When we publish a deep-sky themed astrophoto, many will surely wonder what they exatly see in the picture. In the case of Moon, Jupiter, Saturn photos, there is not much to explain about the given image (of course there are many interesting things about them, but the sight usually speaks for itself, and this of course does not detract from the merits of these photos), but rarely photographed objects like this usually reveal only a few obvious things about themselves to non-photographers. Furthermore, the ambiguity of the circumstances and astrophysical background of the making also gives many the feeling that the images are “colored”, “kitsched” arbitrarily or according to supposed aesthetic considerations. While in some cases this is done intentionally or accidentally, and of course subjectivity cannot be completely ruled out, it is never the primary intention for most of us (otherwise, there are many examples of foreign photographers mentioned above).
So I was thinking about explaining a little bit about exactly what we’re seeing and why we’re seeing it in the picture with my latest photo.
The first and most important, almost fundamental thing is to strive for astrophysical fidelity when processing images. What is not in the picture we don’t put there, what is there (in space), we don’t erase. And we don’t do it the way it can’t be out there. All aesthetic issues – saturation, contrast, composition – can only follow.
My image depicts the emission nebula complex NGC6914 and its environment. This area is located in the Milky Way disc, in the constellation Swan. Usually in the arms of the Milky Way, where the material of our galaxy is concentrated, there are a large number of stars and massive amount gas and dust that surround them. Areas like this typically photographed in Milky Way often glow in red, as you can see in the picture. This is not because of the convention of photographers, but because ultraviolet radiation from the surrounding hot stars brings the atoms of the aforementioned gases into an energized state. In this case, an electron “circulating” around the nucleus enters a shell of higher energy. When this state ceases, the electron returns to its stable orbit and the previously transmitted energy is emitted in the form of light. This light has a well-defined and constant wavelength that is clearly characteristic of the material. In the case of hydrogen, the radiation of the first energized state is 656,281 nanometers. And this is a red color and this is called Hydrogen alpha radiation. And why are almost all such images red? Because hydrogen is the most common material in our universe, its most elemental building block, the stars are also born of these hydrogen clouds, they are still born, so we will most likely find such hydrogen-rich areas in the arms of the Milky Way. But not only hydrogen can emit light when energized, but also oxygen or sulfur, for example, but visible light is created in kitchen fluorescent lamps on a similar principle. Furthermore, hydrogen has not only one such state, but several that are no longer red. Such luminous nebulae are otherwise called emission nebulae. The gas composition of an emission nebula strongly influences its color. It takes a long examination and precise follow-up to return them as realistic as possible. This is further complicated by the fact that the colors are strongly distorted by atmospheric conditions, altitude above the horizon and light pollution, so after a certain point, the photographer’s subjectivity actually enters the picture.
Why don’t we see these colors with the naked eye too? On one hand, because such an area does not emit enough light for our eyes to see in color, moreover, for most of these wavelengths, our eyes are almost completely blind. For example, Hydrogen alpha falls at the very edge of the range we perceive. In many cases, we write to the technical data that the image was taken with a modified DSLR camera. The modification is also necessary because the factory condition machine is just as sensitive to this range as our eyes. During the process, an infrared filter is built into the machine, which allows this range to pass through with an efficiency of around 90%. Furthermore, long exposures of up to several minutes are needed to collect enough light on the sensor to be able to extract enough information from it.
Now let’s look at the stars. Even my own father, who has been an acclaimed amateur photographer for 40 years, grimaced at one of my similar photos and asked why I should color the stars this way and why I put those kitschy rays on it. Therefore, I think this phenomenon also needs to be explained. The color of the stars is real! In fact, the stars are the surest point of reference for determining the correct color balance for a photo. And if we gather enough light for our photo, these colors can be defined nicely and relatively accurately. This is because color is directly related to the temperature of the star. Put simply, the warmer a body is, the shorter its wavelength emits its heat in the form of electromagnetic radiation. This phenomenon was already known in the 19th century, but its exact description and solution of the problem of temperature radiation was impossible with the tools of traditional physics. The solution was found by Max Planck on the basis of a hypothesis published in 1900, which proved to be true and, incidentally, laid the foundations of quantum physics. So if a star is “cold”, it will turn red. As it warms, its color shifts from spectrum to orange to yellow and then to blue. Astronomers keep an accurate record of the stars in a cataloged in accessible way. In the case of very bright stars, these color differences can be easily recognized with the naked eye (Arcturus vs Vega, Betelgeuse vs Sirius).
As for the rays, they don’t get into the picture either during processing, although it’s accepted abroad to draw spikes on the stars, but I don’t think it’s either ethical or aesthetic. Spikes are a feature of photos taken with Newtonian telescopes. In front of the tube, where the secondary mirror is located, there are usually 4 mirror holder legs that cross the path of light. The “glittering” light on these creates the 4 rays of the star, which are otherwise called diffraction spikes. Since these are not in the sky, they could even be removed, but since the result of this operation is doubtful, it is more common to work them out as aesthetically as possible as part of the composition. The shape of these also reveals the condition of our telescope. If they are asymmetrical or open from the star, it all indicates that the legs are not properly positioned, which can even cause other imperfections. Catadioptrical tubes with or without lenses may not form such a diffraction spike.
However, in addition to the glowing hydrogen and colored stars, there is much more to the picture, and most of the time, this “other” is the real photo subject. In this case, it is no different, as the main theme is the bluish-twisted patch group in the middle of the image and the diagonally extending dark band. These are the so-called reflection nebulas, which reflect back or just obscure the light from the surrounding stars. The bluish spots seen here are blue because they are lit by blue hot stars. They look very similar to the dust around the Pleiades anyway. The dark band is interstellar dust and gas that is not illuminated and energized by any star (at least the part facing us) and that evolves through the Milky Way during the evolution of our galaxy under the influence of stellar winds and shock waves and remnants of ancient supernovae. Such forces usually cause a clump in these clouds of matter over and over again, leading to the birth of more and more stars and planets. Surprisingly, hiding behind the dark dust can often be seen so-called Herbig-Haro objects (although my resolution for this is already starting to be scarce), which is a clear and spectacular sign of star formation. With such objects, it is possible to capture just how the material falling into the protostar ejects in the form of a jet through the poles of the star and then glows when it collides with the interstellar medium. Similar objects can also be observed in the dark dust bands of the M78 reflection nebula. In addition, there are a number of exciting details worth browsing and researching in the image, such as the V1515 Cyg star, on which the dust bubble formed around the star can be seen quite clearly, from which it is thought to form its planetary system. But not only on this, but on almost any image we take with telescopes facing the sky worth invetigating because they bring surprising but elusive, incomprehensibly seemingly distant mystical phenomena read in books or seen on TV at arm’s length.
Távcső: 200/800 SkyWatcher Newton
Mechanika: SkyWatcher NEQ6 pro upgraded
Vezetés: Lacerta MGen autoguider
Kamera: Canon EOS 600D
Exposure: 141*5 min on ISO 800
Készítés ideje: from 2017.06.23 to 2017.07.18
Készítés helye: Ágasvár, Mezőfalva, Medvida (Hr), Velika Popina (Hr)