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Guiding RMS error below your imaging resolution

astrophotography
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#1 kvastronomer

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Posted 13 January 2021 - 03:43 PM

Does guiding RMS error below telescope/camera imaging resolution improve image quality?

My imaging resolution is around 1.1 arc seconds per pixel and I can guide with RMS error around 1.0" most of the time.

Would I get a better image trying to guide better? Let's say at RMS < 0.8" or even better

 

Thanks



#2 kvastronomer

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Posted 14 January 2021 - 12:19 PM

Is this a stupid question or a difficult to answer?



#3 Tapio

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Posted 14 January 2021 - 12:41 PM

If you are imaging in high mountain or have exceptionally good seeing then yes.

But usually seeing is 2" or worse so better guiding doesn't help.



#4 imtl

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Posted 14 January 2021 - 12:49 PM

If you are imaging in high mountain or have exceptionally good seeing then yes.

But usually seeing is 2" or worse so better guiding doesn't help.

 

Not entirely true. Guiding is not just about rms numbers. The peak to peak is very important as well as the smoothness of the mount PE. One should aim for rms that is lower than their typical seeing as much as possible and depending on their image scale as well.



#5 Madratter

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Posted 14 January 2021 - 12:49 PM

It isn't simple, hence the lack of answers. The big picture is that other errors like seeing may dominate. But just because they dominate doesn't mean other errors like tracking error don't matter. But it is definitely a diminishing return sort of thing. It is kind of like noise and the mathematics behind it is similar if errors are Gaussian in nature (which in tracking is NOT always a good assumption).


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#6 kvastronomer

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Posted 14 January 2021 - 01:15 PM

I just found excellent explanation on this topic. Probably best article I read this week, and it has formulas :)

 

FWHM = (FWHMoptics 2 + FWHMseeing 2 + FWHMtracking 2) 1/2

 

https://astrojolo.co...guiding-can-be/


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#7 Professor2112

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Posted 14 January 2021 - 01:23 PM

I believe this is a simple way to put it. Someone correct me if I’m wrong, but all other factors aside, if your guiding is better than your image scale(say you’re at 1”/px, guiding is at 0.6-0.8px), then your guiding won’t be your limiting factor because the movements of the setup are less than one pixel.  At that point you’d be limited by seeing or some other wonderful atmospheric effects. 


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#8 Madratter

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Posted 14 January 2021 - 02:13 PM

I just found excellent explanation on this topic. Probably best article I read this week, and it has formulas smile.gif

 

FWHM = (FWHMoptics 2 + FWHMseeing 2 + FWHMtracking 2) 1/2

 

https://astrojolo.co...guiding-can-be/

This adding in quadrature is what I was alluding to above when I said it was similar to noise. Again, the problem with this analysis is that it assumes the errors are Gaussian in nature. That is not necessarily a good assumption.

 

However, it does provide some insight as to why the tracking error cannot be totally ignored if smaller than the seeing, and why improving the tracking error has diminishing returns.

 

For those trying to follow the formula above those are squares, not multiplied by 2.


Edited by Madratter, 14 January 2021 - 02:13 PM.


#9 FlankerOneTwo

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Posted 14 January 2021 - 03:32 PM

Also note that the RMS error reported in PHD2 is the _radius_ of the circle within which roughly 68% of the samples will fall. So one would need RMS error less than one half of the imaging pixel scale for the 68% error circle to fall entirely within a single pixel. And then there's still the other 32% or almost one third of the samples. 
So smaller is better, but one definitely reaches a point of diminishing returns at some point and as pointed out above, depending on your imaging pixel scale, the mechanical guiding may not be your resolution limiting factor.



#10 Der_Pit

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Posted 14 January 2021 - 03:43 PM

Yes, just wanted to write that, too.  RMS is sort-of an average.  Rule of thumb is the real peak-to-peak is 3×RMS, so for an 1" RMS that is ±1.5", or 3" in diameter.  So if you have 1"/px image scale, you'd actually need a 0.33" guiding RMS to have almost all deviations within a pixel.

Also worth a look is asymmetry of guiding.  If your 1" RMS is built on 0.4" DEC and 0.9" RA you would probably still see elongated stars in the result...


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