I have to admit that I did not understand this, even though Roger Cowley explained it to me at a meeting of the Willingboro Astronomy Society. It was not until Joe Stieber was kind enough to supply the following illustration to explain it that I finally "got it."

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The Earth's atmosphere can also act as a prism when light enters the atmosphere from the near vacuum of space. The lower in elevation above the horizon a celestial object is, the greater the angle of incidence to the atmosphere, and the greater the bending of the light, and the greater the dispersion of its constituent colors.

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If you look really closely, you will notice that each star in the double is blue on top and red on the bottom. These colors, however, are not from the stars themselves. The blue and red effect is actually caused by the Earth's atmosphere by prismatic atmospheric dispersion. This is an effect seen in images and visually when an object is viewed or photographed at a low altitude and the light has to go through a lot of the Earth's atmosphere. The air mass acts as a prism, splitting the wavelengths of white light and diffracting the red light to the bottom of the object, and the blue light to the top. At the time this image was taken, Porrima was only about 33 degrees above the horizon.

This image was over-sampled at 0.3127 arc seconds per pixel, so questions of the dispersion falling on a single pixel can be ruled out.

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Two-photon microscopy

Different wavelengths (colors) of light bend different amounts. This is called dispersion. Shorter, more energetic, blue wavelengths bend the most. Longer red wavelengths bend the least.

You might also notice that in the illustration of the prism at the top of this page, the red color is on the top and the blue color is on the bottom when they come out of the prism.

Deep tissue two-photon microscopy

When light travels from one medium to another and enters or leaves at an angle, this speed change and angle of entry cause it to bend and change direction. This is called refraction. The greater the angle of incidence, the greater the angle of refraction.

The Bayer matrix in the camera puts red, green and blue filters over individual pixels in a square with two green, one red, and one blue filter. The color information for each individual pixel in created by interpolating colors from neighboring pixels. So the color resolution is not quite as high as the luminance, which gets luminance information for each individual pixel even though it has a filter over it. In any event, the luminance and color information is pretty good for a normal subject, such as a white star. It is only for narrowband color images that resolution is lost in a Bayer image.

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You will notice that the stars are not circular, but are tear-drop shaped. This is because different wavelengths of light form Airy disks of different sizes. The red Airy disk at bottom is larger than the blue Airy disk at top. The overlapping different sizes cause the tear-drop shape. See the article Atmospheric dispersion and its effect on the Airy pattern by Andre Paquette for a more detailed explanation.

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Microscopyu

"Lucky imaging" was used to mitigate the effects of seeing with only the sharpest video frames from the Canon 550D (T2i) full-resolution 640x480 movie crop mode being selected in Registax.

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We see the effects of atmospheric prismatic dispersion in a telescope as a blue fringe on the top of the object, and the red fringe on the bottom, such as in the example of Porrima below.

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Looking at the pixel matrix that makes up the stars in the image, we can see that for each star, the red-wavelength image is on the bottom and the blue is on the top. In between the two stars, where the red from the top star overlaps the blue from the bottom star, the color is more green, which is a combination of red and blue, which is exactly what we would expect.

With a star diameter of 8 pixels, at 0.3127 arc seconds per pixel, each star has a diameter of about 2.5 arc seconds, a little better than the average seeing because of the lucky imaging technique employed. There are about 4 pixels between the star centers, with a diagonal of each pixel being about 6 microns. This corresponds to the separation of Porrima's components of about 1.7 arc seconds, exactly as we expect.

As observed from the Earth, the two identical mag 3.5, FO spectral class stars had an apparent separation of 1.7 arc seconds when this image was taken on March 15 of 2012. This is a tiny distance and double stars this close can only be seen in a telescope at high power. The two star's combined magnitudes give Porrima an apparent visual magnitude of 2.74.

However, when you look at a star in the sky, these colors are reversed. The blue is on top and red on the bottom. What is going on here?

Multiphotonmicroscopy

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Prismatic atmospheric dispersion, however, is a widely known phenomenon, and I have seen it both visually and in images taken over the years.

We can most easily see this effect with a prism, where light is broken up into its constituent colors - the colors of a rainbow - red, orange, yellow, green, blue, indigo and violet.

On the Digital_Astro group on Yahoo,Howard Ritter pointed out that spectrum is dispersed about 4 arc seconds at an elevation of 33 degrees. And that the spectrum of the two stars would overlap because their separation is 1.7 arc seconds. This seems to be to me the correct explanation.

Under perfect conditions, the C11 at f/10 can form a star with an Airy disk diameter of about 14.6 microns in red light at 600nm, and 11 microns at 450nm. However, many other factors influence the star size, with seeing being the most prominent, and focus, collimation, tracking, exposure length, and ISO coming into play. Seeing was bad to mediocre because of the low elevation of Porrima at the time this image was taken. Focus and collimation were good. Tracking was good because exposure length was short at 1/60th of a second. ISO was at 100 so there should have been little star spreading at this ISO.

It turns out that it is the position of the eye or camera as it views the star's spectrum that makes the colors reversed.

双光子显微镜

When light goes through something other than a vacuum, such as air, or water, or glass, it slows down. The speed of light is different in different media.

Each star image is about 8 pixels across in the original image. Each pixel in the Canon 550D is 4.3 microns. So that means each star is 8 * 4.3 = 34 microns. The difference in theoretical size vs actual size is probably due mostly to exposure and seeing effects, and the anti-aliasing filter in front of the sensor.

The following discussion is about the prismatic atmospheric dispersion that is visible in the image. Some people questioned whether this is what actually caused the color, or if it was an artifact of the camera's Bayer array or an artifact of image processing.

In the image of Porrima originally presented, you can not see individual pixels, even though they are exactly the same as presented here. Because of this, we perceive the individual color pixels as averaged. The top star has just a few more blue pixels and the bottom star just a few more red pixels. When the color saturation was increased a bit in post processing, this exaggerated this effect a little bit. The result is that the top star is perceived as mostly blue, and the bottom star as mostly red, or actually gold in this case because the red is mixed with yellow from thick atmospheric haze. The modified camera's increased red sensitivity also comes into play here in making the red portion of the star more visible.

Porrima, Gamma Virginis, is a binary or double star comprised of two stars that closely orbit each other in a 169-year elongated elliptical orbit. Located in the constellation of Virgo, Porrima is about 38.6 light years away from the Earth.