How are space photos taken?

We live in a fantastic time for astronomers (professionals or amateurs) and astronomy enthusiasts: a time when images of cosmic objects gained a wealth of detail never seen before. This evolution provided both unprecedented data for scientists to analyze and simply fabulous photographs—for lack of a better adjective to describe them. But how are space photos taken?

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  • Instruments such as the Hubble, Spitzer, VLT, Fermi telescopes, probes such as Juno, Cassini and Planck, and observatories such as ALMA provided not only resolution and range. incredible, but they allowed us to see the invisible — they reveal cosmic emissions in lights we can’t see, that is, at wavelengths outside the visible spectrum.

    With images in all waves of the spectrum electromagnetic — microwave, infrared, visible light, ultraviolet, radio, X-rays and gamma rays — we can see objects and phenomena that would have been unimaginable at the beginning of the last century. To cite just one example, scientists would never find evidence of black holes feeding on matter, because this type of event only emits radiation at wavelengths that are not part of visible light.

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    The UGC Galaxy 2013 (Image: Reproduction/NASA, ESA and B. Holwerda)

    Therefore, it is essential that there are telescopes and “specialist” observatories in certain types of issue. For example, Planck is a space telescope equipped with instruments capable of detecting microwave radiation, that is, objects whose light particles travel with the wavelength between 273 mm to 1 mm. This allows this telescope to detect, map and study evidence of the Big Bang called cosmic background radiation. publish on social networks, as if they were ordinary cameras. Scientists first need to download huge files and then calibrate the software for the best analysis. At this point, the images may not yet be colored (depending on the instrument and the type of data it generates).

    In the case of Hubble, for example, the images will never be originally colored, because the telescope takes a “photograph” at just a single wavelength. Then, a file type is generated with black and white images, and only after editing in some image processing software do they finally become aesthetically appealing. But how does it all work? What is this data like and how do they “see” a color image?

    Jupiter (Image: Reproduction/NASA/ESA/A. Simon /MH Wong/OPAL team)

    Below , we summarize the processes of capturing astronomical images at different wavelengths, and how they are processed to look attractive without anything “misleading” being applied. We’ll use a telescope as an example for each type of photo, with preference for those that allow anyone to use their data — yes, you can also create photos of the universe with data from large telescopes!

    Photos of space in visible light

    This is the simplest type of astronomical photo, as it only requires good lenses for the equipment to be used. If you have the necessary knowledge, you can use even medium-sized digital cameras, and you don’t even need to be in a region free from light pollution. Simpler or more robust telescopes can also help take pictures of planets, moons and deep space — clusters, galaxies and nebulae.

    Some large telescopes, observatories and space probes equipped with cameras use visible light to capture images — such as the Juno spacecraft, which studies Jupiter from orbit, and the Perseverance and Curiosity rovers, which are on the surface of Mars. These instruments are equipped with incredible resolution cameras, which allow you to photograph fabulous details, but cannot “see” very far.

    Veil Nebula (Image: Reproduction/ESA/Hubble/NASA/W. Blair)

    The best thing about instruments that use only visible light cameras is that their images can be accessed quickly if the equipment owner makes the data available. NASA, for example, has a policy of always providing all images collected by its space instruments in an accessible way, and any citizen scientist (ordinary people who want to help with some kind of research without having much knowledge) can download and process these images. .

    That’s why we have fantastic photos of Mars and Jupiter, like mosaics of the mounted panoramas. Normally, NASA lets citizen scientists themselves take care of this step—which they do with the greatest pleasure, by the way.

    Invisible spectrum photos

    There are many types of bodies in the cosmos, whether solid, gaseous or even plasma. Each of them behaves in a way, emitting or reflecting a specific type of light. For example, stars shine brightly in visible (white, which can be divided into rainbow colors) or invisible (infrared, ultraviolet, radio, X-ray, gamma, and microwave) light.

    Galaxies are made up of stars, so they also glow in visible light. However, they have many other types of objects and formations besides stars, such as nebulae, clouds of gas and dust, neutron stars, black holes — all glowing with lights at different wavelengths. The sum of all this results in a vibrant and continuous spectacle of the entire electromagnetic spectrum.

    Relativistic jet of the black hole galaxy M supermassive43 (Image: Hubble/STSCI/NASA/Daniele Cavalcante)

    This is because everything that is made of protons, neutrons and electrons (ie matter made of atoms) interacts with light. Or rather, with the fundamental “carrying” particles of light — photons. These particles travel through the universe at a constant speed (the speed of light) until they collide with something else — a gas particle, for example.

    When that happens, the result could be a absorption of that photon and, almost at the same time, the emission of a new photon, which will travel in a slightly different way from the previous one, in another wavelength. In other words, it will be another type of radiation, a different light. In nebulae, for example, the photons emitted by some star will be absorbed by hydrogen atoms, which in turn will compensate for the energy involved in this interaction, releasing another photon, which will have another energy.

    This new energy will have a very specific wavelength, which scientists can easily identify and say that this light came from a cloud of hydrogen. The same goes for all other objects and their respective emissions. Black holes, for example, when feeding on matter, let part of it escape in the form of jets that emit light in the wavelengths of radio, X-ray and gamma rays.

    Electron Capture Supernova 2885zd (Image: Hubble Space Telescope/STScI/Daniele Cavalcante)

    There are several types of interaction between radiation (or light, or photons) and matter (atoms), each generating a type of radiation. For example, atomic or molecular excitation occurs when electrons are displaced from their “orbits” around the nucleus and, when they return, emit the excess energy in the form of light, or X-rays. Ionization, on the other hand, is when electrons are removed from orbitals by radiation, leaving behind what is left of the atom, which is now called ion.

    Infrared and Ultraviolet

    Many areas of space are filled with clouds of gas and dust that block visible light. If we only depended on ordinary lenses, such as those of a digital camera or simple telescopes, we would never see what is behind these clouds, and we would fail to unravel many of the mysteries of the universe. But infrared radiation manages to pass through these obstacles until it reaches us.

    With infrared light telescopes, this radiation can be captured and transformed into an “image” in black and white. In fact, it will be a map, where each pixel represents an infrared emission, with a value that determines the intensity of that small point of light. Put this together in millions of pixels, each carrying a numerical intensity value, and we have a map of the object behind that cloud of gas and dust.

    With the proper software (we’ll talk about them later on in this matter), we can convert these scientific data maps into color images, which represent the cosmic object quite faithfully and with very high precision. Thanks to infrared telescopes we can see stars in formation, the center of galaxies, protoplanetary disks and the formation of new solar systems.

    AG Carinae (Image: Hubble Space Telescope/STScI/Daniele Cavalcante)

    As infrared is basically heat radiation, the telescope must be cooled to near absolute zero, ie to -90º Celsius, so that the data collected does not suffer interference from heat from the telescope itself. One of the best known is NASA’s Spitzer, which was in Earth orbit, as our planet’s atmosphere absorbs the scientifically interesting part of infrared waves (incidentally, the same goes for gamma and ultraviolet ray telescopes, but not for radio and micro -waves; that is, both space and ground telescopes are important).

    Other wavelengths

    Likewise, there are cosmic objects and phenomena that can only be fully understood if seen on radio signals — yes, they too they are a “light”, emitted through photons, but in wavelengths that can be from 1 m to 1 km. The radio allows us to see some events such as the formation of new planets, distant quasars and the mysterious rapid bursts that astronomers are still trying to unravel.

    Radio telescopes are a little different, as their waves are captured by antennas rather than lenses. Furthermore, the size of these waves requires gigantic plates if scientists are to cover a significant area of ​​the universe — which is why observatories like Arecibo are so large. An efficient solution is interferometry, which uses several small or medium dishes as one.

    The top image of the NGC nebula 6895 is a stacking of different wavelengths to transform the data into a “realistic” image; the lower one shows two emission lines of hydrogen, nitrogen and iron at wavelengths suitable for analyzing these elements (Image: Reproduction/STScI, APOD/J. Schmid/tJ. Kastner)

    The microwaves are very useful for studying the cosmic background radiation (CMB, the acronym in English), something the now-retired Planck telescope has done with flying colors. Unlike objects that emit/reflect visible light, infrared or even radio, the CMB does not reveal something with a definite shape, but a map full of spots, but very useful for astronomers.

    Finally, there are telescopes for X-rays and rays gamma, both very important for the observation of active galaxies, that is, that have supermassive black holes that feed on matter and dump plasma jets at almost the speed of light.

    Okay, but what about the images?

    The process for obtaining color and calibrated images from each of these telescopes depends on the type of technology employed. As we saw before, visible light instruments are quite simple: the photons hit the instrument’s lens and the data is sent to a CCD (chips more or less similar to those in digital cameras), which transforms them into electrical signals.

    These signals are sent to a data processing base that scientists can access. Well, it’s a little more complicated than that, especially when it comes to space telescopes that transmit data to Earth, but basically that’s how information from the universe gets to computers.

    Graph showing the wavelengths absorbed by the atmosphere and those that reach the ground (Image: Reproduction/STCI/JHU/NASA

    However, if the telescope is not exclusively for visible light, that image won’t be in color. After all, what would the ultraviolet color be? What would radium look like? Scientists don’t even need color images to work on this data. But there’s still hope for us who want to see nebulae in all of their glory and splendor: the Hubble palette.

    When an astronomer finds the time to use the Hubble telescope (or any telescope that uses infrared and ultraviolet light), a series of filters will be available. your disposal. Each filter allows only light of a single wavelength to reach the sensor. This means that each Hubble “photo” will only be one color: blue, red, green, or any length of infrared and ultraviolet (there are many).

    For a color image, it is I need to have at least the colors green, blue and red — the famous RGB. Therefore, the result of any single Hubble image looks something like the one we see below.

    The NGC galaxy 1566 , at three different wavelengths, captured by Hubble at 2018 (Image: Hubble Space Telescope/STScI/Daniele Cavalcante

    This photo is actually a digital file in the FITS format, specific for storing scientific data. FITS is not exactly an image, but a digital mapping, where each pixel carries information about the light emission it represents, with a value that determines the intensity of that little dot. Put that together in millions of pixels, each carrying a numerical intensity value, and we have a map of the object behind that cloud of gas and dust .

    But it’s still a black and white image. Lucky for us, the astros nomos rarely use Hubble to photograph just one wavelength. After all, they want to know as much as possible about the target object, so they usually capture images in other scientifically interesting “colors”. A lot of wisdom is needed here: we can see visible light with simpler telescopes, but few offer Hubble’s power to see other wavelengths. collect images in at least three lengths — infrared, ultraviolet, and some visible light. Sometimes there are a lot of images in different types of light, but the important thing is to get data on the maximum amount of things happening on that object. If a certain infrared wavelength is ignored, the astronomer could lose valuable information on some chemical element present there.

    If you have at least 3 images of the same object collected through Hubble at intervals of short time (minutes, or even hours between one image and another), each can be turned into one of the RGB colors—red, blue, and green—and stacked in layers. The result will be a very realistic color image, with natural colors that mimic what we might see if our eyes saw everything that is happening on that object. The result you see below. Cool huh?

    The NGC galaxy 1024, at three different wavelengths, captured by Hubble in (Image: Hubble Space Telescope/ STScI/Daniele Cavalcante)

    To make images like this with Hubble data, you simply “dig into” the databases and request the FITS files for the most interesting and widest possible wavelengths, and associate each with one of the three RGB colors. You will also need specific software, such as FITS Liberator or DS9 (developed with a more user-friendly interface for those who are not so concerned with “gross” science).

    In these software, you can turn any of the Hubble images into a red, green, or blue photo. In our example above, the NGC galaxy 642, captured by Hubble in 2013, has been selected and its data has been requested. After receiving the FITS files, the “Hubble palette” was applied, associating the longest wavelengths with red, the shortest ones with blue and the medium ones with green.

    After performing this process, the color image is even exported in PNG format and edited in some image editing software to further enhance details, highlighting the object of interest, removing unwanted lighting and noise. And that’s it, the astronomical image is ready for publication in the media and appreciation of the general public.

    Of course, it is also possible to do the same with data from other instruments, such as the Juno probe, the rovers Curiosity and Perseverance, among others. Each one, however, will require a learning curve and will provide the data in a different way.

    If you are interested in being part of the citizen science of image processing, it is recommended to study about the FITS files, for starters, also available for data from Chandra X-ray telescopes. Also check out the FITS Liberator documentation. If you prefer to process visible light images, NASA invites anyone to “play” with the Juno spacecraft files. There’s a lot more out there, but this is a great start!

    Source: NASA

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