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How to figure out a measure of seeing in arc seconds?

Astrophotography Imaging
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#1 kvastronomer

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Posted 19 May 2022 - 05:52 PM

I often see here on CN people reporting their seeing conditions in arc seconds. Where do I lookup that number?

I'm using NINA for capturing and PixInsight for processing.

 

Thanks in advance for explaining.



#2 rgsalinger

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Posted 19 May 2022 - 06:10 PM

I just look at the HFR as reported and figure out from the image scale what that amounts to by multiplying the two together. It's not scientific, but it seems to correlate well with what gets reported by PI later on as the FWHM of the  image when I run subframe selector. 

 

Rgrds-Ross



#3 Midnight Dan

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Posted 19 May 2022 - 06:21 PM

There's another active thread on this right now with lots of info:

 

https://www.cloudyni...al-seeing-info/

 

-Dan


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#4 KungFood

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Posted 19 May 2022 - 06:23 PM

I've wondered about this as well. I understand that the "high frequency star motion" as reported by PHD2's Guiding Assistant correlates to seeing condition, but am not sure how the conversion from the reported motion to seeing works. The GA reports .3-.35", but there is no way that is my actual seeing. I assume there is some math involved to get to the usual  2-4" conditions I often see reported by folks in urban environs. No clue what that math would be.


Edited by KungFood, 19 May 2022 - 06:25 PM.


#5 kvastronomer

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Posted 19 May 2022 - 08:00 PM

I just look at the HFR as reported and figure out from the image scale what that amounts to by multiplying the two together. It's not scientific, but it seems to correlate well with what gets reported by PI later on as the FWHM of the  image when I run subframe selector. 

 

Rgrds-Ross

Does this mean that FWHM is absolute value that does not depend on our equipment? If you see FWHM = 3.0 and I see the same number our seeing conditions are the same?



#6 rgsalinger

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Posted 19 May 2022 - 10:21 PM

No, that's not what I meant.

 

What I meant is that once you know your equipment you'll find that there's a reasonable range of values that tell you that the seeing is pretty good. Bear in mind that this is not science, just a rule of thumb. So, I have 3 refractors. The WO71 on a good night gets down to around 4 arc seconds for FWHM, the ASKAR 107PHQ will get down to around 2.7 arc seconds and the AP 155 will get down to 2 arc seconds. That's measured with whatever tools I have available to me.

 

With any telescope bigger than 6" it's very likely that you are seeing limited in any case. At my location imaging with my CDK14 gets me down just a small amount from what the AP155 will do because the seeing at the site is between 1.5 and 2 arc seconds. What the big scope gives me is more photons than the smaller scope. 

 

Hope that's helpful and I'm sure that someone out there has a better method, this is good enough for me. If I'm using the WO71 and I'm getting (say) 5 arc seconds, I usually just stop imaging and watch a movie. 

 

Rgrds-Ross


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#7 Drothgeb

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Posted 20 May 2022 - 07:55 AM

The ASIair displays HFR and FWHM. I just gauge seeing based on that. Much over 4”, I don’t image. From 3”-4” is a typical so-so night. I image, but wish conditions were better. Below 3” I consider good. When it’s below 2” (pretty rare) I get excited. This is with a 0.8” pixel scale, and 0.4”-0.5” guiding.

 

I know it’s not actual seeing conditions, but it gives me a good idea of what to expect. 



#8 TelescopeGreg

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Posted 20 May 2022 - 01:10 PM

The trick is differentiating between bad Seeing and simply bad focus from one night (or hour) to another.

 

I generally just look at the sky.  If the stars are really twinkling, I consider finding something else to do.


Edited by TelescopeGreg, 20 May 2022 - 01:28 PM.


#9 KungFood

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Posted 20 May 2022 - 03:32 PM

What I would like to know is, when i see someone say,  "my seeing is 3 arcseconds"...how do they get that number? What metric during a capture session gives that information? I'm assuming the seeing measurement is a derivative, but would love to know what the solid factors towards that derivative are. The only one that I know of that is a direct measurement of atmospheric movement is the "high frequency star motion" measured by PHD2. Is it possible to derive the seeing condition value from this? Or is the (average HFR*image scale) the best way to calculate? If the latter is true, Should the measurement be averaged over a number of frames to eliminate outlier HFR measurements?

 

Or, short of a dedicated measuring device, is it just a well educated guess?

 

I apologize for my tiny brain :)


Edited by KungFood, 20 May 2022 - 03:58 PM.

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#10 dciobota

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Posted 20 May 2022 - 04:11 PM

I personally use the hfr of a non saturated star at a short enough exposure, unguided to not show trailing. So say 5 sec. For me, the asiair app calculates the hfr when I do that.

#11 musicmatters

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Posted 20 May 2022 - 08:25 PM

This site will give you a seeing number in arc seconds

https://www.meteoblu...-states_4207783

#12 rgsalinger

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Posted 20 May 2022 - 11:44 PM

I wish that site was accurate but it's not.It consistently gives sub arc second seeing - remember these are predictions not measurements- for the dark sky site that I use. Not happening in the 12 years I've been out there. I can't tell. if it's more accurate for other areas of the country but is way off where I live. Disappointing. 


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#13 KungFood

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Posted 21 May 2022 - 12:00 AM

Yeah, it said .65" here tonight...I find that excessively improbable.



#14 dciobota

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Posted 21 May 2022 - 12:04 AM

Yup, that site is waaay too optimistic. And those are predictions, not actual measurements.

#15 rgsalinger

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Posted 21 May 2022 - 12:18 AM

I think that if you know what your FWHM (or HFR) usually are then you can know if the seeing is off or on. Not quantitavely but good enough to decide to gather data or not that night. Otherwise you can buy a https://www.sbscient...g-monitor/sm-1/ if you have deep enough pockets. 



#16 KungFood

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Posted 21 May 2022 - 03:29 AM

Honestly, if I had a permanent observatory, I'd very seriously consider one of those. It would be no where near the top of my shopping list, but still.


Edited by KungFood, 21 May 2022 - 03:31 AM.


#17 Wildetelescope

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Posted 21 May 2022 - 01:22 PM

Any night that is clear and I have time, I image, lol. Beggars can’t be choosers. Have imaged in 20 mph winds. For planets, I am more picky.

Jmd

#18 RoscoeD

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Posted 22 May 2022 - 12:40 AM

That's measured with whatever tools I have available to me.

That's the $64,000 question that the OP asked...what tools?




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