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Proportional Permanent Periodic Error Corrector

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#1 rice224534

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Posted 19 January 2014 - 08:48 AM

I found interesting upgrade item for LX200ACF in ebay.com.

The Live Oak Imaging proportional periodic error correction for Meade LX200-ACF Telescope
http://www.ebay.com/...rector-for-M...

They say it eliminates periodic error in RA gear drive.
I goolged this item but I could not find any information.

Anyone know this?

#2 Markigno

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Posted 20 January 2014 - 07:44 AM

Sounds interesting, but this system does not use a high-precision absolute encoder like for example the TDM. Reading the instructions, this device seem to work as a PEC external module memory that slows or accelerates the RA motor to correct the periodic error. The system needs to learn, so you have to perform a manual PEC using the knob on the hand control module, trying to keep a reference star centered for a full cycle (or even more cycles, up to a maximum of 8). As far as I can understand, PemPro can do this automatically and very well, at a much lower price. In addition, with PemPro, you should not act on the telescope with welding etc.
Marco

#3 Christopher Erickson

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Posted 20 January 2014 - 01:45 PM

Pretty-expensive and I am not convinced of its value. If you slow down or speed up the pulses to the motor, I think the mount will see the changes in its own optical encoder pulse count from that motor and will attempt to compensate to cancel it out.

I think using the mount's built-in PEC function has a much-better chance of being useful.

#4 OzAndrewJ

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Posted 20 January 2014 - 03:30 PM

Gday Chris

I think the mount will see the changes in its own optical encoder pulse count from that motor and will attempt to compensate to cancel it out.



Not sure there, as in Polar, the scope just sends a command to track at a certain rate.
The motor card then handles the feedback loop to achieve this.
This "gizmo" doesnt appear to interfere with the board/CPU comms.
If i read it correctly, it appears to involve removing the Xtal from the motorcard, and supplying its own clocking pulses, hence is tweaking the PWM etc to the motor by adjusting the clocking rate of the PIC to control speed????
It wont help with the three turn PEC if the Gbx PE is large relative to worm PE.
They also say it cant be used in AltAz, but not sure if that means PE wise or it means your scope becomes polar only.

Still, an interesting way to skin a cat.

Andrew Johansen Melbourne Australia

#5 Christopher Erickson

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Posted 20 January 2014 - 07:27 PM

Gday Chris

I think the mount will see the changes in its own optical encoder pulse count from that motor and will attempt to compensate to cancel it out.



Not sure there, as in Polar, the scope just sends a command to track at a certain rate.
The motor card then handles the feedback loop to achieve this.
This "gizmo" doesnt appear to interfere with the board/CPU comms.
If i read it correctly, it appears to involve removing the Xtal from the motorcard, and supplying its own clocking pulses, hence is tweaking the PWM etc to the motor by adjusting the clocking rate of the PIC to control speed????
It wont help with the three turn PEC if the Gbx PE is large relative to worm PE.
They also say it cant be used in AltAz, but not sure if that means PE wise or it means your scope becomes polar only.

Still, an interesting way to skin a cat.

Andrew Johansen Melbourne Australia


I could be wrong but it looks like it gets inserted between the motor's DC power feed and the motor control board. Hence my speculation that the motor controller board would try to cancel it out.

Replacing the motor controller board CPU's crystal does make a bit more sense in that respect.

However I am still not sure about its superiority over Meade's built-in PEC function.

#6 OzAndrewJ

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Posted 20 January 2014 - 08:17 PM

Gday Chris

I could be wrong but it looks like it gets inserted between the motor's DC power feed and the motor control board.



You appear to be wrong :grin:
If you read the text, the only thing removed is the Xtal.
The power wires from motor card to motor do not get touched.

What i suspect is the new encoder is used to get an absolute position for the worm.
When you train, the system uses a std mechanism to log PE for playback,
and this model is stored in the new chippies.
The new card now becomes a "dynamic Xtal", ie it provides an adjustable pwm that now becomes the master "external" clocking for the motor card PICs, based on encoder position.
Thus, all standard functions stay preserved, but the timings ( ie motor speed ) can be tweaked via the clocking rate.
I cant think of any other "on card" functions for the motor cards that would be affected by these minor clocking effects.

However I am still not sure about its superiority over Meade's built-in PEC function



It may work ( vs Meades mechanism )????
Recently, i have discovered that the std Meade functionality when tracking in polar can result in the encoders only being read every three seconds ( yes 3 SECONDS ).
This data is used to determine bin start and stop times, and hence can really screw up recording and playback. Even if something like PEMPro is used to generate the model, bin timings for playback can vary quite a bit.
Its one of the reasons i gave up trying to fix the PEC in the LX200s, as i cant see how to force the encoders to be read at a faster rate.

Andrew Johansen Melbourne Australia

#7 rice224534

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Posted 21 January 2014 - 07:28 AM

Thanks for replies. I thought this was poorman's TDM with a high quality encoder but I realized I was wrong. :grin:
But such a upgrade item is very attractive apart from its price. :grin:

#8 Live Oak Imaging

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Posted 27 January 2014 - 08:10 PM

Hello, I am the owner of Live Oak Imaging and the designer of the drive corrector. First of all, I think you could have been better served if you had started the thread under "Mounts". There is much discussion there about these things.

The kit basically replaces the 20MHz clock reference with a variable time reference. The variable time reference is basically a PWM controlling the time that a 18 MHz crystal oscillator and a 22 MHz oscillator are allowed to clock the motor control board. The method emulates a variable crystal with a range of + / - 10%.

The encoder measures the absolute position of the worm. The sensor is read 10 times per second. The sensor has a resolution of 0.35° at the worm and this resolution is multiplied by 180 at the worm gear.

There is much discussion at the "Mount" forum on native periodic error, better worms, polishing worms, etc.

The drive corrector essentially corrects for machining tolerances in the worm. The native periodic error is improved. Gearbox error is not improved. Gearbox periodic errors are generally small compared to worm errors in the LX200.

I'd like to describe just 1 example of a guiding issue with an autoguider when used with conventional PEC. Due to atmospheric lensing, the mount is moving too slow. PEC happens to be speeding up the mount at the same time the autoguider needs to speed up the mount. Since PEC has already signaled the 2X speed, the guide pulse is missed. This is called a collision. Thanks, Dennis

#9 Christopher Erickson

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Posted 27 January 2014 - 08:25 PM

Hello, I am the owner of Live Oak Imaging and the designer of the drive corrector. First of all, I think you could have been better served if you had started the thread under "Mounts". There is much discussion there about these things.

The kit basically replaces the 20MHz clock reference with a variable time reference. The variable time reference is basically a PWM controlling the time that a 18 MHz crystal oscillator and a 22 MHz oscillator are allowed to clock the motor control board. The method emulates a variable crystal with a range of + / - 10%.

The encoder measures the absolute position of the worm. The sensor is read 10 times per second. The sensor has a resolution of 0.35° at the worm and this resolution is multiplied by 180 at the worm gear.

There is much discussion at the "Mount" forum on native periodic error, better worms, polishing worms, etc.

The drive corrector essentially corrects for machining tolerances in the worm. The native periodic error is improved. Gearbox error is not improved. Gearbox periodic errors are generally small compared to worm errors in the LX200.

I'd like to describe just 1 example of a guiding issue with an autoguider when used with conventional PEC. Due to atmospheric lensing, the mount is moving too slow. PEC happens to be speeding up the mount at the same time the autoguider needs to speed up the mount. Since PEC has already signaled the 2X speed, the guide pulse is missed. This is called a collision. Thanks, Dennis


Your product, being specifically-discussed here and intended for a Meade LX200GPS mount, makes this the appropriate forum.

Welcome to Cloudy Nights!

Please take a moment to review the forum rules for vendors, just to keep you out of any trouble going forward.

http://www.cloudynig...p/Number/233117

And thank you for joining Cloudy Nights and being willing to discuss your products!

#10 OzAndrewJ

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Posted 27 January 2014 - 10:35 PM

Gday Mr Live Oak

Nice to hear feedback from the source.

Gearbox periodic errors are generally small compared to worm errors in the LX200.



Not always :cool:
hence why Meade went to the trouble of introducing 3 turn PEC
( which has its own other problems )
I have seen many examples of where the Gbx PE is very large relative to the worm PE, but then tooth spacing errors on the wormwheel also cut in and make a mess of most 3 turn models over time.

when used with conventional PEC. Due to atmospheric lensing, the mount is moving too slow. PEC happens to be speeding up the mount at the same time the autoguider needs to speed up the mount. Since PEC has already signaled the 2X speed, the guide pulse is missed. This is called a collision.



Not sure i understand what you are saying here. In the Meades, when PEC is applied, it doesnt use 2X speed, it uses a modified base tracking rate for a 2.4second slice of time ( one pec bin ). This rate allows for speed adjusts in increments of just under 1% of sidereal .
Any external "guide" requests that arrive merely get queued and then handled when possible ( other than DEC which is a descent to madness to understand what happens ). Ie there are no "collisions" and no missed guide commands in RA.

Andrew Johansen Melbourne Australia

#11 rice224534

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Posted 28 January 2014 - 06:40 PM

Thanks for explaning your product. :D

Although I didn't understand about electric details, I would like to know its efficiency.
If I replace with this upgrade kit, how much PE in RA gear is improved? :question:
Generally, PE for LX200 is said to be 30-60sec without original PEC-on.

#12 Live Oak Imaging

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Posted 28 January 2014 - 09:00 PM

I would like to post some before and after pictures taken under the same conditions. I like to slightly misalign the polar alignment and take a 3 worm cycle time exposure. With known focal length and pixel pitch, measuring and comparing the errors is straight forward from the images. The product was originally developed for my LX50, I have some before and after images with the LX50. I typically use the LX50 as the guide scope and mount for my FSQ106ED which I use for imaging. The LX50 pictures were taken when I only had the ability to record 1 worm cycle. I'm new to this forum, I'm not sure if I can post the large file size pictures here. I might put them onto a website and post a link. I can also email them. It would actually be better to wait for the LX200 ACF pictures with at least 5 cycles of the worm recorded and averaged. I will try my best to have these results this weekend.

#13 Live Oak Imaging

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Posted 06 February 2014 - 09:34 PM

I have not been able to obtain the PE data due to bad weather. I should have an opportunity this weekend or early next week. I have not forgotten and I will post the data as soon as it is available.






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