Maximum Benefit of Increased Guiding Accuracy

[Frankenkeim Dr.]

Aloha!

Could you guys let me know if I am thinking about the "maximum required guiding accuracy", based on location and gear, correctly?

Is it fair to assume that the limiting factor is the larger of:

1) tracking error (SQR(RMSx2+RMSy2)2)

2) gear optics (130/telescope aperture)

3) local seeing conditions

So for example if my local seeing is 2" there is no benefit in getting 1) below 2; or if I use a 70mm scope I get to 1.86" as the optic limit,  and there would be no additional benefit to get my guiding below that value even in excellent seeing conditions?

Am I thinking about this correctly? 

[Lou B]

I am not sure i follow your first equation.  R u taking the absolute value or the average RMS error?

This is only my opinion.  I have not done the analysis to support it.  But, if i had my way, I would figure out the limiting resolution of the optical train and then comfortably exceed it by some margin to make the best use of the equipment.  Then the limitation is seeing.  Given my dream equipment i would like to have a sample that could resolve 2x the best seeing available , there by no loss of resolution. 

Again, only my opinion.  I am sure people do wonders with far less.   

[Frankenkeim Dr.]

Hello!

The first equation is how I believe the total tracking error is calculated. It should match the total RMS error in PHD2 (or so I believe at least :-) ).

Said differently: If I use an 70mm APO would there be any benefit to get my total tracking error below 1.86" (limit of optics = 130/70mm = 1.86")?

I am trying to validate when guiding is "good enough" considering the equipment used and estimated seeing conditions. Is there any benefit to further refine guiding below 1.86" in this example? 

Or for another example: if my seeing conditions are at 2". Is there any benefit to get guiding below 2"?

[bw_msgboard]

I think the problem is more complicated than what you suggest and the guiding constraints are more demanding.  The goal for guiding should be to produce tracking errors that are smaller than the errors introduced by optical defects, mount defects, and seeing conditions.  In other words, guiding shouldn’t be the constraining factor on the quality of the main camera images.  The first thing to consider is the optical performance of the system – and that can’t be done by theoretical calculations like diffraction-limited resolution or things of that nature.  The question is what are the limits of your particular optics from your site.  A good empirical test is to take a series of short, unguided exposures with your main camera on a night of good seeing and measure the FWHM of the field stars.  The exposures need to be short enough that mount tracking errors don’t have any significant effect.  Compute both the average and minimum FWHM values for those exposures and you can use that as a goal.  In a perfect world, your mount and the guiding would produce stars whose size and elongation was that good or nearly that good.  Then it’s a question of translating that into a guiding goal – I have found this document to be pretty useful:

 

https://www.innovationsforesight.com/education/how-much-guiding-error-is-too-much/

 

As you can see, the bottom line metric is total guiding RMS <= ¼ seeing FWHM which I have found over the years to be fairly accurate for my setup.

 

Cheers,

Bruce

 

 

 

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[Paul Trimby]

I've seen it suggested that the total guiding error should be 1/2 the image scale.

Paul Trimby

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[Frankenkeim Dr.]

Thank you Bruce - very interesting article. I will try out your suggestion. 

[Frankenkeim Dr.]

Thank you Paul - this is helpful too