<p>Seeing is typically measured as the FWHM of the seeing disk, but can also be expressed though the <a href="https://en.wikipedia.org/wiki/Fried_parameter" rel="nofollow noreferrer">Fried parameter</a> $r_0$, which measures the size, or strength, of the parcels of gas that cause the turbulence in the atmosphere. The relation between the two is (e.g. <a href="http://adsabs.harvard.edu/abs/1995PASP..107..265V" rel="nofollow noreferrer">Vernin &amp; Munoz-Tunon 1995</a>)
$$
\text{seeing} = 0.98\frac{\lambda}{r_0},
$$
where $\lambda$ is the wavelength of the light.</p>

<p>$r_0$ can by calculated by integrating the "turbulence strength" along the line of sight$^\dagger\!\!$, and  scales as $\lambda^{6/5}\!$. Because of this dependence, the "usual" seeing has only a weak ($\lambda^{-1/5}$) dependence on wavelength.</p>

<p>From <a href="https://en.wikipedia.org/wiki/Astronomical_seeing" rel="nofollow noreferrer">the Wikipedia article on seeing</a>:</p>

<blockquote>
  <p>At visible wavelengths, $r_0$ varies from 20 cm at the best locations to 5 cm at typical sea-level sites</p>
</blockquote>

<hr>

<p>$\dagger$<sub>Actually, $r_0$ refers to a distance corresponding to the line of sight towars zenith; observing at an angle $\zeta$ from zenith introduces a factor $\cos^{3/5}\!\zeta$.</sub></p>

CC BY-SA 3.0
2017-10-16T16:38:33.923Z
2017-10-16T20:10:54.180Z
2017-10-16T20:10:54.180Z
pela
<p>I am very used to working with seeing in arcseconds, but sometimes I come across the seeing in cm, e.g. <a href="http://www.royac.iac.es/seeing.html" rel="nofollow noreferrer">here</a>.</p>

<p>I have looked online for a way to convert from one to the other, but I can't find any. How would this be done?</p>

CC BY-SA 3.0
2017-10-16T15:59:18.753Z
Do you have an example you could point to for seeing stated in cm? The reason I ask is because you can't specify seeing in physical distance units like cm in a general way. You could possibly do it for a specific instrument if you know that instruments [plate scale](http://panisse.lbl.gov/~sed/telescope/obsguide/platescale.html) but really that's just converting the angular size of the seeing to the physical size *for that instrument only*. I've never seen anyone do such a thing though.
zephyr
CC BY-SA 3.0
2017-10-16T16:01:41.883Z
Here for example: http://www.royac.iac.es/seeing.html
Coolcrab
CC BY-SA 3.0
2017-10-16T16:04:03.700Z
And I agree with what you are saying, this is why it is confusing for me. They say that the 'true' seeing which doesn't help
Coolcrab
<observational-astronomy>
Seeing units, arcsec VS cm
Coolcrab
# Seeing units, arcsec VS cm

(Asked by: username_0 on 2017-10-16)

I am very used to working with seeing in arcseconds, but sometimes I come across the seeing in cm, e.g. [here](http://www.royac.iac.es/seeing.html).

I have looked online for a way to convert from one to the other, but I can't find any. How would this be done?

**Tags:** observational-astronomy

> **Comments:**
> **username_1**: Do you have an example you could point to for seeing stated in cm? The reason I ask is because you can't specify seeing in physical distance units like cm in a general way. You could possibly do it for a specific instrument if you know that instruments [plate scale](http://panisse.lbl.gov/~sed/telescope/obsguide/platescale.html) but really that's just converting the angular size of the seeing to the physical size *for that instrument only*. I've never seen anyone do such a thing though.  
> **username_0**: Here for example: http://www.royac.iac.es/seeing.html  
> **username_0**: And I agree with what you are saying, this is why it is confusing for me. They say that the 'true' seeing which doesn't help  

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## 1 answers

### Answer by username_2 on 2017-10-16. Score: 4

Seeing is typically measured as the FWHM of the seeing disk, but can also be expressed though the [Fried parameter](https://en.wikipedia.org/wiki/Fried_parameter) $r\_0$, which measures the size, or strength, of the parcels of gas that cause the turbulence in the atmosphere. The relation between the two is (e.g. [Vernin & Munoz-Tunon 1995](http://adsabs.harvard.edu/abs/1995PASP..107..265V))
$$
\text{seeing} = 0.98\frac{\lambda}{r\_0},
$$
where $\lambda$ is the wavelength of the light.

$r\_0$ can by calculated by integrating the "turbulence strength" along the line of sight$^\dagger\!\!$, and scales as $\lambda^{6/5}\!$. Because of this dependence, the "usual" seeing has only a weak ($\lambda^{-1/5}$) dependence on wavelength.

From [the Wikipedia article on seeing](https://en.wikipedia.org/wiki/Astronomical_seeing):

> At visible wavelengths, $r\_0$ varies from 20 cm at the best locations to 5 cm at typical sea-level sites

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$\dagger$Actually, $r\_0$ refers to a distance corresponding to the line of sight towars zenith; observing at an angle $\zeta$ from zenith introduces a factor $\cos^{3/5}\!\zeta$.


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