High RA error causing oblong stars

[Max]

Hallo everyone,

after several nights of trying around, I can't figure out where my issue is.

My Gear:

Skywatcher EQ6-R Pro

TS 130mm refractor, 60/240 guidescope with ASI 290 Mini guidecam.

I have 2.49" per pixel in the guidecam and my main cam has 0.73" per pixel. So my imaging / guiding ratio is 1:3.43.

Total load on the mount is 13kg.

I'm doing polar alignment using Ekos and it should be spot on. I usually even run it twice to make sure it is solid.

I have quite low DEC error of 0.56" (in the attached log, sometimes even lower)

However, I have strong RA error. Yesterday was particularly bad, I had stable <1" RMS in a previous session. This time I was far from it.

I ran the guiding assistant:

and set the parameters. What was odd, is that the polar alignment error displayed seemed somewhat high. If I didn't mess up PA yesterday, it should be arc-seconds not minutes.

I tried strategically unbalancing the mount against the RA direction, but it didn't seem to make much difference.

Today seems to be another clear night so I can get another experiment done. Can anyone help me with an idea on how to get this figured out?

Thanks

Max

[Max]

Is it possible that my RA algorithm for some reason being set to "Hysteresis" is an issue? I read elsewhere it should be set to Predictive PEC, which would make much more sense.

[bw_msgboard]

Hi Max.  I think your basic problem is that you’ve crippled PHD2’s ability to manage the periodic error in the mount.  You did that by horsing around with all the guiding parameters, especially setting the RA aggressiveness to 40%.  I think you made the beginner mistake of not distinguishing because cause and effect.  The RA is oscillating because of the tracking error in the mount, not because of the guiding.  You should reset all the guiding parameters to their default values and leave them alone.  Then you can get a high-quality assessment of your system by following this procedure:

 

https://www.dropbox.com/s/os1thorvswmzaul/How%20to%20create%20a%20baseline%20for%20guiding%20results%20using%20OpenPHD2.pdf?dl=0  

 

There’s no need to rebuild the profile, just be sure all the guiding parameters are at their default.  The polar alignment error is irrelevant, a few arc-minutes of error will have no effect on what you’re doing.  Once you’ve done all this, you can then try using the PPEC algorithm, setting the period to the native worm period for your mount.  You should also try to keep your guide exposures down to 1-2 seconds to help manage the periodic error – you can typically get away with those shorter exposures with multi-star guiding enabled.

 

Good luck,

Bruce

 

 

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[steve]

Sorry Max. forgot to post to the group..

-------- Forwarded Message --------

Subject:	Re: [open-phd-guiding] Re: High RA error causing oblong stars
Date:	Sun, 14 Feb 2021 11:09:08 +0100
From:	steve <stev...@gmail.com>
To:	Max <maximilia...@gmail.com>

Hi

Yeah for EQ6, I'd definitely recommend PPEC for RA with an initial period of 480s. If that fails, try a fixed period of 122s. Even better,  do both and compare.

Don't forget to also compare with EXOS' internal guider via SEP-multistar with GPG for RA. Period(s) the same.

Cheers

[Max]

Hello there,

thanks for your replies. I tried yesterday night, but before I could read your comments. Anyways. I set things to predictive PEC and left the parameters, except the worm period untouched. I have set this to 479 based on this table: https://github.com/OpenPHDGuiding/phd2/wiki/Mount-Worm-Period-Info

My performance was a lot better now. However, when my target went down west the performance declined. I think I should maybe have rebalanced the mount after the meridian flip to be slightly east heavy?!

In my earlier log I reduced the RA aggressiveness because I thought it was overcorrecting. Anyways, this time I left everything untouched. Only in between I tried increasing exposure time to 3s, but didn't notice improvement so went back to 2s.

What I noticed is that there is constant small oscillation on RA. How could I fix this?

Here's my log from yesterday night:

https://openphdguiding.org/logs/dl/PHD2_logs_FgDC.zip

I guided for nearly 3 hours, performend an meridian flip, then guided for another nearly 3hours, then switched target for my mosaic (just very little slew from IC 417 to NGC1931) then guided another 3h.

What is odd, is that after the last slew, performance degraded. During the first two guiding runs I was at around 0.88", in the end I was at 1.45" RMS. Noting here that the object was close to the horizon at the end of the last period.
 

What I noticed is that each time I started guiding again, it recalibrated. It makes sense after the meridian flip, but when I just moved a few arc minutes to the next mosaic panel, why would it recalibrate?

Eventually I would like to get closer to 0.5" RMS to be happy. I've read of others achieving that level with the EQ6R-Pro.

Thanks a lot,

Max

[Max]

Reading through the dropbox link posted by Bruce, I basically did exactly that, and the log  in my link above does now constitute the baseline guiding result.

[bw_msgboard]

Hi Max, see below.

 

---

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Max

Sent: Sunday, February 14, 2021 4:36 AM
To: Open PHD Guiding

Subject: Re: Re: [open-phd-guiding] Re: High RA error causing oblong stars

 

Hello there,

 

thanks for your replies. I tried yesterday night, but before I could read your comments. Anyways. I set things to predictive PEC and left the parameters, except the worm period untouched. I have set this to 479 based on this table: https://github.com/OpenPHDGuiding/phd2/wiki/Mount-Worm-Period-Info

 

My performance was a lot better now. However, when my target went down west the performance declined. I think I should maybe have rebalanced the mount after the meridian flip to be slightly east heavy?!

 

I think you have to look a little more closely at the last guiding session.  There were two huge RA excursions right at the end, probably because you were pointing down in the weeds.  If we exclude that last 5 minutes of guiding, your total RMS was 0.87 arc-sec.

 

In my earlier log I reduced the RA aggressiveness because I thought it was overcorrecting. Anyways, this time I left everything untouched. Only in between I tried increasing exposure time to 3s, but didn't notice improvement so went back to 2s.

 

What I noticed is that there is constant small oscillation on RA. How could I fix this?

 

You have a lot of uncorrected periodic error in the mount:

 

 

The big contributor is at the 480sec worm period but the 120-sec component common to these mounts contributes as well.  PPEC can only do so much – it’s taking this 14 arc-sec peak-peak oscillation down to about 0.76 arc-sec RMS.  You may be getting all that the mount can deliver unless you can make mechanical adjustments or get a high-quality periodic error correction programmed.  

 

Here's my log from yesterday night:

 

https://openphdguiding.org/logs/dl/PHD2_logs_FgDC.zip

 

I guided for nearly 3 hours, performend an meridian flip, then guided for another nearly 3hours, then switched target for my mosaic (just very little slew from IC 417 to NGC1931) then guided another 3h.

 

What is odd, is that after the last slew, performance degraded. During the first two guiding runs I was at around 0.88", in the end I was at 1.45" RMS. Noting here that the object was close to the horizon at the end of the last period.
 

What I noticed is that each time I started guiding again, it recalibrated. It makes sense after the meridian flip, but when I just moved a few arc minutes to the next mosaic panel, why would it recalibrate?

 

This is evidently an un-wanted Ekos “feature” – it is repeatedly clearing the PHD2 calibrations.  You’ll have to ask them why they’re doing this.

 

Eventually I would like to get closer to 0.5" RMS to be happy. I've read of others achieving that level with the EQ6R-Pro.

 

No two of these mounts behave identically so comparisons are going to be difficult when you’re down below 1 arc-sec total RMS.

 

Good luck,

Bruce

 

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/f8d7fa14-5f7a-4822-b955-37415ccb81fan%40googlegroups.com.

[Niels Rackwitz]

With respect to the ECOS/recalibration question, there is a setting to prevent this from happening. It’s a bit hard to find.Take a look behind the „Options…“ button in the Guide tab.

 

Am 2/14/21 um 12:23 schrieb bw_msgboard <bw_m...@earthlink.net>:

Hi Max, see below.

 

---

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Max
Sent: Sunday, February 14, 2021 4:36 AM
To: Open PHD Guiding
Subject: Re: Re: [open-phd-guiding] Re: High RA error causing oblong stars

 

Hello there,

 

thanks for your replies. I tried yesterday night, but before I could read your comments. Anyways. I set things to predictive PEC and left the parameters, except the worm period untouched. I have set this to 479 based on this table: https://github.com/OpenPHDGuiding/phd2/wiki/Mount-Worm-Period-Info

 

My performance was a lot better now. However, when my target went down west the performance declined. I think I should maybe have rebalanced the mount after the meridian flip to be slightly east heavy?!

 

I think you have to look a little more closely at the last guiding session.  There were two huge RA excursions right at the end, probably because you were pointing down in the weeds.  If we exclude that last 5 minutes of guiding, your total RMS was 0.87 arc-sec.

 

In my earlier log I reduced the RA aggressiveness because I thought it was overcorrecting. Anyways, this time I left everything untouched. Only in between I tried increasing exposure time to 3s, but didn't notice improvement so went back to 2s.

 

What I noticed is that there is constant small oscillation on RA. How could I fix this?

 

You have a lot of uncorrected periodic error in the mount:

 

<image001.jpg>

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/B29054E050A84153AEC6198DA3B067BA%40HomeDesktop.

[Max]

Thanks a lot for the feedback.

Do you think using the PEC inside the mount (I think there is a way to program a PEC curve into the mount) would help reducing the periodic error?

What kind of mechanical adjustments are possible?

Kind regards,

Max