Why is RA RMS twice DEC RMS?

[Magnus Larsson]

Hi!

I recently upgraded from a C8 to a C11 on my Losmandy G11, and am now struggling to get guiding back to good standards. What I get now, is elongated stars (among other things). So a very specific question: why is RA RMS 2x DEC RMS, even in high frequency star movement when doing a GA? See the attatched screenshot, that is from the GA run in the end of the attached log file. THere high frequency movement is twice as much in RA.... how come? How much does that depend on my mount and if so, what could cause that in my mount?

Magnus

[Brian Valente]

Hi Magnus

nice to see you here :)

>>>So a very specific question: why is RA RMS 2x DEC RMS, even in high frequency star movement when doing a GA? 

I wouldn't put a lot of weight on what you are looking at there. First, you are talking 5/100ths of a pixel. Second, if you look at your guidelogs, both axis look choppy to me:

>>>How much does that depend on my mount and if so, what could cause that in my mount?

great question, looking at the periodicity of your unguided results (i.e., is there any periodic error in the mount that would account for this):

I really don't see anything significant in the raw mount performance that would explain it

You do have a pretty significant polar misalignment that would help you to correct.

I don't know if I have a definitive answer for you, looking at DEC and RA together it just seems like poor seeing conditions. 

hth

Brian

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Brian Valente

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[Magnus Larsson]

Hi!

Thanks. I'm aware of the PA misalignment. So you attribute the choppiness to bad seeing, OK, so not mount issues - that's helpful. But should not star high frequency movement be higher when there is bad seeing?

I'm curious about the RA being 2x DEC RMS more generally, because that is what I tend to get, so any ideas on how to improve that is helpful - not least since as you note, there seems to be not much of high frequency things going on in RA - according to the frequency analysis. It's more that the choppiness that are, even when seeing is better, seems to be more in RA than in DEC. 

Best, 

Magnus

[wave...@talktalk.net]

Hi, this RA to Dec ratio in the high frequency statistic is one I see constantly in the UK, where the seeing is never particularly good. I'd say the 2:1 figure is typical, though I was most surprised a few days ago to see it at almost exactly 3:1. Also notable here was the Dec RMS which was much lower than its usual figure. Assuming these rapid star motions are primarily caused by seeing rather than by the mount, could the way the jet steam operates be a big factor? Since the jet stream is moving West to East, won't the diffraction cells that cause scintillation be affected more in that direction, essentially in the RA? As we know, the jet stream has dipped much further South than normal (especially over the central USA) in recent weeks.

Cheers,

- Jack Thewlis

[Mtn Goat]

When my G11 was new my RA/Dec ratio was about 1.2 : 1. My old iOptron also got a ratio like that or better. After just a few months I was lucky when my G11 ratio was 2:1. It can certainly be worse.

To say that’s due to ‘seeing’ means the seeing aberrations are only in the RA axis. The jet stream is not in the same place even from week to week at least in the USA.  I would challenge: find one astronomical or meteorological study that associates ‘Seeing’ conditions w/ the jet stream. One might as well blame it on the aether.

On Mar 5, 2021, at 11:29 AM, 'wave...@talktalk.net' via Open PHD Guiding <open-phd...@googlegroups.com> wrote:



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[bw_msgboard]

I’m not signing up to the idea that the RA:Dec rms differences are typically caused by seeing – I doubt that’s what is going on.  But I think there’s a sound mathematical basis (the Kolmogorov model) for saying that high winds in the troposphere (where the jet streams are) definitely affect astronomical seeing.  We’re not talking about aether here. J  The high wind speeds coupled with the high altitude mean that the coherence times at that layer are very short so you will see a lot of high-frequency “jitter”.  That isn’t the only contributor to seeing, especially for amateurs, but it definitely has an important effect.  Here’s a reference that goes into some useful detail:

 

https://www.innovationsforesight.com/education/astronomical-seeing-tutorial/

 

If you’re still dubious about this, you’ll probably have to drill down to the next level and look at the empirical support for the Kolmogorov model and how it’s used by professional observatories to design and tune their AO devices.

 

Cheers,

Bruce

 

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[Brian Valente]

>>>I’m not signing up to the idea that the RA:Dec rms differences are typically caused by seeing – I doubt that’s what is going on. 

I hope i didn't come across as saying that!

What i did say (or hoped to say) was:

- there doesn't appear to be any obvious mechanical source that would explain that (in terms of periodicity)

- both axis look equally noisy in high frequency when reviewing all the guidelog entries 

- the 2:1 may be overstated (it was just in the one GA run, correct?) 

- RA being more than DEC is a typical challenge we all face

One challenge is i don't see a way to view the high frequency RMS in the guided sessions via the guidelog viewer

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[Mtn Goat]

The article never speaks in terms of jet-streams or anything of such large scale. They state upper atmospheric effects are on the order of millisecond effects.

The authors of the article conclude in their section:  Seeing effect on auto-guiding

“Seeing is random in nature which means that seeing is made of different temporal and spatial components... Close to the ground (in the boundary layer), turbulence effects are more correlated across most of the imaging camera FOV.... The low altitude turbulence is also slower (low frequencies) and therefore auto-guiding correction rate in the range of one to few seconds may mitigate the boundary layer seeing contribution.... However for seeing effects from higher altitude layers this is not likely.

The model had nothing to do with jet-streams. All it takes is one comment to infect social media with some meme that jet-streams cause bad RA to Dec guiding ratios.

On Mar 5, 2021, at 12:29 PM, bw_msgboard <bw_m...@earthlink.net> wrote:



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[bw_msgboard]

I think you’ve missed my point.  Here is the comment I was reacting to:

 

“I would challenge: find one astronomical or meteorological study that associates ‘Seeing’ conditions w/ the jet stream”

 

You didn’t place any qualifiers on that, the statement says nothing about RA and Dec ratios.  It appears to be a theoretical argument and one that I think is wrong.  If you examine the math in the Kolmogorov model, you will find that the seeing effects are functions of both the altitude and the wind speed.  Look at the Wikipedia article on astronomical seeing:

 

https://en.wikipedia.org/wiki/Astronomical_seeing

 

Do a search for ‘wind’ and you will find useful information – for example:

 

If turbulent evolution is assumed to occur on slow timescales, then the timescale t₀ is simply proportional to r₀ divided by the mean wind speed.

 

So if we connect the dots, the jet stream represents high wind speeds in the troposphere (a high altitude layer), therefore it must contribute to astronomical seeing.

 

None of this has anything to do with RA and Dec rms values and I said at the start of my response that I didn’t think those ratios had anything to do with seeing in general.

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[Mtn Goat]

Note my qualifier, “... find one astronomical or meteorological study -> w/ jet streams”

On Mar 5, 2021, at 2:34 PM, bw_msgboard <bw_m...@earthlink.net> wrote:



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[Magnus Larsson]

Hi!

Very interesting! Thanks a lot!

Yes, of course, the high frequency RMS does not a such impact my images. But do I get it that you guys also get a general 2:1 ratio in DEC and RA RMS, as sort of a default result? But still getting round stars...? Previously, I've not had an issue with it but after upgrading to a C11, I do get elongated stars, at least with longer exposures (like 10 mins). That's what I want to improve. Of course, it could be other thing, such as flexure (I use a guidescope) - so far, I've had the idea that it was to with the higher RMS in RA, as the elongation also is in that direction, not in DEC. 

Magnus

[wave...@talktalk.net]

Hi Magnus, switching to an OAG on my 10" LX200 has halved the guiding RMS to below 1 arcsec, so I believe it would probably be worth the investment. Having said that, the rapid star movements haven't changed and the RA:Dec ratio remains around 2:1. However, I noted a 3:1 ratio (Dec was much reduced) about ten days later, using the same rig. This raised the question, why? A change in the atmosphere was the logical answer and the recently highlighted jet stream came to mind. Knowing nothing about the subject, I thought it worth asking. Clearly, this opened a can of worms, but that wasn't my intention.

Cheers,

- Jack

[Brian Valente]

>>>But do I get it that you guys also get a general 2:1 ratio in DEC and RA RMS, as sort of a default result?

i wouldn't say it's a default result

I would say having RA higher than DEC is a typical starting point

RA Is doing all the work and moving all the time, whereas DEC only moves when you request it, right?

What's acceptable really depends on the image scale, resulting star eccentricity, and what you find acceptable

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[George Shoup]

My G11 is in the range of (1-2):1

2:1 is on a bad night with wind

Sent from my iPhone

On Mar 6, 2021, at 2:55 PM, Brian Valente <bval...@gmail.com> wrote:



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[Magnus Larsson]

Hi!

George, have you done anything in particular to reach that nice balance? Beyond polar alignment, balanced well but east heavy, and so on? Do you use PEC?

When using PEC, I get better results, but my PEC curve seems to not work more than one night at the time, for now (I'll keep working on that). 

Magnus

[George Shoup]

Magnus:

I sent my guide logs to Brian Valente at Losmandy and he suggested the PPEC settings to use in PHD.  That helped a lot.  No PEC.  

 I ran the guiding assistant and accepted all the recommendations.  I always use 2 - 2.5 sec exposure times on the guide camera. 

 

Sometimes wind gusts play havoc with the calibration run.  I recalibrate when the guide performance is poor.

George

Sent from my iPhone

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[Magnus Larsson]

Thanks!!

So whick PPEC-settings do you use? 

I just recently realized that by default, PPEC had "auto adjust period" checked, which perhaps explained why it did not help me much :) Otherwise, my main issue with PPEC is that I do a lot of variables, meaning a lot of slews, then some 300- 600 sec exposures for the faint ones. There I need guiding (on the other hand, round stars are less critical when doing variables). And with just a few exposures, PPEC might not have sufficient time to adjust. So I try PEC. So far more try than succeed... :)

Best, 

Magnus

[George Shoup]

Magnus:

Here is what Brian (Losmandy) recommended for Predictive PEC settings for my G11:

Predictive weight 60

Reactive weight 70

Minimum move 0.2 pixels - but the box is blacked out so I'm not sure PHD uses this value

Period length 239.38

Disable or no check for auto adjust period

Retain model 40

Let me know if these values work for you.  It seems these values should be consistent for most new mounts.

I bought my G11 new about a year ago.  

George 

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[George Shoup]

It does appear 2 or three periods  (600 sec approx) are required for the RA to settle out.

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