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INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

How does PEC/PPEC & GPG work to help RA guiding

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Hi Rene, I have spend some time with research about PPEC. The fact is, that it is not usable in Ekos if you also want to use guiding. Those two functions ara fighting against each other. But if I summarize it, how to use PPEC:
1) You will need to record correction curve. For this, you will need to switch from INDI to ASCOM, because INDI driver for EQDir is simple, and it doesn't have this capability.
2) Once your curve is recorded, you can smooth it in PECPrep. This step is optional, but I would recommend to do it.
3) You will need to replay the (smoothed) curve from EQDir under ASCOM. At the same time, you will need to learn the mount the curve. This can be done in Ekos, with one of those two buttons in PPEC tab.
4) Finally, you can enable PPEC during your imaging session. RA periodic error will be automatically corrected by the mount itself, as the correction curve is now recorded in its EEPROM.
As I said, it is way too complicated, and GPG for this purpose is definitely better solution, as it is able to cooperate with guiding.
3 years 2 months ago #66091

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Thanks for that. So are you saying I should use GPG in Ekos instead of PEC/Prep/PemPro? So what is the periodic error tab in the indi control panel used for. if anything?
3 years 2 months ago #66212

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The tab could be used for two things:
1) Start programming PPEC into the mount
2) Enable PPEC once it is programmed in the mount
Once you click on one of these two options in that tab, it will simply turn ON (or OFF) a flag in the mount's electronic. Nothing more, nothing less.
Last edit: 3 years 2 months ago by hades.
3 years 2 months ago #66219

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So if I use GPG during my guiding, does it retain the adjustments from session to session or does it have to recalculate? I’m hoping it retains the information so I don’t have to waste time each imaging session.

Rene
3 years 2 months ago #66293

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Unfortunately it doesn't store that information, and needs to start from scratch each time. That's why I recommend at least saving the mount period.

When I introduced this to KStars, I asked GPG's developer, Edgar Klenske, who's developed this as part of his PhD, the same question. He said that was possible but never done, and not a trivial task.
I think this is unlikely to happen in the near term.
3 years 2 months ago #66295

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Thanks Hy,

Silly question but how do I save the mount period? :)

Rene
3 years 2 months ago #66296

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I think that GPG could be saved for next session. But you need to have a mount with encoders or at least with an index on the RA axis, e.g.: SW EQ6-R. Because the algorithm needs to know at which RA possition is the mount. This is how it works for PPEC.
3 years 2 months ago #66302

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If you go to the GPG Options page (described above) and enter the mount period in "Major Period", and click apply, then KStars should remember it.
If "Estimate Period" is not checked, then the period should not be updated by GPG.
As I said, either look up the period if it's on the PHD2 Wiki page above, or keep a good estimate from a successful KStars GPG session that lasted
a long time, e.g. over an hour since the last slew or other GPG reset.

3 years 2 months ago #66305
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I have a question about the GPG period in seconds. I noticed that it must be an integer. If the period of my mount is 239.5 seconds, then after an hour (15 periods of 240 seconds) it will be 15*0.5=7.5 seconds off a multiple of 240. Does that mean that my GPG will then be out of sync by 7.5 seconds if I enter 240 as the period, and don't click the "estimate period" checkbox (as recommended)? If it does, can the integer period be changed to a floating-point number?
1 year 7 months ago #85520

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Henk,

The period was changed to a float input, I believe in 3.5.8
invent.kde.org/education/kstars/-/merge_requests/542

Hy
The following user(s) said Thank You: Henk Aling
1 year 7 months ago #85522

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One last question: How is a linear trend accounted for? IMHO it should be considered separately as there will always be some trend due to polar misalignment, atmospheric refraction, clutch slippage, etcetera. A trend is a non-periodic signal so when not properly accounted for, it may to additional errors rather than reduced errors.
1 year 7 months ago #85688

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Henk,

Check out Klenske's thesis here:
www.research-collection.ethz.ch/bitstrea...quence=2&isAllowed=y

In particular, look at chapter 6, "Software Implementation: PHD2 Guiding". We use the same GPG source library that PHD2 does, which was written by Klenske et al.
invent.kde.org/education/kstars/-/tree/m..._IS_gaussian_process

In particular, he does mention how there's both periodic error correction as well as correction for drifts built into GPG. Also, of course, there is standard control built in to the guider to account for other types of "random error".

Hy
1 year 7 months ago #85707

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