systems rule !
Questions:
1) What was the total cost beyond the price of the basic G-11 which you invested in the precision gear and the TM encoder and system ? I am talking about the part cost only (what you had to buy from add-on vendors).

2) Are you familliar with the direct drive mounts from ASA (such as the DM80 etc.)? These use a similar principle where the Motors and encoders are directly installed on the Mount's shafts and the closed loop system (software installed on the user's laptop) is very fast doing about 100 position check cycles per second.
They claim amazing accuracy that practically eliminates the need to autoguide - users report unguided exposures of up to 30 minutes being very successful !
The only caviat is the mount costs a bundle !

#6 sfruhl

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Posted 24 April 2012 - 10:17 AM

Costs:
1997 G11 w/Dig Drive $1300
Sell Dig drive -$200
SiTech Controller $350 - (now $400)
2 Pittmans $300
Gurley Encoder $600
O-vision Gear $500
Misc Parts, Belts,etc.$100
Total $2850

I must admit that this is not my first SiTech Conversion. My 36cm Dob and the club's 70cm fork mount Newt both use SiTech. It is a really versatile system.

SiTech is used on Plane Wave mounts with the same encoders.

Yes. I have drooled over the ASA mounts but they are way over my budget. Dan Gray, the owner of SiTech, has experimented with Direct Drive motors and has prototyped them with version 2 of the controller. The Alt-Az Initiative is looking at them. http://www.altazinitiative.org/

Sitech also has some nice pointing software and plate resolving software built in. My nominal point accuracy is about 2 arc-min.

#7 gdd

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Posted 24 April 2012 - 10:36 PM

Hi Stephen,

The SiTech/TM did exceedingly well when used with the Ovision, but I thought it would do better than 8 arcsec p-p for the stock Losmandy worm. Another encoder based solution, the TDM with 500,000 encoder ticks, I think is supposed to get about 2 arcsec p-p when used on a G-11 without Ovision or the OPW. Do you think more fussing with the settings improve the performance?

Thanks,

Gale

#8 sfruhl

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Posted 25 April 2012 - 09:16 AM

Hi Gale,

Possibly. For that matter, just a plain new worm may have done better. This gear was 15+ years old.

When ever you make a correction, be it from guiding or from the encoder, it accelerates the mount and makes it ring. Smaller more frequent accelerations are better than whacking it with a hammer (via guiding) every 10 sec or so. In my case, the encoder is pinged every 60 to 65 ms by the controller. If the gear is inducing errors and the encoder trying to fix them, I could see it getting in a harmonic and making it worse than it should be. Fussing with may have helped.

I have heard from others in the SiTech world that they have gotten better results with the stock gear than I. I just report mine.

Tick Management supports the 320K and the 500K Gurley encoders. TM is a special little treatment that is done for these encoders because the physical characteristics are well known and easily corrected for errors. (I went with the 320K because it is $100 cheaper.) I have not seen the TDM work but the principle is the same. The encoder may not be as close to the controller as SiTech so it may have more latency but I do not know.

As always, your mileage may vary.

Clear Skies,
Steve

#9 John Wunderlin

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Posted 26 April 2012 - 10:16 AM

Is this basically a do-it-yourself version of Telescope Drive Master?

#10 jrcrilly

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Posted 26 April 2012 - 10:43 AM

Is this basically a do-it-yourself version of Telescope Drive Master?


Probably more appropriate to say that the TDM is a turnkey version of the SiTech. The SiTech has been available for some time while the TDM is a recent development.

#11 malachykidd

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Posted 26 April 2012 - 12:47 PM

Probably more appropriate to say that the TDM is a turnkey version of the SiTech. The SiTech has been available for some time while the TDM is a recent development.


Also, looking at the docs, the SiTech is a complete telescope controller-- it would replace Gemini on a Losmandy or Mountain Instruments mount, for example-- while TDM is an add-on that complements any existing controller, and integrates (usually) through the mount's auto-guider port.

'Tick management' is an option for the SiTech, one that adds significantly to the cost, but also provides an impressive improvement to tracking performance.

I'm interested in both, as I have an LX200 that could benefit from TDM, and a G-11 that could use something like the SiTech.

Justin

#12 poita

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Posted 01 May 2012 - 07:37 PM

Is it possible to add the same system to an EQ6?

#13 malachykidd

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Posted 02 May 2012 - 01:03 AM

Is it possible to add the same system to an EQ6?


The SI Tech system is a servo controller, while the EQ6 and Atlas EQ-G use stepper motors. It would require replacing the motors and intermediate gears, which would be a lot of work.

An alternative would be the Telescope Drive Master, which performs similar magic on RA tracking and periodic error, but without replacing the stock controller and motors. The downside is cost, at something like US$1700 for the controller and another $100-300 for the adapter (depending on mount). On the plus side, it could be used (in theory) on almost any telescope mount, so long as it has a guider port and can have an adapter fabricated for the encoder.

http://www.mda-telescoop.com/

It's silly, really, but I'm seriously considering one for use with my LX200 and, maybe, Atlas EQ-G. Fortunately, I have a few other purchases I want to make, first, so I can defer this one a bit.

Justin

#14 gdd

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Posted 02 May 2012 - 08:24 AM

The Telescope Drive Master (TDM) is also "closed loop"?

Gale

#15 malachykidd

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Posted 02 May 2012 - 01:43 PM

The Telescope Drive Master (TDM) is also "closed loop"?

Gale


Yes. Like the SI Tech system, it makes use of a very high resolution encoder attached to the RA shaft of the mount. Unlike the SI Tech system, it sends corrections to the mount through the guider port. According to the test report on their site, prepared by Jerry Hubbell, the TDM reduced the tracking error of an EQ6 to +/- 0.7 arc-seconds, from a nominal PE of 40 arc-seconds peak-to-peak.

Justin






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