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Periodic Error Correction

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#1 alancorey1979

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Posted 26 July 2021 - 11:48 AM

I am soon to be the proud owner of a new Sky Watcher EQ6-R Pro mount... I got my bi-annual bonus at work and decided this year instead of spending it on stupid stuff like bills and food, I am finally going to by this mount that I have been wanting for 5 years. However, it is unfortunately on back order so I am taking the time to really dig into the weeds on how to operate it and whatnot and a question I have for the seasoned astrophotographers pertains to periodic error correction.

 

From my understanding, PEC is the correction of the predictable errors found in worm drive gears but the mount needs a reference point and a calibration point for the PEC to be effective. However, I cannot find any information on how often that calibration needs to occur. Do I need to recalibrate it every time I set my telescope up or is the calibration stored permanently? Polar alignment is never perfect so I am wondering how one night's alignment that could be very good vs a following night's alignment that is ok but not great impacts that calibration. Also, I read that in order to get an accurate calibration, the worm drive needs to make one complete rotation... does that mean one complete rotation of the mount which is way more than one rotation of the gear, or is one rotation of the gear which is way less than one rotation of the mount?

 

Or am I completely over thinking all of this?



#2 DuncanM

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Posted 26 July 2021 - 12:04 PM

I am soon to be the proud owner of a new Sky Watcher EQ6-R Pro mount... I got my bi-annual bonus at work and decided this year instead of spending it on stupid stuff like bills and food, I am finally going to by this mount that I have been wanting for 5 years. However, it is unfortunately on back order so I am taking the time to really dig into the weeds on how to operate it and whatnot and a question I have for the seasoned astrophotographers pertains to periodic error correction.

 

From my understanding, PEC is the correction of the predictable errors found in worm drive gears but the mount needs a reference point and a calibration point for the PEC to be effective. However, I cannot find any information on how often that calibration needs to occur. Do I need to recalibrate it every time I set my telescope up or is the calibration stored permanently? Polar alignment is never perfect so I am wondering how one night's alignment that could be very good vs a following night's alignment that is ok but not great impacts that calibration. Also, I read that in order to get an accurate calibration, the worm drive needs to make one complete rotation... does that mean one complete rotation of the mount which is way more than one rotation of the gear, or is one rotation of the gear which is way less than one rotation of the mount?

 

Or am I completely over thinking all of this?

EQ6-R worm period is 8 minutes (one rotation of the worm, not the worm gear). There is a sensor that records the start of each 8 minute cycle so that it can be reliably timed and PEC can start at the correct instant. Once trained, the mount will retain the PEC indefinitely. 

 

To train the PEC circuit you will need to upload a smoothed PE curve to the mount and that will require software like EQMOD or PEMPro. A smoothed curve has the effects of atmospheric turbulence and poor polar alignment removed from the recording before uploading to the mount's internal firmware. 


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#3 StarBurger

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Posted 26 July 2021 - 12:08 PM

Very good question. If the wheel was consistent error-wise around it's circumference then I guess that just performing PEC for one rotation of the worm would suffice.

As I read the instructions that came with my AVX mount for PEC my mind turned to jello and dribbled out my ears. 

Do I need to perform PEC at every session? No way is the worm going to be at the same known position against the wheel.

If the balance changes? At different attitudes in RA and DEC? Hot and cold?

These questions are what convinced me to go to auto guiding to avoid the dark arts of PEC.



#4 rgsalinger

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Posted 26 July 2021 - 12:50 PM

PEC and guiding are complementary. As an example, I recently acquired a mount which had a slightly out of spec worm. By using PEC, I was able to get the guiding RMS down to well under .5 seconds. Without PEC it clocked in much higher. PEC operates at a much higher cadence than guiding, which is why (I think) it's complementary to guiding. 

 

One worm rotation does not suffice because the observations will be affected by seeing, wind, etc. So, the idea is (I use PEMPRO and the built in routine in the SKYX) is to capture 3 or 4 cycles and average them out. . Then you store the computed result into the mount and you're good to go.

 

Mounts need to have some index built into them so that the corrections can be syncronized with the worm rotation angle. 

 

Normally, you run PEC once and just leave it alone. I would, of course, check periodically that it was actually improving my guiding for any number of reasons. In fact, PEC cannot help you track any better than the seeing itself will allow. 

 

Rgrds-Ross


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#5 andysea

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Posted 26 July 2021 - 01:42 PM

Does that mount have encoders now? I remember the older version did not have encoders but it relied on the steppers for positioning. Same end result, different technology.

#6 alancorey1979

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Posted 26 July 2021 - 05:14 PM

Does that mount have encoders now? I remember the older version did not have encoders but it relied on the steppers for positioning. Same end result, different technology.

yes sir. One set of encoders. Thanks for asking that question.... it reminded me to ask the sales rep at Highpoint Scientific that specific question.



#7 DuncanM

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Posted 26 July 2021 - 05:43 PM

yes sir. One set of encoders. Thanks for asking that question.... it reminded me to ask the sales rep at Highpoint Scientific that specific question.

The EQ6-R doesn't have encoders. It uses stepper motors with the motor controller counting the steps to determine mount position. There is a worm position sensor to track the beginning of each 8min worm cycle.



#8 alancorey1979

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Posted 26 July 2021 - 06:21 PM

The EQ6-R doesn't have encoders. It uses stepper motors with the motor controller counting the steps to determine mount position. There is a worm position sensor to track the beginning of each 8min worm cycle.

I ordered the EQ6-R Pro. Highpoint Scientific stated specifically that it has one set of encoders.



#9 OzAndrewJ

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Posted 26 July 2021 - 06:23 PM

Gday Ross

PEC and guiding are complementary.

One caveat here with the Synta mounts.

I have an AZEQ5 and couldnt get it to work properly, so did a lot of testing.

If "mount" PEC is ON, then sending a simple speed adjust will get over ridden if a PEC bin changes mid guide,

thus resulting in slightly truncated guides.

To get around this, the handbox code was changed to turn PEC OFF, do the guide then turn PEC ON.

EQMod doesnt do this but GreenSwamp does, so if you use EQMod,

it can affect guiding ( esp calibrating in PHD if the system uses long pulses )

The EQ6R uses 100 PPEC bins, so the problem is not as bad as for the AZEQ5 ( which has 1200 PPEC bins )

Sooooooo, If you use PHD + EQMod and see dodgy calibrates,

just turn PPEC OFF, then Calibrate, then turn PPEC ON.

 

Andrew Johansen Melbourne Australia



#10 DuncanM

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Posted 26 July 2021 - 06:37 PM

I ordered the EQ6-R Pro. Highpoint Scientific stated specifically that it has one set of encoders.

I own one and it doesn't have encoders. Skywatcher's current specs and owner's manual don't mention encoders:

 

https://www.skywatch...ducts/eq6-r-pro


Edited by DuncanM, 26 July 2021 - 06:42 PM.



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