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#1 cclark

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Posted 30 December 2012 - 03:39 PM

I am trying to decide if my mount (CGEM DX) would benefit from an aeroquest worm & ring gear replacement. My maine objective is to get a smoother PE curve that is more easily guided (a PE reduction would be nice, but I am mainly looking for smoother). I am using PECPrep to do a little analysis and could use some help understanding the data.

Many of my previous PE recordings and guide logs show a jumpy RA graph so I am staring to look at ways to smooth it out some. I have done a hyper tune and upgraded the bearings as well as adjusted the worm/ring mesh and spur gear mesh several times, but so far I have not had much of an impact on the smoothness in the graphs (other aspects of the mount have improved greatly - eas of balancing, etc. just not the RA smoothness)

I did a run last night to see what my uncorrectable 8/3 error looks like since some people report it being quite large relative to the worm error.

Can anyone comment on the graphs and let me know your opinion on whether a worm/ring upgrade would be helpful?

Fist image - PHD log data imported (my guide star was at a Dec of 0 to simplify)

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#2 cclark

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Posted 30 December 2012 - 03:40 PM

Second image - After selecting auto filter

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#3 cclark

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Posted 30 December 2012 - 03:40 PM

Third image - frequency graph

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#4 orlyandico

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Posted 30 December 2012 - 09:30 PM

Your 8/3 is the 178.

Looks like it is about 24% of the fundamental which is (pardon my language) frackin' good. On my CGEM the 8/3 is larger than the fundamental.

Looks like your total PE is about 24" which is (pardon my language again) frackin' phenomenal for a CGEM. Of that, your fundamental would be about 16" and your 8/3 about 6"

Now Ed says that Aeroquest worms are spec'ed to 5" total - but my Aeroquest is about 16" - still better than my stock worm which is 30" - but in your case I suspect the Aeroquest would not give you much improvement. But then it's only $100-ish so might as well try.

On the other hand, perhaps Ed meant 5" total for the Aeroquest worm and ring gear - which is substantially more than $100-ish.

My question is - what is your RMS guiding? if it is below 1" then there's no point trying to reduce the PE further.

My CGEM has 40" of total PE, of which 22" is due to the 8/3 and about 16" is the fundamental (they are not additive). I can get the PE down to about 25" with PEC - which is still worse than your CGEM DX's native PE.

But without PEC, I can guide at about 1.8" RMS at best, going to 2.7" at worse (easy enough - just look at the RMS value on PHD, and multiply by the pixel scale of your guider).

For reference, my AP600 with its 9" total PE, gives me 0.5" to 1" RMS guiding, and my Mach1 (with its 5" total PE) gives anywhere from 0.3" to 0.8" RMS, depending on conditions, load, balance, etc.

So unless you really want to do unguided with the CGEM DX - unlikely with an AT8RC - what really counts is your RMS guiding performance, and not the actual absolute PE. if you can consistently get sub 1" RMS then that's as good as it's going to get.



#5 orlyandico

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Posted 30 December 2012 - 09:41 PM

i must add - the Celestron PECTool is really good for this, no need for PEMPro.

what you do is, guide on a star near 0 Declination, and turn on PEC training. once the training is done, connect your PC to the mount, run PECTool, download the just-trained PEC curve, smooth it, remove any drift (both easy to do with PECTool) then upload the curve back to the mount.

The CGEM/DX only has 88 PEC cells, so the PEC curve is necessarily a bit rough. But it can still effectively reduce PE. For your mount, since the 8/3 is small, you can probably reduce your total PE below 10" with a good PEC training.

At which point.. you should be able to consistently hit sub 1" RMS guiding. An Aeroquest ring and worm will not be able to give you below about 7" to 8" (because of the un-removable 6" due to the 8/3).

#6 cclark

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Posted 30 December 2012 - 10:11 PM

Thank you! Excellent advice. On my last outing my RMS guiding was about 0.39...my guide scope is 430mm and with the SSAG (5.2 micron pixels) this gives me about 2.5 arc sec/pixel...that would mean I am at about 1" right now. This is with a PEC curve from the PECTool, so maybe I am already about at my limit. I was hoping to improve some of the jaggedness of the curve to reduce the number of quick guiding changes, but based on the feedback I will keep trying with my current setup.

Also, after uploading the graphs and looking at them some more I started thinking maybe I had the tension on the worm end float a little too tight. I had adjusted everything when it was a little warmer outside, but it was really cold on the last run ( for Texas anyway). I noticed there was little more noise than last time out so maybe things were a little tight. I remember Ed mentioning that it is common to over tighten the worm bearings, so I will back off that nut a bit and see if it smoothes out.

Great to know about what guiding tolerances to shoot for (1") as I would have been tempted to keep trying to improve past what is reasonable for this mount.

I will also capture some data while with the PEC curve on and also while guiding and compare those to the ranges to mentioned and see if they look good.

I appreciate the feedback.

Thanks,

Chris

#7 orlyandico

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Posted 30 December 2012 - 11:18 PM

hi Chris,

one thing I also noticed with my CGEM (and AP600!) is that they are very sensitive to balance. You must be slightly east-heavy, otherwise the guiding errors double. But you already knew that. :)

In many areas sub 0.5" RMS is not even possible due to the atmosphere, so I guess I got lucky with my 0.3" (which I don't get all the time, BTW). One of the other posters here (andysea) told me he gets about 0.5" with both his Mach1 and Takahashi NJP, hence my estimate of 1" being reasonable for a CGEM - this is just my estimate, not something set in stone. Ed did tell me that the 1.7" figure I was getting with my CGEM was already decent.

One more thing... the sharp jaggies on the guiding graph for the CGEM are due to the motor gearbox, i.e. nothing you can do about it.

Roland has stated that AP uses custom gearboxes (either made-to-order or in-house fabricated - I don't know..) to get around this issue. The spur gears have very fine teeth, which makes them very noisy.

That said, my AP600 is using Vexta spur gearhead steppers (I retrofitted a GoTo to it). These motors are $220 each retail (I paid much less off ebay) and are the exact same motors used on the Takahashi NJP Temma. They are far smoother (less jaggies) than the CGEM Igarashi motors (as should be expected from a motor that costs $220) but they still have more jaggies than the Mach1 motors. Maybe there's something to be said for AP's approach..

Bottom line.. if you're already getting 1" RMS - it's not going to get much better. You are limited by the gearbox now, not the worm and ring gear.

One avenue that I have not explored.. if the final transfer gears between the motor gearbox and the worm were replaced with a belt drive, it might insulate the worm from some of the sharper jaggies.

I know a good number of EQ6 / Atlas users have done this (particularly in the UK, where they even have a kit for it), and the iEQ45 has a belt drive out of the box.

Note - there is one guy here on the CN forum who reported 0.5" RMS guiding with an Atlas (!!!) which, if true, is phenomenal, that being Mach1 territory and all. But.. it also is believable, because the Atlas has a very simple reduction gearbox compared to the CGEM (it has 2 gears and an idler). Less gears in the gearbox equals less jaggies on the guiding graph.

#8 Ray Gralak

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Posted 31 December 2012 - 12:25 AM

Note - there is one guy here on the CN forum who reported 0.5" RMS guiding with an Atlas (!!!) which, if true, is phenomenal, that being Mach1 territory and all. But.. it also is believable, because the Atlas has a very simple reduction gearbox compared to the CGEM (it has 2 gears and an idler). Less gears in the gearbox equals less jaggies on the guiding graph.


RMS is not necessarily a good value to use to judge autoguiding performance. Two mounts that produce equal RMS guiding values, with otherwise equal optics, could produce quite different stellar FWHM results. RMS is best used to judge seeing because it (seeing) is nearly Gaussian in nature. Periodic error and drift are not so Gaussian in nature...

-Ray

#9 orlyandico

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Posted 31 December 2012 - 12:33 AM

actually Ray that was going to be my next suggestion :)

RMS is easy to see and I think more representative than just periodic error.

but Chris' next step should be to measure the FWHM of his stellar images from his AT8RC.

#10 Ray Gralak

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Posted 31 December 2012 - 02:09 AM


Note - there is one guy here on the CN forum who reported 0.5" RMS guiding with an Atlas (!!!) which, if true, is phenomenal, that being Mach1 territory and all. But.. it also is believable, because the Atlas has a very simple reduction gearbox compared to the CGEM (it has 2 gears and an idler). Less gears in the gearbox equals less jaggies on the guiding graph.


RMS is not necessarily a good value to use to judge autoguiding performance. Two mounts that produce equal RMS guiding values, with otherwise equal optics, could produce quite different stellar FWHM results. RMS is best used to judge seeing because it (seeing) is nearly Gaussian in nature. Periodic error and drift are not so Gaussian in nature...

-Ray


Just to give a couple examples...

Consider a mount with essentially perfect tracking (no PE nor drift). The only effect not accounted for is seeing. So, if autoguider exposures are, say, 10 seconds in duration the effects of seeing get averaged and autoguider RMS will be very small.

However, given the same mount, if instead very short autoguider exposures are used, the centroids will be all over the place compared to the 10-second exposures. Autoguiding RMS would thus be much higher for shorter exposures in this case despite identical tracking performance. In fact if you autoguide this way with this mount the average stellar FWHM would be higher with the shorter autoguider exposures because the autoguider is chasing seeing and not true mount tracking errors.

Another example... consider a mount with a lot of periodic error and drift. A long autoguider exposure would in this case cause a higher RMS error than shorter exposures because the tracking error during a long autoguider exposure is usually substantial. Thus if periodic error and drift cannot be corrected then you HAVE to use short autoguider exposures and suffer reduced FWHM because of contributions from random seeing effects.

Ideal performance is obtained when periodic error and drift are compensated for. I've taken 30-minute unguided images where total tracking error was easily less than 1". In image sets when using 10-second autoguider exposures RMS is usually about 0.05 arc-secs on good nights. If you want a minimum RMS number to shoot for that is probably close to it. And it is obtainable without too much effort on a good mount with a good PEC curve and tracking rate modeling.

-Ray Gralak

#11 orlyandico

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Posted 31 December 2012 - 03:22 AM

Thanks Ray, my comparisons of RMS autoguiding was assuming equal guide exposure lengths of 1-2 seconds, since this is a common guide exposure length. I haven't tried 10-second long guide exposures, I think those would be required if using an OAG.

What I have seen with the CGEM and short-ish (1-2 second guide exposure lengths) is that what I thought was seeing (rapid changes in the guiding graph) were actually gear noise. And this gear noise is over 2" p-p even with guiding enabled. I believe that's much larger than would be accounted for by seeing alone..

#12 Ray Gralak

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Posted 31 December 2012 - 10:08 AM

What I have seen with the CGEM and short-ish (1-2 second guide exposure lengths) is that what I thought was seeing (rapid changes in the guiding graph) were actually gear noise. And this gear noise is over 2" p-p even with guiding enabled. I believe that's much larger than would be accounted for by seeing alone..

Hi Orly,

That's possible but because there is Gaussian noise (from seeing) added to whatever tracking errors there are it's difficult to be sure of the actual magnitudes of gear noises.

Plus, when autoguiding you cannot always count on the mount having moved the actual distance it was commanded. If the mount has a native move command with the time (or distance) to move built into the command it is likely more predictable than a mount that has to use timed start/stop responses for moves. That's because Windows is not a real time operating system so there can be variations in the actual start/stop interval, not to mention that the mount might not process the separate start/stop commands predictably. If a time-to-move value can be passed and executed on the mount, unless there is a firmware bug, the mount will be able to move the commanded distance much more precisely.

So, in a series of autoguider moves, if a autoguider command moved the wrong distance there can be an unexpectedly large distance required for the next move, but it is not because of gear noise. A similar situation can happen in two autoguider cycles if seeing causes the centroid to be at one RA extreme in one cycle and at the other extreme in the next cycle (that could be easily 2"). That's why it's a tremendous advantage having a mount accurate enough to do long autoguider exposures to average seeing. This is why people with low-periodic error, high-end mounts still train their mount's PEC and precisely polar align and sometimes run tracking rate correction software. They are doing that to obtain long-exposure autoguided (or unguided) tracking.

Every time an autoguider move is made the image has already been damaged or is going to be damaged because of the move. Thus, you can see that the real goal is (or should be) to perfect tracking to the extent that it minimize the number and magnitude of autoguider moves.

-Ray Gralak

#13 Starhawk

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Posted 31 December 2012 - 11:00 AM

Ray, since you are reading this thread, I have a related question on operating system effects:

Does tracking vary between PC guidance and all-in-one auto guiders like the Celestron NexGuide Autoguider? I'm wondering about how real time processing combines with these devices compared to the much greater processing power of a computer for tracking performance versus PEC correction quality.

-Rich

#14 Ray Gralak

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Posted 31 December 2012 - 12:17 PM

Ray, since you are reading this thread, I have a related question on operating system effects:

Does tracking vary between PC guidance and all-in-one auto guiders like the Celestron NexGuide Autoguider? I'm wondering about how real time processing combines with these devices compared to the much greater processing power of a computer for tracking performance versus PEC correction quality.

-Rich

Hi Rich,

Yes, absolutely there can and probably will be differences, but probably not what you think. Calculating a centroid is a relative easy operation so even a small microprocessor can do it in a timely matter. The computational power of a computer is usually far more than is needed but other operations in the computer can mess up the timing of start/stop moves in some cases compared to a dedicated microprocessor in an all-in-one autoguider. That said, using a computer can allow you to more easily try different autoguider methods.

In any case I believe it is always beneficial to program PEC because in most cases it can preemptively handle periodic error so that the autoguider does not have to after the tracking error has occurred.

BTW, using an autoguider to program PEC has pitfalls. Some PEC utilities do that, including Celestron's PECTool and even a recent version of EQMOD. However, I think it is a mistake to use this method because you cannot count on the autoguider movements being accurate in all cases and the autoguider introduces a phase shift that must be accounted for. A good PEC curve fit of a passive capture is always going to be more accurate and give better results.

-Ray Gralak

#15 Denimsky

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Posted 31 December 2012 - 01:33 PM

Hi Ray,

Thank you for your insightful information.
Could you explain about tracking rate adjustment software?

I haven't heard of this type of software and I'm wondering about when we need them and how they work.
What examples of software are tracking rate adjustment software?

Thank you.

#16 orlyandico

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Posted 31 December 2012 - 01:33 PM

hi Ray,

you are absolutely correct about the phase shift introduced by an AG.

however we're talking about a CGEM here.. with its 88 PEC cells. really, one could train the PEC with the AG, download the curve from the mount, smooth and otherwise massage it, even edit it with MS Excel (it can be saved as a CSV file) and then re-upload it.

I am not dissing PEMPro - far from it, I recognize its the best out there - but for a CGEM with its 88 PEC cells and large 8/3, I think PEMPro is overkill. Things like the centroid calculation shifting etc. and fast gear noises can't be corrected by the CGEM PEC anyway - 88 cells over 479 seconds means a guide correction is only being applied once every 5.44 seconds.

so the cheapest and I suspect fairly effective way to train a CGEM's PEC is to use a long slow scope, and use a 5-second guider exposure.

that said... how many PEC cells does the GTO CP3 have? I have not had success with PEMPro in the past.. the CGEM was.. well, the 8/3 on mine is huge; my AP600 both in its original QMD guise and after I replaced the controller with a Littlefoot, can only be programmed via the ST-4 interface, and I could never get a good PEC training. I finally am at the point where I have a mount that can really benefit from PEMPro... something I'll have to look into because I ruined the built-in PEM training.

#17 Alph

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Posted 31 December 2012 - 03:14 PM

RMS is not necessarily a good value to use to judge autoguiding performance. Two mounts that produce equal RMS guiding values, with otherwise equal optics, could produce quite different stellar FWHM results. RMS is best used to judge seeing because it (seeing) is nearly Gaussian in nature. Periodic error and drift are not so Gaussian in nature...

-Ray



However, when sampling durations and intervals are kept the same and RMS is computed over the same period of time then RMS describes/quantifies guiding performance quite objectively over a given period of time.

RMS computations don’t have to be limited to random normal (Gaussian) variables to be useful or meaningful. Actually PHD computes RMS on a random variable that does not follow the Gaussian distribution. If x and y are normal random variables then x^2+y^2 is chi-square random variable. BTW TPoint computes RMS exactly in the same manner and I suspect your software is no different in this regard. All that can become quickly quite academic and in practice does not matter much as long as RMS is sampled and computed in a consistent manner.

To the OP. My endeavor with Aeroquest gears turned out to be a total disaster. Proceed with caution.

#18 cclark

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Posted 31 December 2012 - 05:11 PM

actually Ray that was going to be my next suggestion :)

RMS is easy to see and I think more representative than just periodic error.

but Chris' next step should be to measure the FWHM of his stellar images from his AT8RC.


Ok, I looked at some of my images from my last session with the RC and ran them through the PSFEstimator script in PixInsight. I took 5 raw images and compared the readings and they were all similar to the results below:

Median FWHM (px) 5.21
MedDev FWHM (px) 1.43
Median FWHM major axis (px) 5.49
MedDev FWHM major axis (px) 1.53
Median FWHM minor axis (px) 4.96
MedDev FWHM minor axis (px) 1.36
Median aspect ratio 0.9
MedDev aspect ratio 0.03
Median azimuth (°) 166
MedDev azimuth (°) 10
Median MAD 1.18E-02
MedDev MAD 6.33E-03
Median 1.84E-02
MedDev 5.08E-04
MRS noise 4.65E-04
SNR weight 1.19E+00

with a FWHM of 5.21px and an arc sec/pixel of .67, it looks like I am at a FWHM of about 3.5". Right now I am guiding every 0.5 sec to try and keep up. I do keep an eye on the dec movement to see if I am chasing the seeing, but usually the RA jumps are much larger than the random movement in the dec axis (observed while doing a drift align - RA tracking & no Dec corrections).

I am interested to see if I am interpreting this correctly. It is starting to look like I am running up against the gearbox errors, but I am still reviewing all of the comments to see if there are other things I can do to improve.

Thanks,

Chris

#19 Ray Gralak

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Posted 31 December 2012 - 06:36 PM

RMS is not necessarily a good value to use to judge autoguiding performance. Two mounts that produce equal RMS guiding values, with otherwise equal optics, could produce quite different stellar FWHM results. RMS is best used to judge seeing because it (seeing) is nearly Gaussian in nature. Periodic error and drift are not so Gaussian in nature...

-Ray



However, when sampling durations and intervals are kept the same and RMS is computed over the same period of time then RMS describes/quantifies guiding performance quite objectively over a given period of time.

Not necessarily true because guiding performance cannot be separated from scintillation. For instance if you have a perfect tracking mount the measured RMS error is different in very good seeing versus very poor seeing, yet the optimal guiding for such a mount is to make NO corrections in either case (because the mount is tracking perfectly). You cannot judge guiding entirely from the RMS number.

-Ray

#20 korborh

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Posted 31 December 2012 - 08:14 PM

Not necessarily true because guiding performance cannot be separated from scintillation.


Scintillation would be random while guiding errors due to gears/worm would me mostly periodic. So it seems one should be able to separate them, no?

You cannot judge guiding entirely from the RMS number.

-Ray


I agree with this (for my mount) based on my own observations of the tracking graph vs. FWHM in resulting images.

#21 orlyandico

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Posted 01 January 2013 - 12:17 AM

hi Chris,

Frank (freestar8n) claims "sub 2" FWHM" achievable with Metaguide and an OAG, on a mid-range mount (CGE, CGE Pro).

You might want to try using MG for guiding instead of PHD and see where that gets you... MG "supposedly" can track fast errors better as it uses video. Frank also mentions that it can pre-emptively correct periodic errors if you input the expected cycle of the PE (e.g. the 8/3).

I hope Frank can weigh in here with his input as well..

#22 Ray Gralak

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Posted 01 January 2013 - 10:09 AM

Not necessarily true because guiding performance cannot be separated from scintillation.


Scintillation would be random while guiding errors due to gears/worm would me mostly periodic. So it seems one should be able to separate them, no?


No you really can't. My example of the perfect tracking mount with different RMS errors under different seeing conditions I think supports this point.

Another example... in lower end mounts there can be a lot of "gear noise" that Orly mentioned earlier. This noise is random so it cannot always be distinguished from seeing anomalies. Some mounts, the the Atlas employ stepper motors which produce measurable vibrations visible in an FFT spectrum if you use a webcam so that you can sample the star fast enough. The stepper frequency is similar to the 1 second autoguider exposures commonly used so (because of the Nyquist sampling theorem) the vibrations cannot be isolated with typical 1-second exposures.


You cannot judge guiding entirely from the RMS number.

-Ray


I agree with this (for my mount) based on my own observations of the tracking graph vs. FWHM in resulting images.


I believe you have to do more than that even. FWHM measurements can be affected by many things including seeing conditions, telescope aperture and quality, focus, brightness of the star, which camera you use, which software you measure with, duration of the exposure, vibrations, etc. I think to measure guiding quality you need to take several very short exposures (or measure many stars in a single short exposure) to get an average "short-exposure FWHM". This short-exposure FWHM is the baseline against which you can measure autoguider performance by comparing it to the average FWHM you get in a longer duration autoguided exposure. How little the FWHM increases I think would be the most accurate determination of autoguiding quality. But even in this case seeing can be an unknown factor so it's important to average many stars.

-Ray Gralak

#23 orlyandico

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Posted 01 January 2013 - 10:16 AM

i think Maxim can guide from multiple stars?

or is it some other software?

p.s. Ray i finally paid for PEMPro :D (based on the assumption that I now have a mount that deserves it...) to my dismay the uncorrected PE according to PEMPro is 2.33" peak-to-peak (across 3 cycles). I'm not so sure if that can be improved... uploading the curve takes a LONG time..

#24 Ray Gralak

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Posted 01 January 2013 - 10:32 AM

hi Ray,

you are absolutely correct about the phase shift introduced by an AG.

however we're talking about a CGEM here.. with its 88 PEC cells. really, one could train the PEC with the AG, download the curve from the mount, smooth and otherwise massage it, even edit it with MS Excel (it can be saved as a CSV file) and then re-upload it.

I am not dissing PEMPro - far from it, I recognize its the best out there - but for a CGEM with its 88 PEC cells and large 8/3, I think PEMPro is overkill. Things like the centroid calculation shifting etc. and fast gear noises can't be corrected by the CGEM PEC anyway - 88 cells over 479 seconds means a guide correction is only being applied once every 5.44 seconds.

so the cheapest and I suspect fairly effective way to train a CGEM's PEC is to use a long slow scope, and use a 5-second guider exposure.

that said... how many PEC cells does the GTO CP3 have? I have not had success with PEMPro in the past.. the CGEM was.. well, the 8/3 on mine is huge; my AP600 both in its original QMD guise and after I replaced the controller with a Littlefoot, can only be programmed via the ST-4 interface, and I could never get a good PEC training. I finally am at the point where I have a mount that can really benefit from PEMPro... something I'll have to look into because I ruined the built-in PEM training.

I wasn't trying to bring PEMPro into the discussion. My comments were from research and actual measurements from many mounts over the years.

That said, I think there are a couple problems that I can see with your arguments. First, an 88-cell PEC table can be affected just as much as a 970-cell table (most AP Mounts) by phase shifting. You still want to make sure the phase is centered correctly. In fact it's probably more important in an 88-cell PEC table because of the potential of a longer "wrong move" that a cell might cause.

But here's the problem with your argument of training the CGEM with an autoguider. As you know the problem with the 8/3 fundamental is that in each cycle it does not repeat in phase. So, just recording PEC to the mount with an AG does not work (well, the PEC curve works in one out of every three worm cycles). Simple averaging of 3 cycles does not work either. You cannot remove the 8/3 fundamental with averaging. If you are doing that it's no wonder PEC isn't working for your CGEM.

But you can use PEMPro to create a PEC curve that excludes only the problematic 8/3 frequency and upload it to the CGEM. Of course the uncorrectable 8/3 frequency will still exist but any correctable periodic error should be removed.

-Ray Gralak

#25 orlyandico

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Posted 01 January 2013 - 10:38 AM

hi Ray,
that's right, but the thing is PECTool allows to export the curve as CSV. So you can massage it, do curve-fitting, etc. in Excel. Strictly speaking this is outside the scope of PECPrep of course... but most of what you can do in PEMPro can also be done with MS Excel and PECTool...

That said, both PEMPro (trial) and the AG with PECTool method worked. As I mentioned earlier I got my 40" PE down to 25". PEMPro and PECTool both performed at about the same level. Since my CGEM's 8/3 is about 20" to 22" its no wonder that the PE couldn't go lower than 25".

The OP seems to have a good CGEM though. PEC training would work very well for him... I still think PECTool is "sufficient" for his needs. Of course PEMPro would be better.






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