The masking effect of the sky scattering of visible light

[jeremyjr]

It is very well known that the sky scattering of visible light decrease with the wavelength, it is bigger in the blue portion of the spectrum( the reason why the sky is blue ) and is lower on the red portion of the spectrum, this scattering is even lower on the infrared section and lower still in the short radio wave section of the spectrum.

A less known effect of this scattering is its masking effect on small objects that are on the atmosphere, we know that in daylight this effect hides the stars and many other astronomical objects, but also that effect mask/hides small objects that are in the atmosphere, as high altitude birds, bugs, balloons, etc.

This masking effect is very easy to test by making atmospheric observations with binoculars and then doing the same observations placing red filters in front of the binoculars, by doing that many more details will be spotted just by placing red filters in front of the binoculars. So you can say that the spotting efficiency of binoculars increase by just placing red filters in front of them.

The effect is even more dramatic when moving to the infrared, you can spot easily Mars in daylight using a camera enabled to receive infrared and using a 950nm IR pass filter.

Moving then to the short radio wave section will allow you to spot many more objects that are masked by the sky scattering of visible light, because simply the scattering is lower in short radio waves.

These simple ideas lead to the design of very effective instruments for optical atmospheric observations.

Anybody that wants to make optical atmospheric observations should take in count the masking effect mentioned above. Then we have three options listed in order of effectiveness to make optical atmospheric observations:

1- Scan the sky using short radio waves, a radar and then aligned/centered with this radar place a telescope or high optical magnification scope, with this configuration any object spotted by the radar will be optically visible in the telescope section.

2- Scan the sky using a modified camera IR enabled, using a 950nm or 1000nm IR pass filter, then also aligned/centered with the field of view(FOV) of this camera place a telescope or high optical magnification scope, with this configuration any object spotted by the infrared camera and centered in its FOV will be optically visible in the telescope. Even when this option is less effective than using radars it is very effective spotting small objects in the sky, for example Mars will be easily spotted in daylight using such a setup, many "local" objects will be spotted too.

3- Scan the sky using a scope with red filters.

This is an example of how very simple ideas from elementary physics actually drive the design of very effective instruments.

The systematic use of the instruments listed in options 1 and 2, mainly option 2 in atmospheric observations actually are helping in the discovery and study of new atmospheric characteristics.

 

[Andy Resnick]

Scattering by aerosols/haze, etc. (including rain) is a well-studied topic. van de Hulst's classic "Light Scattering by Small Particles" has a chapter, for example. Crane's book "Electromagnetic Wave Propagation Through Rain" is another. MODTRAN and HITRAN codes account for haze.

 

[jeremyjr]

Scattering by aerosols/haze, etc. (including rain) is a well-studied topic. van de Hulst's classic "Light Scattering by Small Particles" has a chapter, for example. Crane's book "Electromagnetic Wave Propagation Through Rain" is another. MODTRAN and HITRAN codes account for haze.

Thanks for the references, my post was mainly directed to people making optical atmospheric observations, like high altitude birds watchers, security/military people and even UFO observers(even when that could be considered taboo/no-go zone in sites like this one), there are many objects in the sky that are not the standard mundane objects that a no initiated in atmospheric observations could expect. Building and using systematically any of the two first options, option 2 is actually very easy and relatively cheap to build for any person with very limited technical skills, will reveal many of these non mundane objects, it is really very worth the try.

 

[sophiecentaur]

The effect is even more dramatic when moving to the infrared, you can spot easily Mars in daylight using a camera enabled to receive infrared and using a 950nm IR pass filter.

Also, the spectrum of energy from the Sun means that there is much less power per Hz in the RF region than in the optical region. This can also have an effect on what is effectively the SNR of observations at RF (when the spectrum of the signal from the observed object is different from that of the Sun).

 

[sardar]

I would like to add some additional information and analysis to this content. The phenomenon of sky scattering, also known as Rayleigh scattering, is a well-studied and well-understood phenomenon in atmospheric science. It occurs when sunlight enters the Earth's atmosphere and interacts with gas molecules, causing the light to scatter in all directions. The amount of scattering depends on the wavelength of the light, with shorter wavelengths (such as blue and violet) being scattered more than longer wavelengths (such as red and infrared).

The reason why the sky appears blue is because blue light is scattered more than other colors, making it the dominant color that we see. This is why the sky appears red during sunrise and sunset, when the sunlight has to travel through a longer path in the atmosphere and the shorter-wavelength blue light is scattered away, leaving behind the longer-wavelength red light.

The masking effect mentioned in this content is a result of this phenomenon. As light from objects in the atmosphere (such as birds, bugs, and balloons) enters our eyes, it also has to pass through the scattered light in the atmosphere. This scattered light can act as a "noise" and make it difficult to see these small objects clearly. By using filters that block out certain wavelengths (such as red filters), we can reduce this noise and improve our ability to spot these objects.

However, it is important to note that the masking effect is not the only factor that affects our ability to see these objects. Factors such as distance, size, and contrast also play a role. For example, a small bird flying far away may still be difficult to see even with a red filter, while a larger object closer to us may be easier to spot even without a filter. Additionally, the type of filter used and the specific wavelengths it blocks can also affect the effectiveness of the masking effect.

In terms of instrumentation, while the options listed in the content may be effective in spotting small objects in the atmosphere, they may not always be necessary or practical. For example, using radars and telescopes may be more complex and expensive, while using red filters with a basic scope may suffice for many observations. It is important for scientists to carefully consider the specific goals and needs of their research in order to choose the most appropriate instrumentation.

Overall, the masking effect of sky scattering is a well-known and important phenomenon in atmospheric science, and understanding its effects can lead to more effective and accurate observations. However, it is just one