| article: The Influence of the Atmosphere in Astronomy |
The atmosphere is introducing several errors to the work with a telescope.
All of these are minimal at the Zenith and maximal close to the horizon.
This is simply because looking at the Zenith is involving less molecules of the atmosphere, the so-called air mass.
The same is true if you elevate from sea level to high mountains.
The following values are approximations for a telescope at sea level (worst case).
Reduction of the brightness (Extinction)
The atmosphere is not 100 percent transparent even if there is no visual fog.
Therefore a small amount of light is blocked before it reaches the telescope.
The following table displays this loss in magnitudes for white light:
Distance to Zenith
in Degrees | Reduction
of Magnitude |
| 0 | 0.00 |
| 10 | 0.00 |
| 20 | 0.01 |
| 30 | 0.03 |
| 40 | 0.06 |
| 50 | 0.12 |
| 60 | 0.23 |
| 70 | 0.45 |
| 80 | 0.99 |
| 85 | 1.77 |
| 87 | 2.61 |
Refraction and therefore change of visible position
Independant of the *Seeing* which is described later there is an ever-present refraction.
It bends the light coming from a star towards the ground.
The effect is that the seen position of the star is higher than the actual one according to a star catologue.
Or in other words: A GoTo mount which is not compensating for that effect is pointing too low even it would position perfectly in mechanical terms.
The following table displays this effect (which can reach the diameter of the full moon!):
Distance to Zenith
in Degrees | Refraction |
| 0 | 0' 00" |
| 10 | 0' 11" |
| 20 | 0' 22" |
| 30 | 0' 35" |
| 40 | 0' 51" |
| 50 | 1' 11" |
| 60 | 1' 45" |
| 70 | 2' 45" |
| 75 | 3' 42" |
| 80 | 5' 31" |
| 85 | 10' 15" |
| 88 | 19' 17" |
| 89 | 25' 36" |
| 90 | 36' 38" |
Wavelength dependend refraction - Dispersion
Depending on the wavelength the light is refracted in different way by the atmosphere.
This means that blue features in the image are moved to the top and red features are moved to the bottom.
This effect can be harmful in "1-shot-color" high resolution astrophotography, i.e. with WebCams.
It is also good to know about that when you are judging the remaining chromatic aberration in a refractor telescope.
Dispersion of red and blue
To estimate the size of this effect in seconds of arc we can use a table with the refractive index as a function of the wavelength.
It is taken from the book Allen's Astrophysical Quantities, Arthur N. COx, Springer 2000.
Wavelength λ
in nm | Refractive
Index | Atmospheric
Refraction |
| 300 - UV | 307.6 | 63.34" |
| 400 - B | 298.3 | 61.46" |
| 550 - G | 293.1 | 60.39" |
| 650 - R | 291.5 | 60.06" |
| 900 - IR | 289.6 | 59.67" |
The given values are for 273.15 K, dry air and a pressure of 1013 hPa.
Actual values are differing with the humidity in the air, the temperature and the pressure.
Even more these values are by no means constant throughout the atmosphere for a given situation.
The following table is nothing more than a first guess to show the order of the effect in the worst case (dry air).
Distance to Zenith
in Degrees | R - B
visual |
B - IR
typical CCD |
| 0 | 0.0" | 0.0" |
| 10 | 0.2" | 0.3" |
| 20 | 0.5" | 0.6" |
| 30 | 0.8" | 1.0" |
| 40 | 1.2" | 1.5" |
| 50 | 1.2" | 2.1" |
| 60 | 2.4" | 3.1" |
| 70 | 3.8" | 4.9" |
| 80 | 7.9" | 9.9" |
Look at the (horrible) Mars image below.
It was sampled at 0.16 arc sec / peixel and Mars was at a distance to Zenith of about 75 degrees.
I used an IR-X (Infra Red Blocking) Filter so the image was taken in visual light only.
The blue shift can be counted to be 10 pixels hence giving a total of 1.6 seconds of arc.
This is by factor 3 better than calculated in the worst case table above.
Mars in 'high resolution', bad seeing and no image enhancement like Multiple Unsharp Masking but nicely showing color aberration at a Zenith Distance of 75 Degrees
Usually the seeing (see next chapter) just above horizon is in the same order like the dispersion.
Hence you will most probably see a mixture of both effects and not a clear seperation of colors.
If you are interested in how to correct your image please check this article: Atmospheric Dispersion Correction .
Seeing (Refraction modulation in cells of air which are then working like lenses)
The Kolmogorov Turbulence Model is describing the air under turbulent conditions as a carrier of parcels.
In these parcels the air is under pressure equilibrium and therfore these parcels have different densities.
For detail and the mathematical model behind it see Lawson, 1999
The seeing can vary from 10 arc secs to 0.5 arc secs.
Rarely it can be even better or worse.
(Well: right above a campfire in the wintertime it is *much* worse!)
At Paranal, the location of the ESO VLT telescopes, there is 0.3 arc secs under best conditions.
While the cells of air are moving in a turbulent way there is a change in the refraction index of that certain direction (imagine a tube of air you are looking through) acting like a lens.
The size of these cells is in the order of some centimeters to some dozends of centimeters (appr. 1 to 8 inches).
Typically In small apertures the star or moon will jump arround while in bigger ones (>10 inches) the object will be blurred.
Very often it is a mixture of both effects.
In a photopgraph with long exposure the airy disk of a star will be enlarged and detail on the moon will be smeared (scintillation and blurring).
You can also say: The stars are twinkling, which is reflecting the higher frequency component of the seeing.
Or: The the star is jumping arround, which is reflecting the lower frequency.
The next video is showing Arcturus acquired with very short exposure to "freeze" the seeing and to show both effects.
Low and high Frequency seeing effects disturbing Arcturus
What looks so nice to the naked eye is a serious enemy to astronomers.
For planetary observations you - in that case - decrease the magnification to 200x or lower.
Planetary photography with film is useless because no detail can be recorded in the long exposures needed.
Using WebCams can still be rewarding because the very short exposures needed for CCDs.
From time to time there is a chance in catching a 'lucky frame' among the 95% or more bad frames.
Deep Sky photography can still be done but is restricted to shorter focal lengths (wide angle) and the larger objects like M42, M31 or M45.
A suitable list of Deep Sky objects ordered by size can be found here.
It is ironic to know that often when the sky is foggy there is very good seeing because of the steady air...
Conclusion
If you are putting everything together the best way to observe or photograph the objects is when they are at their closest position to the Zenith.
A table of popular deep sky objects ordered by the month when they are highest in the sky can be found here.
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