Guiding RMS versus image train resolution

[Don Pierce]

I just wanted to see if this makes sense.

I understand that if my guiding RMS is about 75% of my resolution then I am in good shape.

That is when I am in good - excellent seeing conditions.

One example - at 0.96 arcsec/pix resolution I want my total rms at or below 0.74arcsec

2nd example - without reducer my resolution is 0.68 arcsec/pixel so I want guiding at or below 0.5arcsec

Of course I would prefer lower rms but it is tough getting good seeing conditions all the time.

I used the CCD suitability calculator in Astronomy.tools to determine my image resolution.

[bw_m...@earthlink.net]

Hi Don.  I don’t know where your 75% number comes from but it’s up to you.  To be a bit more systematic, you can start by measuring the FWHM values of stars in your images under various seeing conditions.  You do that by using short exposure times, say 10 sec, so that guiding isn’t really a factor.  If you average the FWHM values over a number of frames, you can get your own estimate of the seeing conditions in arc-sec assuming that the optics are performing well.  You have to decide, in other words, whether you are optically or seeing-limited.  With the much longer exposures you take, the star sizes will increase to some extent – the question is how much and whether the amount is acceptable to you. 

 

Here is another take on the subject if you’re interested:  https://www.innovationsforesight.com/education/how-much-guiding-error-is-too-much/

 

This is more in line with my experience, for me your 75% target is probably too generous.

 

Regards,

Bruce

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[Donald Polidori]

My understanding is that guiding performance/expectations should be judged against seeing conditions, not imaging scale.  The rule of thumb I always hear is that you want your guiding RMS to be around 1/3 your seeing.  If you get this , you are likely "seeing-limited" in guiding performance.  In fact, as the seeing changes throughout the night you can see your guiding RMS change accordingly.  Another way to check if "seeing-limited" is that your Dec and RA error should be similar and the errors on the graph should appear random without any cyclical patterns.

Imaging scale vs. seeing will dictate if you are over or under sampling.  Imaging scale of your GUIDE camera does play a role in guiding, but sounds like you are referring to the main camera scale.

[Don Pierce]

Good points thanks

[Don Pierce]

Thanks for the tips. I will try to put that to practice.